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<p class="MsoNormal" style="margin-left:4.8pt">Clang's behavior seems correct. The general goal of stepping is to stop at every statement boundary.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Which means, the ideal is to stop at both `if` statements. Neither compiler allows that. Clang's behavior is closer to the ideal
than GCC, even if it looks inconsistent. But I would not be inclined to complain about either behavior; they are idiosyncrasies of how each compiler chooses to handle these expressions at –O0.<o:p></o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> cfe-dev [mailto:cfe-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>John McCall via cfe-dev<br>
<b>Sent:</b> Thursday, August 24, 2017 9:38 AM<br>
<b>To:</b> Frozen<br>
<b>Cc:</b> cfe-dev<br>
<b>Subject:</b> Re: [cfe-dev] Evaluate constant condition of if statement has no effect<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Aug 24, 2017, at 3:01 AM, Frozen <<a href="mailto:bluechristlove@163.com">bluechristlove@163.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Firstly, I briefly clarify the logic of Info.noteSideEffect. Info.noteSideEffect will set EvalStatus.HasSideEffects = true; and make hasUnacceptableSideEffect function always
return true so that our <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">EvaluateAsInt alway return false when LHS can not evaulate.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Yes, I understand the implementation.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Then I want to express one significant point of it, that is debug. Let us compare it with GCC in the following example.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">int main()<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">{<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> int x;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (1 || x) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> printf("LHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (x || 1) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> printf("RHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> }<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">}<br>
<br>
When we use GCC to debug it, we will get the following result:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) b main<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Breakpoint 1 at 0x10000620: file a.c, line 5.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) r<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Starting program: a.out<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Breakpoint 1, main () at a.c:5<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">5
<span class="apple-converted-space"> </span> printf("LHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Missing separate debuginfos, use: debuginfo-install glibc-2.17-78.ael7b.ppc64le<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) step<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">LHS<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">9
<span class="apple-converted-space"> </span> printf("RHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) step<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">RHS<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">11 }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">And if we use clang, we will get:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) b main<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Breakpoint 1 at 0x10000614: file a.c, line 5.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) r<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Starting program: a.out<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Breakpoint 1, main () at a.c:5<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">5
<span class="apple-converted-space"> </span> printf("LHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Missing separate debuginfos, use: debuginfo-install glibc-2.17-78.ael7b.ppc64le<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) step<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">LHS<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">8
<span class="apple-converted-space"> </span> if (x || 1) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) step<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">9
<span class="apple-converted-space"> </span> printf("RHS\n");<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">(gdb) step<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">RHS<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">11 }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
We can see that we will stop at line 8 and can not stop at line 9 directly like GCC or LHS is constant. This is somehow not unified behaviour and make uses feel strange.<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Clang's behavior seems correct. The general goal of stepping is to stop at every statement boundary.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">John.<o:p></o:p></p>
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<p class="MsoNormal"><br>
<br>
<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
</span><span style="font-size:10.5pt;font-family:"MS Gothic"">在</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> 2017-08-24 13:59:06</span><span style="font-size:10.5pt;font-family:"MS Gothic"">，</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">"John
McCall" <<a href="mailto:rjmccall@apple.com">rjmccall@apple.com</a>> </span><span style="font-size:10.5pt;font-family:"MS Gothic"">写道：</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
<br>
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">On Aug 24, 2017, at 1:26 AM, Frozen <<a href="mailto:bluechristlove@163.com">bluechristlove@163.com</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Not. Original implementation used the default parameter value, i.e. No Side Effects<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> /// EvaluateAsInt - Return true if this is a constant which we can fold and<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> /// convert to an integer, using any crazy technique that we want to.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> bool EvaluateAsInt(llvm::APSInt &Result, const ASTContext &Ctx,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> <b>SideEffectsKind AllowSideEffects = SE_NoSideEffects</b>) const;<br>
<br>
But, now, we should<span class="apple-converted-space"> </span><b>allow side effects</b><span class="apple-converted-space"> </span>and pass the computation to the function </span><b><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">Info.noteSideEffect().<span class="apple-converted-space"> </span></span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">As the
comment of source code said:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">bool DataRecursiveIntBinOpEvaluator::<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> VisitBinOpLHSOnly(EvalResult &LHSResult, const BinaryOperator *E,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> bool &SuppressRHSDiags) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (E->isLogicalOp()) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> bool LHSAsBool;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (!LHSResult.Failed && HandleConversionToBool(LHSResult.Val, LHSAsBool)) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // We were able to evaluate the LHS, see if we can get away with not<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // evaluating the RHS: 0 && X -> 0, 1 || X -> 1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (LHSAsBool == (E->getOpcode() == BO_LOr)) {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> Success(LHSAsBool, E, LHSResult.Val);<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> return false; // Ignore RHS<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> }<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> } else {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> LHSResult.Failed = true;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // Since we weren't able to evaluate the left hand side, it</span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // might have had side effects.</span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (!Info.noteSideEffect())</span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> return false;</span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // We can't evaluate the LHS; however, sometimes the result<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // is determined by the RHS: X && 0 -> 0, X || 1 -> 1.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> // Don't ignore RHS and suppress diagnostics from this arm.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> SuppressRHSDiags = true;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> return true;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> }<o:p></o:p></span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">So we might have had side effects when constant-evaluation in if stmt condition , so we should use </span><b><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">Expr::SE_AllowSideEffects,<span class="apple-converted-space"> </span></span></b><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">then use <b>Info.noteSideEffect()<span class="apple-converted-space"> </span></b>to
compute the result. <o:p></o:p></span></p>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Info.noteSideEffect() is not given the expression to evaluate, nor does it call back to IR-generation in a way that would allow us to emit the side effect. A more promising
approach would be to modify IRGen's logic for emitting branches on conditions, which is already a special-case expression emitter due to the desire to directly branch on && and || instead of branching on a phi. That emitter has special-case logic for constant
branches, but from the generated code it looks like we're not applying that in all positions, just at the top level. Recognizing when we're about to branch on a constant i1 would be sufficient to catch things like this and allow IRGen to reap the (small)
benefits of emitting less silly code. I don't think there's much point in trying to recognize (X || true) especially, but we could do that, too, if you have a good argument.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">John.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
<br>
<o:p></o:p></span></p>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
</span><span style="font-size:10.5pt;font-family:"MS Gothic"">在</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> 2017-08-24 02:02:23</span><span style="font-size:10.5pt;font-family:"MS Gothic"">，</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">"John
McCall" <<a href="mailto:rjmccall@apple.com">rjmccall@apple.com</a>> </span><span style="font-size:10.5pt;font-family:"MS Gothic"">写道：</span><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
<br>
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
<div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">On Aug 23, 2017, at 1:58 PM, bluechristlove <<a href="mailto:bluechristlove@163.com">bluechristlove@163.com</a>> wrote:<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
<div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">This is not specific example. If we don’t fix it, user will feel strange why LHS of if stmt condition could do constant folding but RHS couldn’t.<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">From the source code view’s point, this is our compiler bug.<o:p></o:p></span></p>
</div>
</div>
</div>
</div>
</blockquote>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">In the ExprConstant.cpp:<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> case Job::BinOpKind: {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> ...<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (!VisitBinOpLHSOnly(Result, Bop, SuppressRHSDiags)) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> Queue.pop_back();<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> ...<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> job.Kind = Job::BinOpVisitedLHSKind;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> enqueue(Bop->getRHS());<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> case Job::BinOpVisitedLHSKind: {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> ...<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> Result.Failed = !VisitBinOp(job.LHSResult, RHS, Bop, Result.Val);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> Queue.pop_back();<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">If we can not evaluate LHS successfully, we will evaluate RHS (function VisitBinOp). In the function VisitBinOp:<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (E->isLogicalOp()) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool lhsResult, rhsResult;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool LHSIsOK = HandleConversionToBool(LHSResult.Val, lhsResult);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool RHSIsOK = HandleConversionToBool(RHSResult.Val, rhsResult);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (LHSIsOK) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (RHSIsOK) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (E->getOpcode() == BO_LOr)<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return Success(lhsResult || rhsResult, E, Result);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> else<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return Success(lhsResult && rhsResult, E, Result);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> } else {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (RHSIsOK) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // We can't evaluate the LHS; however, sometimes the result</span></b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // is determined by the RHS: X && 0 -> 0, X || 1 -> 1.</span></b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (rhsResult == (E->getOpcode() == BO_LOr)) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return Success(rhsResult, E, Result);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">We can see that our previous implementation has consider this condition of RHS constant folding. But because of ignore the parameter Expr::SE_AllowSideEffects in the function ConstantFoldsToSimpleInteger:<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">bool CodeGenFunction::ConstantFoldsToSimpleInteger(const Expr *Cond,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> llvm::APSInt &ResultInt,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool AllowLabels) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // FIXME: Rename and handle conversion of other evaluatable things<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // to bool.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> llvm::APSInt Int;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (!Cond->EvaluateAsInt(Int, getContext(),<span class="apple-converted-space"> </span><b>Expr::SE_AllowSideEffects)</b>) -------> allow side effect<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return false; // Not foldable, not integer or not fully evaluatable.<o:p></o:p></span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">We will not make the RHS constant folding happened.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">bool DataRecursiveIntBinOpEvaluator::<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> VisitBinOpLHSOnly(EvalResult &LHSResult, const BinaryOperator *E,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool &SuppressRHSDiags) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">….<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (E->isLogicalOp()) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> bool LHSAsBool;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> E->getLHS()->dumpColor();<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (!LHSResult.Failed && HandleConversionToBool(LHSResult.Val, LHSAsBool)) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // We were able to evaluate the LHS, see if we can get away with not<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // evaluating the RHS: 0 && X -> 0, 1 || X -> 1<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (LHSAsBool == (E->getOpcode() == BO_LOr)) {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> Success(LHSAsBool, E, LHSResult.Val);<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return false; // Ignore RHS<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> } else {<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> LHSResult.Failed = true;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // Since we weren't able to evaluate the left hand side, it<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> // might have had side effects.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> if (!Info.noteSideEffect()) { --------> HERE!!!</span></b><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> return false;<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""> }<o:p></o:p></span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">The detail source code analysis could be find in the comment 2 of <a href="https://bugs.llvm.org/show_bug.cgi?id=34229"><span style="font-family:"Arial","sans-serif"">https://bugs.llvm.org/show_bug.cgi?id=34229</span></a><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">So, I think we should fix it. <o:p></o:p></span></p>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Are you arguing that we should generally ignore side-effects when constant-evaluating an if condition?<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">John.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><br>
<br>
<o:p></o:p></span></p>
<div>
<div>
<div>
<div>
<div>
<div>
<div style="margin-left:-15.0pt">
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
</div>
</div>
</div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
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<p class="MsoNormal" style="background:#EFEFEF"><b><span style="font-size:9.0pt;font-family:"PMingLiU","serif"">发件人：</span></b><span style="font-size:9.0pt;font-family:"PingFang SC","serif""> <a href="mailto:rjmccall@apple.com" target="_blank">John McCall</a><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:#EFEFEF"><b><span style="font-size:9.0pt;font-family:"PMingLiU","serif"">发送时间：</span></b><span style="font-size:9.0pt;font-family:"PingFang SC","serif""> 2017-08-23 01:30:45<o:p></o:p></span></p>
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<p class="MsoNormal" style="background:#EFEFEF"><b><span style="font-size:9.0pt;font-family:"MS Mincho"">收件人：</span></b><span style="font-size:9.0pt;font-family:"PingFang SC","serif""> <span class="apple-converted-space"> </span><a href="mailto:bluechristlove@163.com" target="_blank">Frozen</a><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:#EFEFEF"><b><span style="font-size:9.0pt;font-family:"MS Mincho"">抄送：</span></b><span style="font-size:9.0pt;font-family:"PingFang SC","serif""> <span class="apple-converted-space"> </span><a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><o:p></o:p></span></p>
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<p class="MsoNormal" style="background:#EFEFEF"><b><span style="font-size:9.0pt;font-family:"MS Mincho"">主</span></b><b><span style="font-size:9.0pt;font-family:"PMingLiU","serif"">题：</span></b><span style="font-size:9.0pt;font-family:"PingFang SC","serif""> Re:
[cfe-dev] Evaluate constant condition of if statement has no effect<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">On Aug 21, 2017, at 12:28 AM, Frozen via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Simple Example:<br>
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int main()<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">{<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> int x;<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> if (x || 1) {}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">Clang can not evaluate this condition and will emit IR like this:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">define signext i32 @main() #0 {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">entry:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %retval = alloca i32, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %x = alloca i32, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> store i32 0, i32* %retval, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %0 = load i32, i32* %x, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %tobool = icmp ne i32 %0, 0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> br i1 %tobool, label %if.then, label %lor.lhs.false<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">lor.lhs.false: ; preds = %entry<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> br i1 true, label %if.then, label %if.end<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">if.then: ; preds = %lor.lhs.false, %entry<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> br label %if.end<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">if.end: ; preds = %if.then, %lor.lhs.false<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %1 = load i32, i32* %retval, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> ret i32 %1<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">}<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">However, when we swap the position of LHS and RHS(i.e. if (1 || x), Clang can recognize it:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">define signext i32 @main() #0 {<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">entry:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> %x = alloca i32, align 4<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif""> ret i32 0<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial","sans-serif"">}<br>
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I also find the root issue and propose one potential solution in comment 2 of this link: <a href="https://bugs.llvm.org/show_bug.cgi?id=34229">https://bugs.llvm.org/show_bug.cgi?id=34229</a><br>
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Any idea?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">There will always be some example of something that we generate less efficiently at -O0 than we could. Why is this example specifically worth optimizing?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"PingFang SC","serif"">John.<o:p></o:p></span></p>
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