<div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Mon, Jul 24, 2017 at 9:02 AM Peter Lawrence <<a href="mailto:peterl95124@sbcglobal.net">peterl95124@sbcglobal.net</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><blockquote type="cite"><div>On Jul 21, 2017, at 10:55 PM, Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>> wrote:</div><br class="m_-3161789555371559420Apple-interchange-newline"><div><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">2017-07-21 22:44 GMT-07:00 Peter Lawrence <span dir="ltr"><<a href="mailto:peterl95124@sbcglobal.net" target="_blank">peterl95124@sbcglobal.net</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><span style="font-size:14px">Mehdi,</span><div><span style="font-size:14px"> Hal’s transformation only kicks in in the *presence* of UB</span></div></div></blockquote><div><br></div><div>No, sorry I entirely disagree with this assertion: I believe we optimize program where there is no UB. We delete dead code, code that never runs, so it is code that does not exercise UB.</div></div></div></div></div></blockquote><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">Mehdi,</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures"> I had to read that sentence several times to figure out what the problem</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">is, which is sloppy terminology on my part</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255);min-height:16px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">Strictly speaking the C standard uses “undefined behavior” to describe what</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">happens at runtime when an “illegal” construct is executed. I have been using</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">“undefined behavior” and UB to describe the “illegal” construct whether it is</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">executed or not.</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255);min-height:16px"><span style="font-variant-ligatures:no-common-ligatures"> </span><br class="m_-3161789555371559420webkit-block-placeholder"></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">Hence I say “Hal’s transform is triggered by UB”, when I should be saying</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">“Hal’s transformation is triggered by illegal IR”.</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255);min-height:16px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">All I can say is I’m not the only one being sloppy, what started this entire </span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">conversation is the paper titled “Taming Undefined Behavior in LLVM”, while</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">the correct title would be “Taming Illegal IR in LLVM”. (I think we are all</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255)"><span style="font-variant-ligatures:no-common-ligatures">pretty confident that LLVM itself is UB-free, or at least we all hope so :-).</span></div><div style="margin:0px;line-height:normal;background-color:rgb(255,255,255);min-height:16px"><span style="font-variant-ligatures:no-common-ligatures"></span></div></div><div>I believe you are being sloppy when you say "we optimize program </div><div>where there is no UB”, because I believe you mean "we optimize program </div><div>under the assumption that there is no UB”. In other words we recognize</div><div>“Illegal” constructs and then assume they are unreachable, and delete </div><div>them, even when we can’t prove by any other means that they are</div><div>unreachable. We don’t know that there is no (runtime) UB, we just assume it.</div></div></div><div style="word-wrap:break-word"><div><div><br></div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div>The example Hal showed does not exhibit UB, it is perfectly valid according to the standard.</div><div><br></div></div></div></div></div></blockquote><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Whether it exhibits UB at runtime or not is not the issue, the issue is what </div><div>a static analyzer or compiler can tell before runtime, see below</div></div></div><div style="word-wrap:break-word"><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> <br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><span style="font-size:14px">, and</span></div><div><span style="font-size:14px">it does not matter how that UB got there, whether by function inlining</span></div><div><span style="font-size:14px">or without function inlining.</span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px">The problem with Hal’s argument is that the compiler does not have</span></div><div><span style="font-size:14px">a built in ouija board with which it can conjure up the spirit of the</span></div><div><span style="font-size:14px">author of the source code and find out if the UB was intentional</span></div><div><span style="font-size:14px">with the expectation of it being deleted, or is simply a bug.</span></div><div><span style="font-size:14px">Function inlining does not magically turn a bug into not-a-bug, nor</span></div><div><span style="font-size:14px">does post-inlining simplification magically turn a bug into not-a-bug.</span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px">Let me say it again: if the compiler can find this UB (after whatever</span></div><div><span style="font-size:14px">optimizations it takes to get there) then the static analyzer must</span></div><div><span style="font-size:14px">be able to do the same thing, forcing the programmer to fix it</span></div><div><span style="font-size:14px">rather than have the compiler optimize it.</span></div></div></blockquote><div><br></div><div>This is again incorrect: there is no UB in the program, there is nothing the static analyzer should report.</div></div></div></div></div></blockquote><div><br></div><div><br></div></div></div><div style="word-wrap:break-word"><div><div>Hal’s example starts with this template</div><div><br></div><div><div><blockquote type="cite" style="background-color:rgb(255,255,255)"><div><div>template <typename T></div><div>int do_something(T mask, bool cond) {</div><div> if (mask & 2)</div><div> return 42;</div><div><br></div><div> if (cond) {</div><div> T high_mask = mask >> 48; // UB if sizeof(T) < 8, and cond true</div><div> if (high_mask > 5)</div><div> do_something_1(high_mask);</div><div> else</div><div> do_something_2();</div><div> }</div><div><br></div><div> return 0;</div><div>}</div></div></blockquote></div><div><div></div></div></div><div><br></div><div>Which is then instantiated with T = char,</div><div>and where it is impossible for either a static analyzer or a </div><div>compiler to figure out and prove that ‘cond’ is always false.</div><div><br></div><div>Hence a static analyzer issues a warning about the shift,</div><div>while llvm gives no warning and instead optimizes the entire</div><div>if-statement away on the assumption that it is unreachable.</div><div><br></div><div>Yes a static analyzer does issue a warning in this case.</div><div><br></div><div><br></div><div>This is not the only optimization to be based on assumption</div><div>rather than fact, for example type-based-alias-analysis is</div><div>based on the assumption that the program is free of this sort</div><div>of aliasing. The difference is that a user can disable TBAA</div><div>and only TBAA if a program seems to be running incorrectly </div><div>when optimized and thereby possibly track down a bug, but</div><div>so far there is no command line option to disable UB-based-</div><div>analysis (or ‘illegal-IR-based” :-), but there really needs to be.</div><div><br></div><div>Do we at least agree on that last paragraph ?</div></div></div></blockquote><div><br>We likely agree it's good to have tools to help developers identify/diagnose UB in their programs. And we have that: -fsanitize=undefined (not only does it effectively disable many UB-based optimizations (because it makes them not undefined - by conditionalizing the code to check that UB isn't reached, as such) - it even provides pretty diagnostics (of course you can't actually continue running the program - if the line after the diagnostic will dereference a null pointer - there's no non-null pointer we can magic-up, so execution must stop))<br> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><div><br></div><div><br></div><div>Peter Lawrence.</div><div><br></div><div><br></div><div><br></div></div></div><div style="word-wrap:break-word"><div><br></div><div><br><blockquote type="cite"><div><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div>The compile is still able to delete some code, because of breaking the abstraction through inlining or template instantiation for example (cf Hal example).</div><div><br></div><div>-- </div><div>Mehdi</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px">Or, to put it another way: there is no difference between a compiler</span></div><div><span style="font-size:14px">and a static analyzer [*]. So regardless of whether it is the compiler or</span></div><div><span style="font-size:14px">the static analyzer that finds any UB, the only rational thing to do with</span></div><div><span style="font-size:14px">it is report it as a bug.</span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px">Peter Lawrence.</span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px"><br></span></div><div><span style="font-size:14px">[* in fact that’s one of the primary reasons Apple adopted llvm, to use</span></div><div><span style="font-size:14px"> It as a base for static analysis]</span></div><div><div class="m_-3161789555371559420h5"><div><span style="font-size:14px"><br></span></div><div><br></div><div><br><div><blockquote type="cite"><div>On Jul 21, 2017, at 10:03 PM, Mehdi AMINI <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>> wrote:</div><br class="m_-3161789555371559420m_794137231564833888Apple-interchange-newline"><div><br class="m_-3161789555371559420m_794137231564833888Apple-interchange-newline"><br style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><div class="gmail_quote" style="font-family:Helvetica;font-size:12px;font-style:normal;font-variant-caps:normal;font-weight:normal;letter-spacing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">2017-07-21 21:27 GMT-07:00 Peter Lawrence<span class="m_-3161789555371559420m_794137231564833888Apple-converted-space"> </span><span dir="ltr"><<a href="mailto:peterl95124@sbcglobal.net" target="_blank">peterl95124@sbcglobal.net</a>></span>:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><div style="word-wrap:break-word"><font face="Menlo">Sean,</font><div><font face="Menlo"> Let me re-phrase a couple words to make it perfectly clear</font></div><div><br><div><span><blockquote type="cite"><div><font face="Menlo">On Jul 21, 2017, at 6:29 PM, Peter Lawrence <<a href="mailto:peterl95124@sbcglobal.net" target="_blank">peterl95124@sbcglobal.net</a>> wrote:</font></div><font face="Menlo"><br class="m_-3161789555371559420m_794137231564833888m_2150444843056015504Apple-interchange-newline"></font><div><div style="word-wrap:break-word"><font face="Menlo">Sean,</font><div><font face="Menlo"><br></font></div><div><font face="Menlo">Dan Gohman’s “transform” changes a loop induction variable, but does not change the CFG,</font></div><div><font face="Menlo"><br></font></div><div><font face="Menlo">Hal’s “transform” deletes blocks out of the CFG, fundamentally altering it.</font></div><div><font face="Menlo"><br></font></div><div><span style="font-family:Menlo">These are two totally different transforms.</span></div></div></div></blockquote><blockquote type="cite"><div style="word-wrap:break-word"><div><br></div><div><span style="font-family:Menlo"><br></span></div></div></blockquote><blockquote type="cite"><div><div style="word-wrap:break-word"><div><font face="Menlo">And even the analysis is different,</font></div><div><font face="Menlo"><br></font></div><div><font face="Menlo">The first is based on an *assumption* of non-UB (actually there is no analysis to perform)</font></div></div></div></blockquote></span><font face="Menlo"> the *absence* of UB<br></font><span><blockquote type="cite"><div style="word-wrap:break-word"><div><font face="Menlo"><br></font></div><div><font face="Menlo">the second Is based on a *proof* of existence of UB (here typically some non-trivial analysis is required)</font></div></div></blockquote></span><font face="Menlo"> <span class="m_-3161789555371559420m_794137231564833888Apple-converted-space"> </span>the *presence* of UB<br></font><span><br><blockquote type="cite"><div style="word-wrap:break-word"><div><font face="Menlo">These have, practically speaking, nothing in common.</font></div><div><font face="Menlo"><br></font></div></div></blockquote><div><br></div><div><br></div></span><div><font face="Menlo">In particular, the first is an optimization, while the second is a transformation that</font></div><div><font face="Menlo">fails to be an optimization because the opportunity for it happening in real world</font></div><div><font face="Menlo">code that is expected to pass compilation without warnings, static analysis without</font></div><div><font face="Menlo">warnings, and dynamic sanitizers without warnings, is zero.</font></div><div><font face="Menlo"><br></font></div><div><font face="Menlo">Or to put it another way, if llvm manages to find some UB that no analyzer or</font></div><div><font face="Menlo">sanitizer does, and then deletes the UB, then the author of that part of llvm</font></div><div><font face="Menlo">is in the wrong group, and belongs over in the analyzer and/or sanitizer group.</font></div></div></div></div></blockquote><div><br></div><div>I don't understand your claim, it does not match at all my understand of what we managed to get on agreement on in the past.</div><div><br></div><div>The second transformation (dead code elimination to simplify) is based on the assumption that there is no UB.</div><div><br></div><div>I.e. after inlining for example, the extra context of the calling function allows us to deduce the value of some conditional branching in the inline body based on the impossibility of one of the path *in the context of this particular caller*.</div><div><br></div><div>This does not mean that the program written by the programmer has any UB inside.</div><div><br></div><div>This is exactly the example that Hal gave.</div><div><br></div><div>This can't be used to expose any meaningful information to the programmer, because it would be full of false positive. Basically a program could be clean of any static analyzer error, of any UBSAN error, and totally UB-free, and still exhibit tons and tons of such issues.</div><div><br></div><div>-- </div><div>Mehdi</div></div></div></blockquote></div><br></div></div></div></div></blockquote></div><br></div></div>
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