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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">>
</span>Is there a way I can suppress that expansion? I'd really like to get hold of the IR with the va_args intact.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">IIRC, doing what DefaultABIInfo::EmitVAArg() does should cause va_args to be emitted but this will break varargs for targets that need the expansion.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I'm not sure if this affects your use case or not but I ought to mention it just in case: The fixed arguments in clang-generated IR are target dependent too. For example, some targets add signext/zeroext attributes and some lower structures
into multiple integer arguments and change member accesses to suit.<span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Preston Briggs via llvm-dev<br>
<b>Sent:</b> 29 September 2015 17:31<br>
<b>To:</b> mats petersson<br>
<b>Cc:</b> llvm-dev<br>
<b>Subject:</b> Re: [llvm-dev] varargs, the x86, and clang<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">When I read the LLVM IR manual, the description of the Variable Argument Handling Intrinsics and the va_arg instruction lead me to expect IR that uses va_arg instead of loads and register references.<o:p></o:p></p>
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<p class="MsoNormal">I was try to get hold of the IR before it becomes too machine dependent, i.e., before code generation, but it seems as though va_arg is replaced/expanded very early, before most optimization.<o:p></o:p></p>
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<p class="MsoNormal">Is there a way I can suppress that expansion? I'd really like to get hold of the IR with the va_args intact.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">My other question regarding -arch and -march I think involves a misleading (imo) or out-of-date man page.<o:p></o:p></p>
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<p class="MsoNormal">Using -target is an effective way to get code for other machines.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Preston<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Sep 29, 2015 at 1:06 AM, mats petersson <<a href="mailto:mats@planetcatfish.com" target="_blank">mats@planetcatfish.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">What is it you are actually trying to do?
<br>
<br>
Varargs are architecture dependent, so will need to take into account the calling convention of the architecture it is on.<o:p></o:p></p>
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<p class="MsoNormal">Your errors about 'mips' is probably based on the build of clang on your machine?<o:p></o:p></p>
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<p class="MsoNormal"><br>
--<o:p></o:p></p>
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<p class="MsoNormal">Mats<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On 29 September 2015 at 00:07, Preston Briggs via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">When I use clang on an x86-64 to spit out the LLVM, like this<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">clang -O -S -emit-llvm varargstest.c</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">where varargstest.c looks like this<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">int add_em_up(int count, ...) {</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> va_list ap;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> int i, sum;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> va_start(ap, count);</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> sum = 0;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> for (i = 0; i < count; i++)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> sum += va_arg(ap, int);</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> va_end(ap);</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> return sum;</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">}</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I see LLVM that looks like it's been customized for the x86-64,<o:p></o:p></p>
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<p class="MsoNormal">versus the varargs stuff I was led to expect from the LLVM IR documentation.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">define i32 @add_em_up(i32 %count, ...) #0 {</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">entry:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %ap = alloca [1 x %struct.__va_list_tag], align 16</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %arraydecay1 = bitcast [1 x %struct.__va_list_tag]* %ap to i8*</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> call void @llvm.va_start(i8* %arraydecay1)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %cmp7 = icmp sgt i32 %count, 0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br i1 %cmp7, label %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a>, label %for.end</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""><a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a>: ; preds = %entry</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %gp_offset_p = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i64 0, i64 0, i32 0</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %0 = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i64 0, i64 0, i32 3</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %overflow_arg_area_p = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i64 0, i64 0, i32 2</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %gp_offset.pre = load i32* %gp_offset_p, align 16</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br label %for.body</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">for.body: ; preds = %vaarg.end, %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a></span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %gp_offset = phi i32 [ %gp_offset.pre, %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a> ], [ %gp_offset10, %vaarg.end ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %sum.09 = phi i32 [ 0, %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a> ], [ %add, %vaarg.end ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %i.08 = phi i32 [ 0, %<a href="http://for.body.lr.ph" target="_blank">for.body.lr.ph</a> ], [ %inc, %vaarg.end ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %fits_in_gp = icmp ult i32 %gp_offset, 41</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br i1 %fits_in_gp, label %vaarg.in_reg, label %vaarg.in_mem</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">vaarg.in_reg: ; preds = %for.body</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %reg_save_area = load i8** %0, align 16</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %1 = sext i32 %gp_offset to i64</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %2 = getelementptr i8* %reg_save_area, i64 %1</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %3 = add i32 %gp_offset, 8</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> store i32 %3, i32* %gp_offset_p, align 16</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br label %vaarg.end</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">vaarg.in_mem: ; preds = %for.body</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %overflow_arg_area = load i8** %overflow_arg_area_p, align 8</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %overflow_arg_area.next = getelementptr i8* %overflow_arg_area, i64 8</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> store i8* %overflow_arg_area.next, i8** %overflow_arg_area_p, align 8</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br label %vaarg.end</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">vaarg.end: ; preds = %vaarg.in_mem, %vaarg.in_reg</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %gp_offset10 = phi i32 [ %3, %vaarg.in_reg ], [ %gp_offset, %vaarg.in_mem ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %<a href="http://vaarg.addr.in" target="_blank">vaarg.addr.in</a> = phi i8* [ %2, %vaarg.in_reg ], [ %overflow_arg_area, %vaarg.in_mem ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %vaarg.addr = bitcast i8* %<a href="http://vaarg.addr.in" target="_blank">vaarg.addr.in</a> to i32*</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %4 = load i32* %vaarg.addr, align 4</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %add = add nsw i32 %4, %sum.09</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %inc = add nsw i32 %i.08, 1</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %exitcond = icmp eq i32 %inc, %count</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> br i1 %exitcond, label %for.end, label %for.body</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">for.end: ; preds = %vaarg.end, %entry</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> %sum.0.lcssa = phi i32 [ 0, %entry ], [ %add, %vaarg.end ]</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> call void @llvm.va_end(i8* %arraydecay1)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New""> ret i32 %sum.0.lcssa</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Courier New"">}</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Notice at the bottom of the block labeled "for.body" that there's a test that determines<o:p></o:p></p>
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<p class="MsoNormal">whether to look for an argument on the stack or in a register. I see something similar w/ or w/o the -O flag.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This isn't what I was led to expect by the LLVM IR documentation.<o:p></o:p></p>
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<p class="MsoNormal">Is there a way to avoid this "premature" optimization?<o:p></o:p></p>
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<p class="MsoNormal">I tried things like -arch mips and -march=mips but get complaints<o:p></o:p></p>
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<p class="MsoNormal">about unrecognized flags (-arch) or unknown target architecture CPU 'mips'.<o:p></o:p></p>
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<p class="MsoNormal">Why's that? The man page suggests both should work.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Preston<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><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">_______________________________________________<br>
LLVM Developers mailing list<br>
<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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