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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] repeated memory references for infinity checks"
href="https://bugs.llvm.org/show_bug.cgi?id=43611">43611</a>
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<th>Summary</th>
<td>[X86] repeated memory references for infinity checks
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>andres@anarazel.de
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<pre>Hi,
LLVM, for x64, does generate repeated memory references when checking the
results of floating point math for infinity. Clang generates IR like:
%add = fadd double %a, %b
%0 = tail call double @llvm.fabs.f64(double %add) #3
%cmpinf = fcmp oeq double %0, 0x7FF0000000000000
br i1 %cmpinf, label %if.then, label %if.end, !prof !2
...
if.end: ; preds = %entry
%add2 = fadd double %add, %c
%1 = tail call double @llvm.fabs.f64(double %add2) #3
%cmpinf3 = fcmp oeq double %1, 0x7FF0000000000000
br i1 %cmpinf3, label %if.then9, label %if.end10, !prof !2
...
for
...
sum += b;
if (unlikely(__builtin_isinf(sum)))
overflowerror();
sum += c;
if (unlikely(__builtin_isinf(sum)))
overflowerror();
...
which end up with instructions that reference the NaN/+Inf constants multiple
times:
vaddsd %xmm1, %xmm0, %xmm0
vandpd .LCPI0_0(%rip), %xmm0, %xmm1
vucomisd .LCPI0_1(%rip), %xmm1
jae .LBB0_5
vaddsd %xmm2, %xmm0, %xmm0
vandpd .LCPI0_0(%rip), %xmm0, %xmm1
vucomisd .LCPI0_1(%rip), %xmm1
jae .LBB0_5
...
rather than moving them into a register once.
Here's a godbolt link showing the issue:
<a href="https://godbolt.org/z/MnGGMU">https://godbolt.org/z/MnGGMU</a>
Interestingly llvm generates slightly better code for single precision floats,
moving the NaN constant into a register, but continuing to reference the
positive +Inf from memory.
Thanks to LebedevRI on #llvm, here are the links to llvm-mca analyses for
double/single precision, comparing with GCC generated code (although part of
the cost difference are due to gcc using subq/addq $8, %rsp, and clang using
pushq/popq, and I'm not sure that's accurately costed). Interestingly gcc
generates similar code to clang for single precision code.
<a href="https://godbolt.org/z/_2q9cc">https://godbolt.org/z/_2q9cc</a>
<a href="https://godbolt.org/z/nytVTG">https://godbolt.org/z/nytVTG</a>
What I originally was wishing for was that some pass would recognize that the
isinf statements are redundant, and remove all but the last, but that's a
separate issue. In my case the repeated additions are only apparent after IPO.
- Andres</pre>
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