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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Clang ignores NANs with -ffast-math and -fhonor-nans (or -fno-finite-math-only)"
href="https://bugs.llvm.org/show_bug.cgi?id=33510">33510</a>
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<th>Summary</th>
<td>Clang ignores NANs with -ffast-math and -fhonor-nans (or -fno-finite-math-only)
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.9
</td>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>eborisch@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>## Summary: In contrast with GCC -- and apparently its own internal
## attributes states -- clang ignores NANs when compiling with :
## -ffast-math and (-fhonor-nans or -fno-finite-math-only)
## Test code: (nanfunc.c)
int testfunc(float x) {return x == x;}
## TEST 1: clang -O3 -S -emit-llvm -o - nanfunc.c
## This version checks for NANs (fcmp ord)
## Outputs are trimmed to the interesting parts
define i32 @testfunc(float) local_unnamed_addr #0 {
%2 = fcmp ord float %0, 0.000000e+00
%3 = zext i1 %2 to i32
ret i32 %3
}
attributes #0 = { norecurse nounwind readnone ssp uwtable
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8"
"target-cpu"="core2"
"target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3,+x87"
"unsafe-fp-math"="false" "use-soft-float"="false" }
## TEST 2: clang -O3 -ffast-math -S -emit-llvm -o - nanfunc.c
## This version converts (x==x) to (1) as expected for no NANs allowed.
## Note all the changes in attributes from -ffast-math, especially
## no-nans-fp-math=true
define i32 @testfunc(float) local_unnamed_addr #0 {
ret i32 1
}
attributes #0 = { norecurse nounwind readnone ssp uwtable
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true"
"no-signed-zeros-fp-math"="true" "stack-protector-buffer-size"="8"
"target-cpu"="core2"
"target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3,+x87"
"unsafe-fp-math"="true" "use-soft-float"="false" }
## TEST 2: clang -O3 -ffast-math -fhonor-nans -S -emit-llvm -o - nanfunc.c
## This version still(!) converts (x==x) to (1). Note in the attributes
## that it has parsed the option and set no-nans-fp-math=false, but it
## doesn't seem to have followed that restriction.
define i32 @testfunc(float) local_unnamed_addr #0 {
ret i32 1
}
attributes #0 = { norecurse nounwind readnone ssp uwtable
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="true" "stack-protector-buffer-size"="8"
"target-cpu"="core2"
"target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3,+x87"
"unsafe-fp-math"="true" "use-soft-float"="false" }
## TEST 3: clang -O3 -ffast-math -ffinite-math-only -S -emit-llvm -o -
nanfunc.c
## Same functional output as TEST 2; a few additional optimizations
## attributes change (no-infs-fp-math, signed-zeros)
; Function Attrs: norecurse nounwind readnone ssp uwtable
define i32 @testfunc(float) local_unnamed_addr #0 {
ret i32 1
}
attributes #0 = { norecurse nounwind readnone ssp uwtable
"disable-tail-calls"="false" "less-precise-fpmad"="false"
"no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
"no-infs-fp-math"="true" "no-jump-tables"="false" "no-nans-fp-math"="true"
"no-signed-zeros-fp-math"="true" "stack-protector-buffer-size"="8"
"target-cpu"="core2"
"target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3,+x87"
"unsafe-fp-math"="true" "use-soft-float"="false" }
## Note: this is different from GCC's behavior, where -ffast-math elides
## the (x==x) check, but '-ffast-math -fno-finite-math-only' does not.</pre>
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