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[SimplifyLibCalls] Fix infinite loop with fast-math optimization. 

One of the fast-math optimizations is to replace calls to standard double
functions with their float equivalents, e.g. exp -> expf. However, this can
cause infinite loops for the following:

  float expf(float val) { return (float) exp((double) val); }

A similar inline declaration exists in the MinGW-w64 math.h header file which
when compiled with -O2/3 and fast-math generates infinite loops.

So this fix checks that the calling function to the standard double function
that is being replaced does not match the float equivalent.

Differential Revision: https://reviews.llvm.org/D31806

llvm-svn: 301304
This commit is contained in:
Andrew Ng 2017-04-25 12:36:14 +00:00
parent ab0446332e
commit 1606fc0bf9
2 changed files with 78 additions and 0 deletions
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18
llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
View File

@ -926,6 +926,24 @@ static Value *optimizeUnaryDoubleFP(CallInst *CI, IRBuilder<> &B,
if (V == nullptr) if (V == nullptr)
return nullptr; return nullptr;
// If call isn't an intrinsic, check that it isn't within a function with the
// same name as the float version of this call.
//
// e.g. inline float expf(float val) { return (float) exp((double) val); }
//
// A similar such definition exists in the MinGW-w64 math.h header file which
// when compiled with -O2 -ffast-math causes the generation of infinite loops
// where expf is called.
if (!Callee->isIntrinsic()) {
const Function *F = CI->getFunction();
StringRef FName = F->getName();
StringRef CalleeName = Callee->getName();
if ((FName.size() == (CalleeName.size() + 1)) &&
(FName.back() == 'f') &&
FName.startswith(CalleeName))
return nullptr;
}
// Propagate fast-math flags from the existing call to the new call. // Propagate fast-math flags from the existing call to the new call.
IRBuilder<>::FastMathFlagGuard Guard(B); IRBuilder<>::FastMathFlagGuard Guard(B);
B.setFastMathFlags(CI->getFastMathFlags()); B.setFastMathFlags(CI->getFastMathFlags());

60
llvm/test/Transforms/Util/libcalls-fast-math-inf-loop.ll Normal file
View File

@ -0,0 +1,60 @@
; RUN: opt -S -instcombine -o - %s | FileCheck %s
; Test that fast math lib call simplification of double math function to float
; equivalent doesn't occur when the calling function matches the float
; equivalent math function. Otherwise this can cause the generation of infinite
; loops when compiled with -O2/3 and fast math.
; Test case C source:
;
; extern double exp(double x);
; inline float expf(float x) { return (float) exp((double) x); }
; float fn(float f) { return expf(f); }
;
; IR generated with command:
;
; clang -cc1 -O2 -ffast-math -emit-llvm -disable-llvm-passes -triple x86_64-unknown-unknown -o - <srcfile>
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-unknown"
; Function Attrs: nounwind
define float @fn(float %f) #0 {
; CHECK: define float @fn(
; CHECK: call fast float @expf(
%f.addr = alloca float, align 4
store float %f, float* %f.addr, align 4, !tbaa !1
%1 = load float, float* %f.addr, align 4, !tbaa !1
%call = call fast float @expf(float %1) #3
ret float %call
}
; Function Attrs: inlinehint nounwind readnone
define available_externally float @expf(float %x) #1 {
; CHECK: define available_externally float @expf(
; CHECK: fpext float
; CHECK: call fast double @exp(
; CHECK: fptrunc double
%x.addr = alloca float, align 4
store float %x, float* %x.addr, align 4, !tbaa !1
%1 = load float, float* %x.addr, align 4, !tbaa !1
%conv = fpext float %1 to double
%call = call fast double @exp(double %conv) #3
%conv1 = fptrunc double %call to float
ret float %conv1
}
; Function Attrs: nounwind readnone
declare double @exp(double) #2
attributes #0 = { nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { inlinehint nounwind readnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-features"="+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { nounwind readnone }
!llvm.ident = !{!0}
!0 = !{!"clang version 5.0.0"}
!1 = !{!2, !2, i64 0}
!2 = !{!"float", !3, i64 0}
!3 = !{!"omnipotent char", !4, i64 0}
!4 = !{!"Simple C/C++ TBAA"}