https://bugs.llvm.org/show_bug.cgi?id=46077
Bug ID: 46077
Summary: __builtin_isnan/__builtin_isfinite should not produce
poison with -ffinite-math-only
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: matthew.arsena...@amd.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
With -ffinite-math-only, I would intuitively expect __builtin_isnan to always
return false, and __builtin_isfinite to always return true. This would allow
compiling a fast and slow version of the same math library sources, where the
special case paths are naturally optimized out in the fast variant of the
library.
However, if I look at the IR produced and reading the LangRef, I believe these
will currently return poison.
For these basic test functions:
bool test_isnan(float x) {
return __builtin_isnan(x);
}
bool test_isfinite(float x) {
return __builtin_isfinite(x);
}
I currently get this IR:
define hidden zeroext i1 @test_isnan(float %0) #0 {
%2 = alloca float, align 4, addrspace(5)
store float %0, float addrspace(5)* %2, align 4, !tbaa !5
%3 = load float, float addrspace(5)* %2, align 4, !tbaa !5
%4 = fcmp nnan ninf uno float %3, %3
ret i1 %4
}
define hidden zeroext i1 @test_isfinite(float %0) #0 {
%2 = alloca float, align 4, addrspace(5)
store float %0, float addrspace(5)* %2, align 4, !tbaa !5
%3 = load float, float addrspace(5)* %2, align 4, !tbaa !5
%4 = call nnan ninf float @llvm.fabs.f32(float %3) #2
%5 = fcmp nnan ninf one float %4, 0x7FF0000000000000
ret i1 %5
}
The description for fast math flags states that the return value is poison if
an input is a nan/inf with the corresponding flag. This is trivially violated
in the isfinite case since a compare to a constant infinity.
I think the correct solution is to not emit the nnan/ninf flags when emitting
these builtins.
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