https://bugs.llvm.org/show_bug.cgi?id=47393
Bug ID: 47393
Summary: Conversion from uint64_t to double produces wrong
result for rounding downward
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: chf...@gmail.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
When floating-point rounding direction is set to "towards negative infinity"
(FE_DOWNWARD) and conversion from uint64_t(0) to double is performed the result
is -0.0 instead of +0.0.
Architecture: x86_64
Clang version: reproducible in all known versions, including trunk
Options: happens for all combinations of -O0, -O2, -frounding-math,
-fsigned-zeros.
Code (-lm is needed)
enum { FE_DOWNWARD = 0x400 };
extern int fesetround(int rounding_direction);
__attribute__((noinline))
double ui64_to_f64(unsigned long x)
{
return (double)x;
}
int main()
{
fesetround(FE_DOWNWARD);
double z = ui64_to_f64(0);
return __builtin_signbit(z) != 0; // expected 0
}
Can be played with at https://godbolt.org/z/jvPGqW.
I believe the main issue is in LLVM.
Without -frounding-math the Clang uses uitofp instruction. The reference says
this should be fine in this context as "it is rounded using the default
rounding mode".
With -frounding-math the
call double @llvm.experimental.constrained.uitofp.f64.i64(i64 %3, metadata
!"round.dynamic", metadata !"fpexcept.ignore")
is used, but the result is wrong nevertheless.
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