u64 to f32 conversion is inaccurate on Windows 32bit

On Windows, the x87 control word has a different default value than on other platforms. This not only introduces floating point precision issues, but also results in inconsistent behavior with other platforms. I tried to avoid using x87 by adding the -msse2 option when compiling for Windows 32bit, but the compiler still generates binaries using only x87.

The code below outputs differently on Windows 32bit than on other platforms:

#include <stdio.h>
#include <stdint.h>

int main() {
    int64_t n = 0b0000000000111111111111111111111111011111111111111111111111111111;
    printf("%lld, %.0f, %.0f", n, (float)n, (float)(uint64_t)n);

    return 0;
}

rust-lang/rust#105626

[llvmbot]

@llvm/issue-subscribers-backend-x86

[topperc]

Here's a patch to use library call for Windows instead of the broken inline expansion.

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index c0aeed8e45fb..adf236d5d7d0 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -21880,6 +21880,11 @@ SDValue X86TargetLowering::LowerUINT_TO_FP(SDValue Op,
   }
 
   assert(SrcVT == MVT::i64 && "Unexpected type in UINT_TO_FP");
+
+  // Windows default precision control does not support 80-bit FP.
+  if (Subtarget.isOSWindows())
+    return SDValue();
+
   SDValue ValueToStore = Src;
   if (isScalarFPTypeInSSEReg(Op.getValueType()) && !Subtarget.is64Bit()) {
     // Bitcasting to f64 here allows us to do a single 64-bit store from

I also tried clang.exe test.c -mno-x87 -msse -msse2 but it still only uses x87. I tested it with LLVM for Windows 32bit.

[Alcaro]

I also tried clang.exe test.c -mno-x87 -msse -msse2 but it still only uses x87. I tested it with LLVM for Windows 32bit.

-mfpmath=sse

No clue why -msse/-msse2 doesn't imply that. It doesn't affect ABI, it still passes arguments on stack and return in x87.

[topperc]

On Windows 32bit, I also tried clang.exe test.c -mno-x87 -msse -msse2 but it still only uses x87.

-mno-x87 is not correctly implemented and is poorly tested. It was added after a lot of code was written and a lot of that hasn’t been updated.

There’s no sse2 instruction that can do unsigned 64-bit integer to float on a 32 bit target. So we use x87. But the algorithm only works if x87 is configured for full precision. If not it rounds incorrectly.

[phoebewang]

MSVC has the same problem: https://godbolt.org/z/4sbn8dq1T

MSVC has the same problem: https://godbolt.org/z/4sbn8dq1T

MSVC x86 does not seem to produce different results depending on whether or not it has sign, so it is less likely to be an issue. It might be better to just match MSVC's behavior than to use library calls, even though we lose a tiny bit of precision.

Fixed in a6e3027