X87ReducedPrecision breaks FILD m64int -> FISTP m64int roundtrip

What Game The Witcher 3: Wild Hunt (classic version)

The issue itself is not game-specific though. I reduced it to a tiny 32bit reproducer below.

Describe the bug X87ReducedPrecision breaks a valid FILD m64int -> FISTP m64int conversion.

I originally hit this in setup_redlauncher.exe. The installer contains copy helpers that use x87 FILD+FISTP to move 64bit data.

With X87ReducedPrecision=1, strings copied through these helpers get corrupted and the installer fails with Path not found.

The issue also reproduces without Wine or Proton.

To Reproduce

.section .data
src:
    .quad 0x0068005c003a005a
dst:
    .quad 0

.section .text
.global _start
_start:
    fildq src
    fistpq dst

    movl src, %eax
    cmpl dst, %eax
    jne bad

    movl src+4, %eax
    cmpl dst+4, %eax
    jne bad

    movl $1, %eax
    xorl %ebx, %ebx
    int $0x80

bad:
    movl $1, %eax
    movl $1, %ebx
    int $0x80

Build:

as --32 x87copy.s -o x87copy.o
ld -m elf_i386 x87copy.o -o x87copy

Run without reduced precision:

FEX_X87REDUCEDPRECISION=0 ./x87copy
echo $?

Result:

0

Run with reduced precision:

FEX_X87REDUCEDPRECISION=1 ./x87copy
echo $?

Result:

1

The original installer contains copy helpers like this:

fild   QWORD PTR [eax]
fild   QWORD PTR [eax+0x8]
fild   QWORD PTR [eax+0x10]
fild   QWORD PTR [eax+0x18]

...

fistp  QWORD PTR [edx+0x18]
fistp  QWORD PTR [edx+0x10]
fistp  QWORD PTR [edx+0x8]
fistp  QWORD PTR [edx]

So x87 is effectively being used to copy 8-byte chunks.

Expected behavior FILD m64int followed by FISTP m64int, with no arithmetic in between, should preserve the original 64bit integer value.

I'm not sure whether this is considered expected behavior though.

System information:

• OS: Debian arm64 guest
• CPU/SoC: Apple M3 Pro
• Video driver version: N/A
• RootFS used: Ubuntu 24.04 FEX RootFS
• FEX version: FEX-2604-226-g99662b7
• Thunks Enabled: Yes

Additional context

• Is this an x86 or x86-64 game: x86
• Does this reproduce on AArch64 with Radeon/Intel/Nvidia: Untested
• Is this a Vulkan game: No