| Issue |
109987
|
| Summary |
clang fastcall x86 abi does not match gcc on unix
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
m-lw
|
Some uses of fastcall in the x86 abi no longer match what gcc has always generated on unix. This changed in llvm 16 and is probably the result of the fix for #57737. Possibly this fix should have only applied to MS Windows targets?
E.g. compiling this code with the -O option on FreeBSD 13.3 with clang version 19.0.0git (snapshot from 2024-05-10):
```
typedef union { long long l ; double d ; } res_val_union;
#define FASTCALL_ATTRIBUTE __attribute__ ((fastcall))
static res_val_union fastcall_results_vector[100] ;
res_val_union* fastcall_type_results_5(void) { return fastcall_results_vector ;}
int FASTCALL_ATTRIBUTE fastcall_ilc_5(long long arg_1, signed char arg_2)
{
fastcall_results_vector[0].l = (long long)arg_1;
fastcall_results_vector[1].l = (long long)arg_2;
return 50;
}
```
gives this code:
```
fastcall_ilc_5: # @fastcall_ilc_5
.cfi_startproc
# %bb.0: # %entry
pushl %ebp
.cfi_def_cfa_offset 8
.cfi_offset %ebp, -8
movl %esp, %ebp
.cfi_def_cfa_register %ebp
movl 8(%ebp), %eax
movl 12(%ebp), %edx
movl %edx, fastcall_results_vector+4
movl %eax, fastcall_results_vector
movsbl %cl, %eax
movl %eax, fastcall_results_vector+8
sarl $31, %eax
movl %eax, fastcall_results_vector+12
movl $50, %eax
popl %ebp
.cfi_def_cfa %esp, 4
retl $8
```
whereas in gcc version 15.0.0 20240630 (experimental) it gives:
```
fastcall_ilc_5:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %eax
movl 12(%ebp), %edx
movl %eax, fastcall_results_vector
movl %edx, fastcall_results_vector+4
movsbl 16(%ebp), %eax
movl %eax, fastcall_results_vector+8
cltd
movl %edx, fastcall_results_vector+12
movl $50, %eax
popl %ebp
ret $12
```
Note that the second argument is in `%cl` in clang but is on the stack in gcc.
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