| Issue |
76958
|
| Summary |
Clang miscompiling code when using -fms-hotpatch and optimizations
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
MolecularMatters
|
When using the -fms-hotpatch option (which is a requirement for Live++), the tailcall/sibling call optimizations will lead to clang miscompiling function calls.
Example code:
`typedef unsigned long long size_t;
extern "C" {
void* mi_new(size_t size);
}
void *mi_new_test(size_t count)
{
return mi_new(count);
}
void *builtin_malloc_test(size_t count)
{
return __builtin_malloc(count);
}
`
Using -O1, the code will compile to the following:
`"?mi_new_test@@YAPEAX_K@Z": # @"?mi_new_test@@YAPEAX_K@Z"
jmp mi_new # TAILCALL
"?builtin_malloc_test@@YAPEAX_K@Z": # @"?builtin_malloc_test@@YAPEAX_K@Z"
jmp malloc # TAILCALL`
However, adding -fms-hotpatch, the code will miscompile to this:
`"?mi_new_test@@YAPEAX_K@Z": # @"?mi_new_test@@YAPEAX_K@Z"
xchg ax, ax
"?builtin_malloc_test@@YAPEAX_K@Z": # @"?builtin_malloc_test@@YAPEAX_K@Z"
xchg ax, ax`
As a workaround, specifying -fno-optimize-sibling-calls will produce the correct (but suboptimal) following code:
`"?mi_new_test@@YAPEAX_K@Z": # @"?mi_new_test@@YAPEAX_K@Z"
sub rsp, 40
call mi_new
nop
add rsp, 40
ret
"?builtin_malloc_test@@YAPEAX_K@Z": # @"?builtin_malloc_test@@YAPEAX_K@Z"
sub rsp, 40
call malloc
nop
add rsp, 40
ret`
Please refer to this godbolt compiler explorer repro: https://gcc.godbolt.org/z/r86nYbqvo
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