| Issue |
76879
|
| Summary |
Clang -fms-hotpatch elides memory allocation calls
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
tycho
|
I built my application using clang-cl with optimizations enabled and the `/hotpatch` flag (to try out Live++). It compiled and linked successfully, but when I try to run it, the very first memory allocation hits an `int 3` instruction. It seems all the memory allocation functions (default `malloc`, default `new`/`new[]`, custom `malloc`/`new`, etc) are just a `nop` followed by several `int 3`s. It seems to have assumed it's unreachable code or something?
For example, a somewhat reduced test case (and here's a [Godbolt link](https://gcc.godbolt.org/z/Ed8fd5arh)):
```c++
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);
}
```
When buit with `-O0 -target x86_64-pc-windows-msvc19.38.33133 -fms-hotpatch`, it yields fairly reasonable (if unoptimized) code:
```
"?mi_new_test@@YAPEAX_K@Z": # @"?mi_new_test@@YAPEAX_K@Z"
sub rsp, 40
mov qword ptr [rsp + 32], rcx
mov rcx, qword ptr [rsp + 32]
call mi_new
nop
add rsp, 40
ret
"?builtin_malloc_test@@YAPEAX_K@Z": # @"?builtin_malloc_test@@YAPEAX_K@Z"
sub rsp, 40
mov qword ptr [rsp + 32], rcx
mov rcx, qword ptr [rsp + 32]
call malloc
nop
add rsp, 40
ret
```
When `-O2` (or indeed any `-O` flag higher than `0`) is used, it emits 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
```
What happened here?
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