[Bug c/114437] New: Inline asm with "+m,r" operand ignores input value

[xog4nar4--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114437

Bug ID: 114437
   Summary: Inline asm with "+m,r" operand ignores input value
   Product: gcc
   Version: 13.2.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: xog4n...@a-n.cc
  Target Milestone: ---

In the following snippet, GCC seems to assume that `arg` is not read by the asm
statement and optimizes out previous stores when compiling on -O3:
```
extern int external_fn(int);
extern int arg;

int fn() {
arg = 123;
asm volatile("" : "+m,r"(arg) : : "memory");
return external_fn(arg);
}
```

Resulting asm (123 is not stored, leaving `arg` uninitialized):
```
 :
   0:   f3 0f 1e fa endbr64
   4:   8b 3d 00 00 00 00   movedi,DWORD PTR [rip+0x0]# a

   a:   e9 00 00 00 00  jmpf 
```

The asm snippet comes from the `DoNotOptimize` function in Google Benchmark,
where it is intended to prevent the compiler from optimizing away parts of a
micro-benchmark:
https://github.com/google/benchmark/blob/main/include/benchmark/benchmark.h#L518


Godbolt reproduction: https://godbolt.org/z/3W97dM9eW

[Bug middle-end/114437] Inline asm with "+m, r" operand ignores input value

[xog4nar4--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114437

--- Comment #3 from xog4n...@a-n.cc ---
Just to confirm, is this an incorrect constraint for the intended goal of
convincing the compiler that the value is used and modified, without actually
doing either and preserving the value, or am I just using it wrong in this
particular example? I'm somewhat surprised since this inline asm snippet is
used in multiple well-known micro-benchmarking libraries.


Is the following version correct?
```
asm volatile("" : "=m,r"(arg) : "0,0"(arg) : "memory");
```

[Bug middle-end/114437] Inline asm with "+m, r" operand ignores input value

[xog4nar4--- via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114437

xog4n...@a-n.cc changed:

   What|Removed |Added

 Resolution|DUPLICATE   |INVALID

--- Comment #5 from xog4n...@a-n.cc ---
I would prefer to use something else, but I'm not aware of any less hacky way
to ensure that the compiler does not interfere with a benchmark. If there's any
explicitly supported way to achieve that in GCC, I'm all ears, but I don't
assume so given that each and every C++ benchmarking library I know uses inline
asm.