[cfe-dev] Inline Assembly Comments

[Manuel Rigger via cfe-dev]

Hi,

I'm investigating how inline assembly is used in C projects. While doing
so, I found that a number of projects include comments as part of their
inline assembly snippets. The code snippet below is an example:

#include <stdint.h>

uint64_t atomic_swap_long(volatile uint64_t *p, uint64_t v) {
  __asm __volatile(
    "xchgq %1,%0; # atomic_swap_long"
: "+r" (v),   /* 0 */
 "+m" (*p)); /* 1 */
  return v;
}


When compiling with GCC (gcc -O1 -S) the following code is produced for the
inline assembly snippet on my system:

 #APP
# 4 "inline-assembly-comment.c" 1
xchgq (%rdi),%rax; # atomic_swap_long
# 0 "" 2
#NO_APP


With Clang 3.8 and below, an empty line is inserted instead of the comment:

#APP
xchgq %rsi, (%rdi)
#NO_APP


With Clang 3.9 and 4.0, the newline disappeared but the comment is still
not part of the assembly code.

The comment is retained on the LLVM IR level:

%5 = call i64 asm sideeffect "xchgq\09$1,$0; # atomic_swap_long",
"=r,=*m,0,*m,~{dirflag},~{fpsr},~{flags}"(i64* %4, i64 %3, i64* %4)


Is this intended? Should I open a bug report?

- Manuel
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