[cfe-dev] Inline Assembly Comments

[Hal Finkel via cfe-dev]

On 07/13/2017 06:51 AM, Manuel Rigger via cfe-dev wrote:
> 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?

Please feel free to file a bug report. Retaining the comments seems like 
a useful thing to do.

  -Hal

> - Manuel
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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