[cfe-dev] Some assembly fails in clang and succeeds in gcc

[Reid Kleckner]

This is http://llvm.org/bugs/show_bug.cgi?id=18916


On Wed, Aug 6, 2014 at 3:42 PM, Yuri <yuri at rawbw.com> wrote:

> On 08/06/2014 13:05, Reid Kleckner wrote:
>
>> So, 1: is supposed to be a label?  I suspect that is not going to work.
>>  Consider:
>>
>> $ cat t.cpp
>> int main() {
>>   asm ("1: jmp 1");
>> }
>>
>> $ clang -cc1 t.cpp -S -o - | grep -A10 main:
>> _main:
>>         #APP
>> Ltmp0:
>>         jmp     1  # Not right...
>>         #NO_APP
>>         movl    $0, %eax
>>         retl
>>
>>
> However, in my case the corresponding jump label is 1b, and generated
> assembly is right:
>
> $ cat t.cpp
> int main() {
>   asm ("1: jmp 1b");
>
> }
>
> $ clang -cc1 t.cpp -S -o - | grep -A10 main:
> main:
>         #APP
> .Ltmp0:
>         jmp     .Ltmp0
>         #NO_APP
>         movl    $0, %eax
>         retq
> .Ltmp1:
>         .size   main, .Ltmp1-main
>
>
> I limited the testcase to this small form.
> $ cat testcase.c
> void func(void *state) {
> __asm__ __volatile__
> (
>         "\n" "1" ":" "jb 1b;"
>         ".att_syntax prefix;"
>         :
>         : "c" (state)
>         : "memory", "cc", "%eax"
> );
> }
>
>
> void use() {
>   func(0);
> }
>
> $ clang testcase.c
> testcase.c:5:15: error: invalid instruction mnemonic 'prefix'
>         "\n" "1" ":" "jb 1b;"
>               ^
> <inline asm>:2:21: note: instantiated into assembly here
> 1:jb 1b;.att_syntax prefix;
>                     ^~~~~~~
> 1 error generated.
>
>
> gcc-4.8.0 compiles it, and clang rev.214386 doesn't.
>
> Yuri
>
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