Re: [PATCH] longlong.h: Do no use asm input cast for clang

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 30/11/22 20:24, Segher Boessenkool wrote:
> Hi!
> 
> On Wed, Nov 30, 2022 at 03:16:25PM -0300, Adhemerval Zanella via Gcc-patches wrote:
>> clang by default rejects the input casts with:
>>
>>   error: invalid use of a cast in a inline asm context requiring an
>>   lvalue: remove the cast or build with -fheinous-gnu-extensions
>>
>> And even with -fheinous-gnu-extensions clang still throws an warning
>> and also states that this option might be removed in the future.
>> For gcc the cast are still useful somewhat [1], so just remove it
>> clang is used.
> 
> This is one of the things in inline asm that is tightly tied to GCC
> internals.  You should emulate GCC's behaviour faithfully if you want
> to claim you implement the inline asm GNU C extension.

Agree, that's why I just make it a no-op for clang which indicates that
it does not seem much use of this extension.

> I understand that the casts should be no-ops on the asm side (maybe they
> change the sign) and they are present as type-checking.  Can we implement
> this type-checking in a different (portable) way?  I think the macro you use
> should be named like __asm_output_check_type (..) or so to indicate the
> intended purpose.

I do not think trying to leverage it on clang side would yield much, it
seems that it really does not want to support this extension.  I am not
sure we can really make it portable, best option I can think of would to
add a mix of __builtin_classify_type and typeof prior asm call (we do
something similar to powerp64 syscall code on glibc), although it would
still require some gcc specific builtins.

I am open for ideas, since to get this header to be clang-compatible on
glibc it requires to get it first on gcc.

> 
>> --- a/include/ChangeLog
>> +++ b/include/ChangeLog
> 
> That should not be part of the patch?  Changelog entries should be
> verbatim in the message you send.
> 
> The size of this patch already makes clear this is a bad idea, imo.
> This code is already hard enough to read.

Indeed, I forgot that CL entries were not part of the commit.