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Subject: Re: Inline assembly and value extensions
To: Florian Weimer <fweimer@redhat.com>, gcc-help@gcc.gnu.org
Cc: Segher Boessenkool <segher@kernel.crashing.org>
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From: Harald van Dijk <harald@gigawatt.nl>
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Date: Mon, 5 Oct 2020 14:39:52 +0100
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On 05/10/2020 14:29, Florian Weimer wrote:
> * Harald van Dijk via Gcc-help:
> 
>> In the x32 ABI, pointers do not have a single mode. They are SImode,
>> except when passed as parameters or returned, in which case they are
>> DImode (see the ix86_promote_function_mode function I mentioned). This
>> is really the source of the problems.
> 
> I think that GCC cannot optimize away unecessary sign and zero
> extensions is also a contributing factor, no?

Hmm, yes, that is a good point too. An ugly way to try to inform GCC 
that the extension is unnecessary would be

   void *return_a_pointer(void) {
     unsigned long long result;
     asm("movl $0x11223344, %%eax" : "=a"(result));
     if (result & 0xFFFFFFFF00000000ULL)
       __builtin_unreachable();
     return (void *)result;
   }

This is not something I would want to have to write explicitly, but it 
does not help anyway at the moment, even in this case GCC emits the 
extra extension: <https://godbolt.org/z/vPb1Mr>. I think here the code 
does give enough information to GCC that it could in theory optimize it 
away.

(clang does not emit the extra extension, but that is because LLVM 
implements this ABI incorrectly, so not a fair comparison. I hope to be 
able to submit a patch to them for that at some later time.)