Inline assembly and value extensions

[Harald van Dijk]

On 22/08/2020 16:30, Harald van Dijk wrote:
> Hi,
> 
> I am trying to port some x86-64 inline assembly to work properly for the 
> x32 ABI and I am running into a small problem. The x32 ABI specifies 
> that pointers are passed and returned zero-extended to 64 bits. When a 
> pointer variable is defined by an inline assembly statement and then 
> returned by a function, I do not see a way to inform GCC that the result 
> is already zero-extended, that GCC does not need to zero-extend it again.
> 
> A silly example:
> 
>   void *return_a_pointer(void) {
>     void *result;
>     asm("movl $0x11223344, %%eax" : "=a"(result));
>     return result;
>   }
> 
> This function gets an extra "movl %eax, %eax" between the hand-written 
> movl and the generated ret, which can be seen online at 
> <https://godbolt.org/z/T8bGPo>. This extra movl is there to ensure the 
> high bits of %rax are zero, but the initial movl already achieves that. 
> How can I inform GCC that it does not need to emit that extra movl?
> 
> Likewise, is there an easy way to provide an inline assembly statement 
> with a zero-extended pointer input? This one I am able to work around, 
> as it is possible to instead of passing in a pointer value p, pass in an 
> integer value (uint64_t)(uint32_t)p, but the workaround is kind of hard 
> to read and I would like to avoid that if possible.
> 
> I looked the documentation for either relevant inline assembly 
> constraints or relevant variable / type attributes, but was unable to 
> find any. The most promising search result was the mode attribute, I was 
> hoping it might be possible to give result a mode(DI) attribute, but the 
> compiler rejects that.

I have now found that forcing a different mode appears to be exactly how 
the zero-extension of arguments and return values is implemented: that 
is what ix86_promote_function_mode does.

The fact that this is not an option through variable attributes or 
inline assembly constraints looks like an unfortunate limitation of the 
inline assembly functionality, there is currently just no way to do what 
I am after. I very much hope to be proved wrong, but will try to just 
pick a workaround that does not look too bad.

> Is there another approach that I can use instead?
> 
> Cheers,
> Harald van Dijk