Re: Function Return ABI (was Re: Argument passing and ABI: GCC vs clang)

[Vineet Gupta <vineet.gupta@linux.dev>]

On 2/19/26 6:41 PM, Alexei Starovoitov wrote:
>> It seems the existing clang ABI is not symmetrical. The function
>> arguments promotion is handled in callee, but function return is handled
>> in caller [1] which is a bit atypical IMO and would need to be handled
>> in gcc as well.


Ignore this as it is clearly wrong (my jetlag and waking up at 3 am is 
to blame :-)

With PR/124171 gcc-bpf will promote args in caller same as llvm (not 
callee)


> hmm. I thought we already concluded that for clang-x86 and clang-bpf
> argument extension is handled in*caller*.
> The same thing for returns. It's a caller responsibility.
> gcc-bpf doing args in the callee only is broken and has to be fixed.


Correct, agreed.


>> Since we are on the topic of ABI change, I wanted to surface that as
>> well before we go off and tackle that.
>>
>> Simple example
>>
>> _Bool bar_bool(void);
>>
>> int ret_caller(void) {
>>         if (bar_bool() != 1) return 0; else return 1;
>> }
>>
>> char ret_callee(my_t *s)
>> {
>>       return s->c;    // c is a char in a struct
>> }
>>
>> ret_caller:
>>       call    bar_bool
>>       r0 &= 0xff        <-- ret promotion in caller
>>       exit
>>
>> ret_callee:
>>       r0 = *(u8 *) (r1+6) <-- no ret promotion in callee
>>       exit
> Exactly as it should be because x86 will populate 8-bit sub
> registers in the callee and bpf side has to do r0 &= 0xff
> in the caller.