Bug in identical code folding?

[Georg-Johann Lay]

Am 02.12.25 um 23:40 schrieb Andrew Pinski:
> On Tue, Dec 2, 2025 at 12:45 PM Georg-Johann Lay via Gcc
> <gcc@gcc.gnu.org> wrote:
>>
>> Playing with a new avr function attribute, I stumbled upon this problem:
>>
>> On avr, all varargs arguments are passed on the stack, even the named
>> ones. So I added a "rargs" function attribute that passes named args in
>> registers if possible.  One test case looks like this:
>>
>> #define RA __attribute__((rargs))
>>
>> void call_rafun_cast1 (void (*f)(char, ...))
>> {
>>       ((RA void(*)(char,...)) f) (10);
>> }
>>
>> void call_rafun_cast0 (RA void (*f)(char, ...))
>> {
>>       ((void(*)(char,...)) f) (20);
>> }
>>
>> The code compiles as expected:
>>
>> // rargs: no push/pop
>> call_rafun_cast1:
>>          movw r30,r24     ;  20  [c=4 l=1]  *movhi/0
>>          ldi r24,lo8(10)  ;  14  [c=4 l=1]  movqi_insn/1
>>          ijmp             ;  7   [c=0 l=1]  call_insn/2
>>
>> // default: push/pop.
>> call_rafun_cast0:
>>          ldi r18,lo8(20)  ;  15  [c=4 l=1]  movqi_insn/1
>>          push r18                 ;  7   [c=4 l=1]  pushqi1/0
>>          movw r30,r24     ;  23  [c=4 l=1]  *movhi/0
>>          icall            ;  8   [c=0 l=1]  call_insn/0
>>           ; SP += 1       ;  9   [c=8 l=1]  *addhi3_sp
>>          pop __tmp_reg__
>>          ret              ;  19  [c=0 l=1]  return
>>
>> Notice that cast0 has a push / pop of the argument as expected, while
>> cast1 hasn't, also as expected. This is all fine.  Now change the code
>> slightly so that the function bodies become the same (pass 10 instead of
>> 20):
>>
>> void call_rafun_cast0 (RA void (*f)(char, ...))
>> {
>>       ((void(*)(char,...)) f) (10);
>> }
>>
>> The code compiles to:
>>
>> call_rafun_cast0:
>>          rjmp call_rafun_cast1    ;  7   [c=0 l=1]  call_insn/3
>>
>> which is wrong IMO.  The bodies of the functions are the same, but the
>> prototypes are not, so performing ICF is wrong.
>>
>> I am not completely sure about a function pointer cast adding / removing
>> a function attribute.  Though GCC seems to compile it as expected
>> without ICF.
>>
>> So is this an ICF bug or am I missing some target hook?
> 
> Looking into the code, and looking into how fastcall works for i686
> (since replacing rargs with fastcall works there), the target hook
> comp_type_attributes is needed to make sure to return false for the
> attribute mismatch on the types with/without the attribute.
> 
> Thanks,
> Andrew Pinski

Hi Andrew,

thank you for the pointer.  Though it doesn't help.  For the test 
program, the TARGET_COMP_TYPE_ATTRIBUTES hook isn't even called.

Johann


>> For now I only changed the cumulative args hooks, but there are some
>> VA_LIST and VA_ARG hooks in
>> https://gcc.gnu.org/onlinedocs/gccint/Register-Arguments.html where I
>> don't know what to do with them.
>>
>> All unnamed args are passed the same way in either case; the attribute
>> only affects passing of named args.
>>
>> GCC is current trunk.
>>
>> Johann
>>