[llvm-dev] willreturn

[Jeroen Dobbelaere via llvm-dev]

> I'm confused.
> 
> Are you interested in adding `willreturn` to a function via clang?

I think so (also getting a little confused :( ) .. 
What should the behavior be of a '__attribute__((const))' function ?
Is the progress guaranteed ?

See https://www.godbolt.org/z/GoPYhM


// extern "C" ...
extern int ptrfun1(int, int*, int*) __attribute__((const));

int b[5];

int foo() {
    int a[10];
    int index = ptrfun1(5, &a[0], &b[0]);
    a[index]=10;
    return a[index];
}

clang-11 is able to optimize this away

clang-trunc is mixed:
- for this case, the call will not optimize it away. (as far as I see, since D94106)
- But if you do not use the return value upfront, it will be optimized away.
    ...
    int index = 3; 
    ptrfun1(5, &a[0], &b[0]);
    ...

If a '__attribute__((const))' function is allowed to not progress, then not all llvm passes are aware of this
with the current mapping on llvm attributes and imho, we'll need an attribute to indicate that we want
to have progress. Or, clang should map '__attribute__((const))' to 'readnone willreturn'.

Greetings,

Jeroen Dobbelaere


> -----Original Message-----
> From: Johannes Doerfert <johannesdoerfert at gmail.com>
> Sent: Wednesday, February 17, 2021 18:11
> To: Jeroen Dobbelaere <dobbel at synopsys.com>; llvm-dev at lists.llvm.org; Florian
> Hahn <florian_hahn at apple.com>
> Subject: Re: willreturn
> 
> I'm confused.
> 
> Are you interested in adding `willreturn` to a function via clang?
> 
> 
> On 2/17/21 11:05 AM, Jeroen Dobbelaere wrote:
> > After the '[local] treat calls that may not return as being alive' patch,
> https://urldefense.com/v3/__https://reviews.llvm.org/D94106__;!!A4F2R9G_pg!LFI
> Ix70hH4oLQtVkzqKiChp_3Ubjedy7AJEChQFkMZjrjdF1WwkKgf1qBaJL0GzkK45Vdo4q$
> >
> > it seems not to be possible in clang to indicate that a 'C' function or an
> 'extern "C"' function will progress.
> > At least, I did not find an attribute that indicates this. (
> __attribute__((const))  is not sufficient)
> >
> > Is there already a clang attribute to indicate this ?
> >
> > Thanks,
> >
> > Jeroen Dobbelaere
> >