RE: [PATCH 1/3]middle-end: Add the ability to let the target decide the method of argument promotions.

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Monday, May 16, 2022 12:36 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; rguenther@suse.de;
> jeffreyalaw@gmail.com
> Subject: Re: [PATCH 1/3]middle-end: Add the ability to let the target decide
> the method of argument promotions.
> 
> Tamar Christina <tamar.christina@arm.com> writes:
> > Hi All,
> >
> > Some targets require function parameters to be promoted to a different
> > type on expand time because the target may not have native
> > instructions to work on such types.  As an example the AArch64 port
> > does not have native instructions working on integer 8- or 16-bit
> > values.  As such it promotes every parameter of these types to 32-bits.
> 
> This doesn't seem specific to parameters though.  It applies to any
> 8- or 16-bit variable.  E.g.:
> 
> #include <stdint.h>
> uint8_t foo(uint32_t x, uint32_t y) {
>     uint8_t z = x != 0 ? x : y;
>     return z + 1;
> }
> 
> generates:
> 
> foo:
>         cmp     w0, 0
>         and     w1, w1, 255
>         and     w0, w0, 255
>         csel    w0, w1, w0, eq
>         add     w0, w0, 1
>         ret
> 
> So I think the new behaviour is really a modification of the PROMOTE_MODE
> behaviour rather than the PROMOTE_FUNCTION_MODE behaviour.
> 
> FWIW, I agree with Richard that it would be better not to add a new hook.
> I think we're really making PROMOTE_MODE choose between
> SIGN_EXTEND, ZERO_EXTEND or SUBREG (what LLVM would call “any
> extend”) rather than the current SIGN_EXTEND vs. ZERO_EXTEND choice.

Ah, I hadn't realized this also applied to locals.. ok I can modify PROMOTE_MODE then,
but I also need the actual SSA_NAME and not just the type so will have to pass this along.

From a practical point of view.. the actual hook however is implemented by 34 targets,
would I need to CC maintainers for each of them or would global maintainer approval
suffice for these mostly mechanical changes?

Cheers,
Tamar

> 
> Thanks,
> Richard