Re: [PATCH][RFC] tree-optimization/107389 - use __builtin_assume_alignment at -O0

[Richard Biener <rguenther@suse.de>]

On Mon, 7 Nov 2022, Jakub Jelinek wrote:

> On Mon, Nov 07, 2022 at 10:02:11AM +0100, Richard Biener wrote:
> > The following adds a fold_builtins pass at -O0, keying off some
> > unwanted optimization and setting pointer alignment of the result
> > of __builtin_assume_alignment before removing the call.  This
> > allows libatomic calls to be elided at -O0 on s390 for
> > 
> > __uint128_t foo(__uint128_t *p)
> > {
> >   return __atomic_load_n((__uint128_t *)__builtin_assume_aligned (p, 16), 0);
> > }
> > 
> > not sure how to reliably test this though.
> > 
> > Thoughts?
> 
> Do we really need a separate pass for it?
> Can't we do it say during gimplification?

gimplification would be too early for always inline - of course since
we don't do any copy propagation the source pattern this works reliably
are limited, mostly when used directly as arguments like in the example
above.

So yes, the specific case in question would work when we elide
__builtin_assume_aligned during gimplification at -O0 (or during
the GIMPLE lower pass).

Would you prefer that?  Richard, would that work for you?

Thanks,
Richard.

> > 	PR tree-optimization/107389
> > 	* tree-pass.h (pass_fold_builtins_O0): Declare.
> > 	* passes.def (pass_fold_builtins_O0): Schedule.
> > 	* tree-ssa-ccp.cc (pass_fold_builtins_O0): Define.
> > 	(pass_fold_builtins): Template, add gate.
> > 	(pass_fold_builtins::execute): Apply alignment to the
> > 	target of __builtin_assume_aligned at -O0.  Disable most
> > 	optimizations for the -O0 pass instance.
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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