[PATCH] Fix PR92462

Richard Biener rguenther@suse.de
Mon Nov 18 10:09:00 GMT 2019


On Mon, 18 Nov 2019, Jakub Jelinek wrote:

> On Mon, Nov 18, 2019 at 10:44:14AM +0100, Richard Biener wrote:
> > The following adjusts the find_base_{term,value} heuristic when
> > looking through ANDs to the case where it is obviously not
> > aligning a base (when the LSB is set).
> 
> What is specific about the LSB?  I mean & 2 is obviously not an aligning
> AND either.

It aligns 0x1 and 0x3 ;)

> Shouldn't we recurse only if the CONST_INT_P operand has
> some set bits followed by clear bits, i.e. after the != 0 check
> compute ctz_hwi and verify that INTVAL >> ctz == -1?

I thought of more advanced heuristics but I guess that
any contiguous set of bits with LSB unset might be aligning
if the programmer knows upper bits are zero anyways?  So I fear
the == -1 test would not work reliable?

But since find_base_term/value are broken anyways I guess we
can live with that...

Richard.



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