Re: [PATCH] combine: Fix simplify_comparison AND handling for WORD_REGISTER_OPERATIONS targets [PR109040]

[Jeff Law <jeffreyalaw@gmail.com>]

On 4/6/23 04:31, Jakub Jelinek wrote:

> 
> If we want to fix it in the combiner, I think the fix would be following.
> The optimization is about
> (and:SI (subreg:SI (reg:HI xxx) 0) (const_int 0x84c))
> and IMHO we can only optimize it into
> (subreg:SI (and:HI (reg:HI xxx) (const_int 0x84c)) 0)
> if we know that the upper bits of the REG are zeros.  
But in WORD_REGISTER_OPERATIONS, that inner AND variant operates on a 
full word.  So I think they're equivalent.  But maybe I'm getting myself 
confused again.



> 
> Now, this patch fixes the PR, but certainly generates worse (but correct)
> code than the dse.cc patch.  So perhaps we want both of them?
I think the dse patch has value independently of this discussion, though 
I think it's more of a gcc-14 thing.

> 
> As before, I unfortunately can't test it on riscv-linux (could perhaps try
> that on sparc-solaris on GCC Farm which is another WORD_REGISTER_OPERATIONS
> target, but last my bootstrap attempt there failed miserably because of the
> Don't bootstrap at midnight issue in cp/Make-lang.in; I'll post a patch
> for that once I test it).
I can spin it here when the time comes.

jeff