Re: [PATCH] Fix simplify_shift_const_1 once more (PR rtl-optimization/70429)

[Jeff Law <law at redhat dot com>]

On 03/29/2016 11:21 AM, Jakub Jelinek wrote:
> Hi!
>
> This is a case similar to the LSHIFTRT I've fixed recently.
> But, unlike LSHIFTRT, which can be handled by masking at the outer level,
> ASHIFTRT would need outer sign extension, so most of the time 2 outer
> operations in addition to the kept two inner shifts, which is IMHO very
> unlikely to ever be successfully combined on any target nor actually
> beneficial.  So this patch just avoids that optimization for ASHIFTRT
> if there are different modes.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2016-03-29  Jakub Jelinek  <jakub@redhat.com>
>
> 	PR rtl-optimization/70429
> 	* combine.c (simplify_shift_const_1): For ASHIFTRT don't optimize
> 	(cst1 >> count) >> cst2 into (cst1 >> cst2) >> count if
> 	mode != result_mode.
>
> 	* gcc.c-torture/execute/pr70429.c: New test.
But isn't the point of this code that cst1 >> cst2 turns into a compile 
time constant just leaving one runtime shift of the result by count?


Jeff