Re: [PATCH][combine] PR rtl-optimization/68651 Try changing rtx from (r + r) to (r << 1) to aid recognition

[Bernd Schmidt <bschmidt at redhat dot com>]

On 12/14/2015 01:25 PM, Kyrill Tkachov wrote:
> PR 68651 is a code quality regression for GCC 5 and GCC 6 that was
> introduced due to updated rtx costs
> for -mcpu=cortex-a53 that affected expansion.  The costs changes were
> correct (to the extent that rtx
> costs have any meaning) and I think this is a deficiency in combine that
> should be fixed.

Thinking a bit more about this, I'm actually not sure that this isn't a 
backend problem. IMO the costs could and maybe very well should be 
represented such that a left shift by 1 and an add have the same cost, 
and the insn pattern for the shift should emit the add if it is cheaper. 
If there are multiple ways of expressing an operation, then how it is 
represented in RTL is essentially irrelevant to the question of how much 
it costs.


Bernd