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Re: cortexa57_extra_costs' alu.shift_reg
- From: Andrew Pinski <pinskia at gmail dot com>
- To: GCC Mailing List <gcc at gcc dot gnu dot org>, Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>
- Date: Sat, 11 Jan 2020 02:07:51 -0800
- Subject: Re: cortexa57_extra_costs' alu.shift_reg
- References: <CA+=Sn1mjDo1Hp4tynXuM_PsKgC_jMueKZiZsj1jg=is4T=3UUg@mail.gmail.com>
On Sat, Jan 11, 2020 at 2:02 AM Andrew Pinski <email@example.com> wrote:
> I was looking into reassoc (for PR 93131) and I noticed that the
> alu.shift_reg is set to COSTS_N_INSNS (1). This prevents an
> optimization where we combine some if statements into shifts. I
> looked into the Corext A57 software optimization guide and saw that
> shift with a register has a latency of 1 which is the same as a shift
> with an immediate. I looked in the history but it was there to begin
> with. Is there a reason for this and would it be acceptable if we
> change a57's costs to be correct?
> I looked into A65, A72, A75, A76, A77 and N1 software optimization
> guides too and they have a similar thing of a latency of one cycle and
> those are the ones which use the cortexa57_extra_costs variable too.
> Note I could not find the A73 guide though.
I forgot to mention this causes lshift_cheap_p to return false which
is not true for these cores.
> Andrew Pinski
>  http://infocenter.arm.com/help/topic/com.arm.doc.uan0015b/Cortex_A57_Software_Optimization_Guide_external.pdf