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Re: [PATCH, ARM] Further improve stack usage in sha512, part 2 (PR 77308)
Bernd Edlinger wrote:
On 12/20/16 16:09, Wilco Dijkstra wrote:
> > As a result of your patches a few patterns are unused now. All the Thumb-2 iordi_notdi*
> > patterns cannot be used anymore. Also I think arm_cmpdi_zero never gets used - a DI
>> mode compare with zero is always split into ORR during expand.
>
> I did not change anything for -mthumb -mfpu=neon for instance.
> Do you think that iordi_notdi* is never used also for that
> configuration?
With -mfpu=vfp or -msoft-float, these patterns cannot be used as logical operations are expanded before combine. Interestingly with -mfpu=neon ARM uses the orndi3_neon patterns (which are inefficient for ARM and probably should be disabled) but Thumb-2 uses the iordi_notdi patterns... So removing these reduces the number of patterns while we will still generate orn for Thumb-2.
> And if the arm_cmpdi_zero is never expanded, isn't it already
> unused before my patch?
It appears to be, so we don't need to fix it now. However when improving the expansion of comparisons it does trigger. For example x == 3 expands currently into 3 instructions:
cmp r1, #0
itt eq
cmpeq r0, #3
Tweaking arm_select_cc_mode uses arm_cmpdi_zero, and when expanded early we generate this:
eor r0, r0, #3
orrs r0, r0, r1
Using sub rather than eor would be even better of course.
Wilco