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Re: [PATCH] [Aarch64] Optimize subtract in shift counts

On August 21, 2017 7:46:09 PM GMT+02:00, Richard Sandiford <> wrote:
>Richard Biener <> writes:
>> On Tue, Aug 8, 2017 at 10:20 PM, Richard Kenner
>> <> wrote:
>>>> Correct. It is truncated for integer shift, but not simd shift
>>>> instructions. We generate a pattern in the split that only
>>>> the integer shift instructions.
>>> That's unfortunate, because it would be nice to do this in
>>> since it's machine-independent, but that has to be conditioned on
>>> SHIFT_COUNT_TRUNCATED, so you wouldn't get the benefit of it.
>> SHIFT_COUNT_TRUNCATED should go ... you should express this in
>> the patterns, like for example with
>> (define_insn ashlSI3
>>   [(set (match_operand 0 "")
>>          (ashl:SI (match_operand ... )
>>                      (subreg:QI (match_operand:SI ...)))]
>> or an explicit and:SI and combine / simplify_rtx should apply the
>> optimization we expect.
>The problem with the explicit AND is that you'd end up with either
>an AND of two constants for constant shifts, or with two separate
>one for constant shifts and one for variable shifts.  (And the problem
>theory with two patterns is that it reduces the RA's freedom, although
>practice I guess we'd always want a constant shift where possible for
>cost reasons, and so the RA would never need to replace pseudos with
>constants itself.)
>I think all useful instances of this optimisation will be exposed by
>the gimple optimisers, so maybe expand could to do it based on
>TARGET_SHIFT_TRUNCATION_MASK?  That describes the optab rather than
>the rtx code and it does take the mode into account.

Sure, that could work as well and also take into account range info. But we'd then need named expanders and the result would still have the explicit and or need to be an unspec or a different RTL operation. 



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