From: Richard Henderson <richard.henderson@linaro.org>
To: gcc-patches@gcc.gnu.org, richard.earnshaw@arm.com,
marcus.shawcroft@arm.com, kyrylo.tkachov@arm.com,
Wilco.Dijkstra@arm.com,
Segher Boessenkool <segher@kernel.crashing.org>,
richard.sandiford@arm.com
Subject: Re: [PATCH v2 3/9] aarch64: Add <su>cmp_*_carryinC patterns
Date: Wed, 1 Apr 2020 10:14:43 -0700 [thread overview]
Message-ID: <1b470fbc-6b17-1717-fb0b-ba0f5d3f8c4d@linaro.org> (raw)
In-Reply-To: <mpto8sb5fxk.fsf@arm.com>
On 4/1/20 9:28 AM, Richard Sandiford wrote:
> How important is it to describe the flags operation as a compare though?
> Could we instead use an unspec with three inputs, and keep it as :CC?
> That would still allow special-case matching for zero operands.
I'm not sure.
My guess is that the only interesting optimization for ADC/SBC is when
optimization determines that the low-part of op2 is zero, so that we can fold
[(set (reg cc) (compare ...))
(set (reg t0) (sub (reg a0) (reg b0))]
[(set (reg cc) (compare ...))
(set (reg t1) (sub (reg a1)
(sub (reg b1)
(geu (reg cc) (const 0)))))]
to
[(set (reg t0) (reg a0)]
[(set (reg cc) (compare ...))
(set (reg t1) (sub (reg a1) (reg b1))]
which combine should be able to do by propagating zeros across the compare+geu.
Though I suppose it's still possible to handle this with unspecs and
define_split, so that
[(set (reg cc)
(unspec [(reg a1) (reg b2) (geu ...)]
UNSPEC_SBCS)
(set (reg t1) ...)]
when the geu folds to (const_int 0), we can split this to a plain sub.
I'll see if I can make this work with a minimum of effort.
r~
next prev parent reply other threads:[~2020-04-01 17:14 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-21 2:42 [PATCH v2 0/9] aarch64: Implement TImode comparisons Richard Henderson
2020-03-21 2:42 ` [PATCH v2 1/9] aarch64: Accept 0 as first argument to compares Richard Henderson
2020-03-31 16:55 ` Richard Sandiford
2020-03-31 17:15 ` Richard Henderson
2020-03-21 2:42 ` [PATCH v2 2/9] aarch64: Accept zeros in add<GPI>3_carryin Richard Henderson
2020-03-21 2:42 ` [PATCH v2 3/9] aarch64: Add <su>cmp_*_carryinC patterns Richard Henderson
2020-03-22 19:30 ` Segher Boessenkool
2020-03-22 20:40 ` Richard Henderson
2020-03-31 18:34 ` Richard Sandiford
2020-03-31 22:44 ` Richard Henderson
2020-04-01 12:37 ` Segher Boessenkool
2020-04-01 16:28 ` Richard Sandiford
2020-04-01 17:14 ` Richard Henderson [this message]
2020-03-21 2:42 ` [PATCH v2 4/9] aarch64: Add <su>cmp<GPI>_carryinC_m2 Richard Henderson
2020-03-21 2:42 ` [PATCH v2 5/9] aarch64: Provide expander for sub<GPI>3_compare1 Richard Henderson
2020-03-21 2:42 ` [PATCH v2 6/9] aarch64: Introduce aarch64_expand_addsubti Richard Henderson
2020-03-21 2:42 ` [PATCH v2 7/9] aarch64: Adjust result of aarch64_gen_compare_reg Richard Henderson
2020-03-22 21:55 ` Segher Boessenkool
2020-03-22 22:21 ` Richard Henderson
2020-03-21 2:42 ` [PATCH v2 8/9] aarch64: Implement TImode comparisons Richard Henderson
2020-03-21 2:42 ` [PATCH v2 9/9] aarch64: Implement absti2 Richard Henderson
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