From: Jeff Law <law@redhat.com>
To: Segher Boessenkool <segher@kernel.crashing.org>, gcc-patches@gcc.gnu.org
Cc: bergner@linux.ibm.com
Subject: Re: [PATCH] combine: Do not combine moves from hard registers
Date: Mon, 22 Oct 2018 22:07:00 -0000 [thread overview]
Message-ID: <59f7ad07-5230-3b65-ed02-7f9e806c383e@redhat.com> (raw)
In-Reply-To: <68abf72a5400b96b9a100966331d3ad2056648e7.1540237620.git.segher@kernel.crashing.org>
On 10/22/18 2:17 PM, Segher Boessenkool wrote:
> On most targets every function starts with moves from the parameter
> passing (hard) registers into pseudos. Similarly, after every call
> there is a move from the return register into a pseudo. These moves
> usually combine with later instructions (leaving pretty much the same
> instruction, just with a hard reg instead of a pseudo).
>
> This isn't a good idea. Register allocation can get rid of unnecessary
> moves just fine, and moving the parameter passing registers into many
> later instructions tends to prevent good register allocation. This
> patch disallows combining moves from a hard (non-fixed) register.
>
> This also avoid the problem mentioned in PR87600 #c3 (combining hard
> registers into inline assembler is problematic).
>
> Because the register move can often be combined with other instructions
> *itself*, for example for setting some condition code, this patch adds
> extra copies via new pseudos after every copy-from-hard-reg.
>
> On some targets this reduces average code size. On others it increases
> it a bit, 0.1% or 0.2% or so. (I tested this on all *-linux targets).
>
> I'll commit this to trunk now. If there are problems, please don't
> hesitate to let me know! Thanks.
>
>
> Segher
>
>
> 2018-10-22 Segher Boessenkool <segher@kernel.crashing.org>
>
> PR rtl-optimization/87600
> * combine.c: Add include of expr.h.
> (cant_combine_insn_p): Do not combine moves from any hard non-fixed
> register to a pseudo.
> (make_more_copies): New function, add a copy to a new pseudo after
> the moves from hard registers into pseudos.
> (rest_of_handle_combine): Declare rebuild_jump_labels_after_combine
> later. Call make_more_copies.
I know we've gone back and forth on this stuff through the years,
particularly for targets with likely-to-spilled classes that are used
for register passing/return values.
I'm certainly willing to go with this as general guidance. I wouldn't
be surprised if we find that things like CSE, fwprop and other passes
need twiddling over time to mimick what you're doing in combine.
jeff
next prev parent reply other threads:[~2018-10-22 20:52 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-22 21:06 Segher Boessenkool
2018-10-22 22:07 ` Jeff Law [this message]
2018-10-22 22:46 ` Segher Boessenkool
2018-10-23 10:54 ` Christophe Lyon
2018-10-23 12:36 ` Christophe Lyon
2018-10-23 13:10 ` Segher Boessenkool
2018-10-23 13:30 ` Christophe Lyon
2018-10-23 13:08 ` Segher Boessenkool
2018-10-23 13:50 ` Christophe Lyon
2018-10-24 1:50 ` Segher Boessenkool
2018-10-24 9:52 ` Christophe Lyon
2018-11-02 22:19 ` Renlin Li
2018-11-02 23:03 ` Segher Boessenkool
2018-11-02 23:54 ` Segher Boessenkool
2018-11-03 2:34 ` Jeff Law
2018-11-05 12:35 ` Renlin Li
2018-11-05 18:16 ` Renlin Li
2018-11-07 23:02 ` Segher Boessenkool
2018-11-09 10:21 ` Richard Earnshaw (lists)
2018-11-05 22:50 ` Segher Boessenkool
2018-10-24 9:38 ` Paul Hua
2018-10-23 16:16 ` Andreas Schwab
2018-10-23 16:28 ` Segher Boessenkool
2018-10-24 12:24 ` Andreas Schwab
2018-10-26 17:41 ` Steve Ellcey
2018-10-26 18:37 ` Segher Boessenkool
[not found] ` <7bc8697f-a09e-627f-b032-eba5ecb682ac@arm.com>
2018-11-08 20:34 ` Segher Boessenkool
2018-11-09 21:12 ` Jeff Law
2018-11-10 4:51 ` Segher Boessenkool
2018-11-10 16:54 ` Jeff Law
2018-11-12 11:57 ` Sam Tebbs
[not found] ` <e9104024-2f6a-6cb8-e366-39b96f7a72f2@arm.com>
2018-11-12 12:54 ` Segher Boessenkool
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