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From: Robin Dapp <rdapp@linux.ibm.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>, Andrew Pinski <pinskia@gmail.com>
Subject: Re: [PATCH] expand: Convert cst - x into cst xor x.
Date: Wed, 7 Sep 2022 14:20:07 +0200	[thread overview]
Message-ID: <3d928191-a2f5-f314-c03b-d4e590282ce9@linux.ibm.com> (raw)
In-Reply-To: <CAFiYyc0dV7LP=LhKcJBuc0M4tqas==FgEqHwpMbW2u6JGTVQrg@mail.gmail.com>

> The question is really whether xor or sub is "better" statically.  I can't
> think of any reasons.  On s390, why does xor end up "better"?

There is an xor with immediate (as opposed to no "subtract from
immediate") which saves an instruction, usually.  On x86, I think the
usual argument for xor is that it's shorter (if flags etc. are not needed).

It's not that I don't want to implement it in the backend, just that I
understood the original PR in a way that it would make sense to have
this conversion available for more targets.  If there are too many
confounding factors that prevent this situation from being statically
costed properly, then sure, not much use in implementing it generally.

Regards
 Robin

  reply	other threads:[~2022-09-07 12:20 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-06  9:42 Robin Dapp
2022-09-06 12:31 ` Richard Biener
2022-09-06 14:01   ` Robin Dapp
2022-09-07 12:08     ` Richard Biener
2022-09-07 12:20       ` Robin Dapp [this message]
2022-09-07 12:45         ` Richard Biener
2022-09-07 16:30           ` Jeff Law
2022-10-21  9:13           ` Robin Dapp

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