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From: Jeff Law <jeffreyalaw@gmail.com>
To: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] RISC-V: Produce better code with complex constants [PR95632] [PR106602]
Date: Thu, 8 Dec 2022 11:15:47 -0700	[thread overview]
Message-ID: <1953155e-7d64-664b-4b88-2fc4cb33acf7@gmail.com> (raw)
In-Reply-To: <mhng-e3367d70-4b53-4aff-bbc8-09bea1e0296a@palmer-ri-x1c9a>



On 12/8/22 10:53, Palmer Dabbelt wrote:
> On Wed, 07 Dec 2022 12:55:17 PST (-0800), rzinsly@ventanamicro.com wrote:
>> Due to RISC-V limitations on operations with big constants combine
>> is failing to match such operations and is not being able to
>> produce optimal code as it keeps splitting them. By pretending we
>> can do those operations we can get more opportunities for
>> simplification of surrounding instructions.
> 
> I saw Jeff's comments.  This is always the kind of thing that worries 
> me: we're essentially lying to the optimizer in order to trick it into 
> generating better code, which might just make it generate worse code. 
> It's always easy to see a small example that improves, but those could 
> be wiped out by secondary effects in real code.  So I'd usually want to 
> have some benchmarking for a patch like this.
> 
> That said, if this is just the standard way of doing things then maybe 
> it's just fine?
Bridge combiner patterns are pretty standard.  The insn's condition of 
cse_not_expected is also in there to minimize the potential for 
surprises by not exposing this too early.

jeff

  reply	other threads:[~2022-12-08 18:15 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-07 20:55 Raphael Moreira Zinsly
2022-12-07 21:13 ` Jeff Law
2022-12-07 21:30 ` Jakub Jelinek
2022-12-08 17:39   ` Palmer Dabbelt
2022-12-08 17:53 ` Palmer Dabbelt
2022-12-08 18:15   ` Jeff Law [this message]
2022-12-08 20:21     ` Palmer Dabbelt

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