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From: "andrew at sifive dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108247] Missed opportunity to generate shNadd on risc-v
Date: Thu, 29 Dec 2022 01:36:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108247

Andrew Waterman <andrew at sifive dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrew at sifive dot com

--- Comment #1 from Andrew Waterman <andrew at sifive dot com> ---
The base-ISA-only code sequence has the same instruction count but smaller =
code
size:

  slli a1, a1, 1
  addw a0, a1, a0
  ret

I guess this example is a proxy for a broader set of missed optimizations, =
some
of which are actually improved by Zba, so maybe my comment is moot.  But it
would be best not to de-optimize code size in pursuit of opportunities to u=
se
the Zba instructions.

(It also occurs to me that the sh1add + sext.w sequence is easier to fuse
because the instructions have the same destination.  But choosing to increa=
se
code size to avail a fusion opportunity is a uarch-specific tuning decision=
.)=