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From: "vineetg at rivosinc dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108987] New: [13 Regression] RISC-V: shiftadd cost model
 bug needlessly preferring syth_multiply
Date: Wed, 01 Mar 2023 19:34:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108987

            Bug ID: 108987
           Summary: [13 Regression] RISC-V: shiftadd cost model bug
                    needlessly preferring syth_multiply
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vineetg at rivosinc dot com
  Target Milestone: ---

gcc trunk is preferring synthetic multiply using shift+add even when they a=
re
costlier than multiply.

unsigned long long f5(unsigned long long i)
{
  return i * 0x0202020202020202ULL;
}

riscv64-unknown-linux-gnu-gcc -c -O2 -march=3Drv64gc_zba

f5:
        slli    a5,a0,8
        add     a0,a5,a0
        slli    a5,a0,16
        add     a0,a0,a5
        slli    a5,a0,32
        add     a0,a0,a5
        slli    a0,a0,1
        ret

With gcc 12.2 this used to be

f5:
        lui     a5,%hi(.LC0)
        ld      a5,%lo(.LC0)(a5)
        mul     a0,a0,a5
        ret

This is a regression introduced by commit f90cb39235c4 ("RISC-V: costs: sup=
port
shift-and-add in strength-reduction"). It introduced the cost for
shift[1-3]+add (to favor SH*ADD) but due to a coding bug ended up doing this
for all shift values, affecting synth multiply among others.

This showed up as dynamic icount regression in SPEC 531.deepsjeng.=