[Bug tree-optimization/98335] [9/10/11/12 Regression] Poor code generation for partial struct initialization

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98335

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:251ea6dfbdb4448875e41081682bb3aa451b5729

commit r12-7616-g251ea6dfbdb4448875e41081682bb3aa451b5729
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Fri Mar 11 17:57:12 2022 +0000

    PR tree-optimization/98335: New peephole2 xorl;movb -> movzbl

    This patch is the backend piece of my proposed fix to PR tree-opt/98335,
    to allow C++ partial struct initialization to be as efficient/optimized
    as full struct initialization.

    With the middle-end patch just posted to gcc-patches, the test case
    in the PR compiles on x86_64-pc-linux-gnu with -O2 to:

            xorl    %eax, %eax
            movb    c(%rip), %al
            ret

    with this additional peephole2 (actually four peephole2s):

            movzbl  c(%rip), %eax
            ret

    2022-03-11  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR tree-optimization/98335
            * config/i386/i386.md (peephole2): Eliminate redundant insv.
            Combine movl followed by movb.  Transform xorl followed by
            a suitable movb or movw into the equivalent movz[bw]l.

    gcc/testsuite/ChangeLog
            PR tree-optimization/98335
            * g++.target/i386/pr98335.C: New test case.
            * gcc.target/i386/pr98335.c: New test case.