[Bug target/122532] New: Compilation with -Oz --- does not reuse known register values

[zero at smallinteger dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122532

            Bug ID: 122532
           Summary: Compilation with -Oz --- does not reuse known register
                    values
           Product: gcc
           Version: 15.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 62691
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=62691&action=edit
Sample code

Consider the attached code compiled with -Oz.  Per Godbolt, gcc 15.2 yields the
following.

test:
        push    66
        pop     rax
        mov     QWORD PTR [rdi], 66
        mov     QWORD PTR [rdi+8], 66
        mov     QWORD PTR [rdi+16], 66
        ret

Note the memory stores do not reuse the known value of rax.  Consequently, the
emitted code is larger than what it could be.  For comparison, clang does this
with -Oz.

test:
        push    66
        pop     rax
        mov     qword ptr [rdi], rax
        mov     qword ptr [rdi + 8], rax
        mov     qword ptr [rdi + 16], rax
        ret