[Bug c/112748] New: memmove(ptr, ptr, n) call optimized out even at -O0 with -fsanitize=undefined

["tavianator at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112748

            Bug ID: 112748
           Summary: memmove(ptr, ptr, n) call optimized out even at -O0
                    with -fsanitize=undefined
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tavianator at gmail dot com
  Target Milestone: ---

This is counter-productive, as I wrote the memmove() specifically to get the
sanitizers to check that ptr really points to a big enough allocation.

$ cat foo.c
typedef __SIZE_TYPE__ size_t;
void *memmove(void *dest, void *src, size_t n);

void foo(void *ptr, size_t n) {
        memmove(ptr, ptr, n);
}
$ gcc -O0 -fsanitize=undefined -S foo.c
$ cat foo.s
        .file   "foo.c"
        .text
        .globl  foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movq    %rdi, -8(%rbp)
        movq    %rsi, -16(%rbp)
        nop
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   foo, .-foo
        .ident  "GCC: (GNU) 13.2.1 20230801"
        .section        .note.GNU-stack,"",@progbits