[Bug middle-end/82885] New: memcpy does not propagate aliasing knowledge

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82885

            Bug ID: 82885
           Summary: memcpy does not propagate aliasing knowledge
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

The following code:

void foo3(char* ch1, const char* ch2) {
    __builtin_memcpy(ch1, ch2, 1024*1024);
    ch1[0] = ch2[2];
    ch1[1] = ch2[2];
}


Produces assembly

foo3(char*, char*):
        push    rbx
        mov     edx, 1048576
        mov     rbx, rsi
        call    memcpy
        mov     rcx, rax
        movzx   eax, BYTE PTR [rbx+2]
        mov     BYTE PTR [rcx], al
        movzx   eax, BYTE PTR [rbx+2] <== This could be removed
        mov     BYTE PTR [rcx+1], al
        pop     rbx
        ret

memcpy works for non aliased data, otherwise it is UB. Knowledge that pointers
do not alias could be propagated further to remove reads from ch2.