[Bug target/70079] New: missed constant propagation in memcpy expansion

[bonzini at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70079

            Bug ID: 70079
           Summary: missed constant propagation in memcpy expansion
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bonzini at gnu dot org
  Target Milestone: ---

int f(char *restrict a, const char *restrict b)
{
        __builtin_memcpy(a, b, 512);
}

$ gcc f.c -O2 - -o f.s

includes the following code:

        movq    %rdi, %rcx
        leaq    8(%rdi), %rdi
        andq    $-8, %rdi                 ;; 1
        subq    %rdi, %rcx                ;; 2
        subq    %rcx, %rsi                ;; 3
        addl    $512, %ecx                ;; 4
        shrl    $3, %ecx                  ;; 5

At 1, rdi = (a + 8) & ~7 = a & ~7 + 8 = a + 8 - (a & 7)
At 2, rcx = a - rdi = a - a - 8 + (a & 7) = (a & 7) - 8
At 3, rsi = b - (a & 7) + 8
At 4, rcx = (a & 7) + 504, which is between 504 and 511
At 5, rcx is always 31.

Not sure how to fix it in RTL optimizers, so I'm leaving this in the target
component for a backend-specific fix.