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

["segher at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |segher at gcc dot gnu.org

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Paolo Bonzini from comment #0)
>         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.

It would seem %rsi is still alive after this, so the %rcx input to "3"
has to be computed, which takes more than one insn so there is no way
combine will be able to handle this.