[Bug middle-end/32662] Significant extra code generation for 64x64=>128-bit multiply

[scovich at gmail dot com]

------- Comment #3 from scovich at gmail dot com  2007-07-07 14:55 -------
While it's nice that the new optimization framework can eliminate the redundant
IMUL instruction(s), why were they being generated in the first place? 

Compiling the simpler foo() without optimizations gives:

_Z3fooPyPKyy:
.LFB2:
        pushq   %rbp
.LCFI0:
        movq    %rsp, %rbp
.LCFI1:
        pushq   %rbx
.LCFI2:
        movq    %rdi, -16(%rbp)
        movq    %rsi, -24(%rbp)
        movq    %rdx, -32(%rbp)
        movq    -24(%rbp), %rax
        movq    (%rax), %rax
        movq    %rax, %rcx
        movl    $0, %ebx          << here
        movq    -32(%rbp), %rax
        movl    $0, %edx          << here
        movq    %rbx, %rsi
        imulq   %rax, %rsi        << here
        movq    %rdx, %rdi
        imulq   %rcx, %rdi        << here
        addq    %rdi, %rsi
        mulq    %rcx
        addq    %rdx, %rsi
        movq    %rsi, %rdx
        movq    %rdx, %rax
        xorl    %edx, %edx
        movq    %rax, %rdx
        movq    -16(%rbp), %rax
        movq    %rdx, (%rax)
        popq    %rbx
        leave
        ret


Barring something really strange it seems like this problem could/should be
fixed at its source, even for 4.1/4.2

Reopen?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32662