[Bug middle-end/44382] Slow integer multiply

["hjl dot tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from hjl dot tools at gmail dot com  2010-06-04 13:08 -------
(In reply to comment #1)
> Because our tree reassoc doesn't re-associate them.
> 

The tree reassoc pass makes it slower:

[hjl@gnu-6 44382]$ cat x.i
extern int a, b, c, d, e, f;
void
foo ()
{
  a = (b * c) * (d * e);
}
[hjl@gnu-6 44382]$ gcc -S -O2 x.i
[hjl@gnu-6 44382]$ cat x.s
        .file   "x.i"
        .text
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        movl    c(%rip), %eax
        imull   b(%rip), %eax
        imull   d(%rip), %eax
        imull   e(%rip), %eax
        movl    %eax, a(%rip)
        ret
[hjl@gnu-6 44382]$ gcc -S -O2 x.i -fno-tree-reassoc
[hjl@gnu-6 44382]$ cat x.s
        .file   "x.i"
        .text
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
.LFB0:
        .cfi_startproc
        movl    b(%rip), %eax
        movl    d(%rip), %edx
        imull   c(%rip), %eax
        imull   e(%rip), %edx
        imull   %edx, %eax
        movl    %eax, a(%rip)
        ret
[hjl@gnu-6 44382]$ 


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44382