[Bug middle-end/28395] Improved division-by-constant code

[vda dot linux at googlemail dot com]

------- Comment #9 from vda dot linux at googlemail dot com  2006-07-16 18:47 
-------
The test program below shows that in this case doing division with div insn
takes more instructions than with mul+shift.

Also mul+shift path has absolutely useless "movl %edx, %eax" insn, shaving that
will make it even smaller. Need to build newer gcc and retest...

# cat t.c
enum { B = 10 };
enum { shift_bits = 35 };
enum { K = (1ULL<<shift_bits)/B + 1 };
unsigned a,b;
void f(unsigned A)
{
asm("#1");
        a = A/B;
asm("#2");
        b = (((unsigned long long)A) * K) >> shift_bits;
asm("#3");
}

# gcc -Os -fomit-frame-pointer -S t.c

# cat t.s
        .file   "t.c"
        .text
.globl f
        .type   f, @function
f:
        pushl   %ebx
#APP
        #1
#NO_APP
        movl    $10, %edx
        movl    %edx, %ebx
        movl    8(%esp), %eax
        xorl    %edx, %edx
        divl    %ebx
        movl    %eax, a
#APP
        #2
#NO_APP
        movl    $-858993459, %eax
        mull    8(%esp)
        movl    %edx, %eax
        shrl    $3, %eax
        movl    %eax, b
#APP
        #3
#NO_APP
        popl    %ebx
        ret
        .size   f, .-f
        .comm   a,4,4
        .comm   b,4,4
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.3"


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28395