------- Comment #2 from ajrobb at bigfoot dot com 2008-08-30 07:21 -------
Thanks for that.
I notice that the 64-bit value is now being accumulated in memory rather than
in registers. To be fair, a full 64-bit accumulator need not be maintained -
only the high 32 bits. Could you compile the following version with the fixed
4.4, please?
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
uint64_t mul2(uint32_t a, uint32_t b) {
return a * (uint64_t)b;
}
void mul32(uint32_t * a, const size_t n, const uint32_t factor) {
ssize_t i = n;
uint32_t hi = 0;
while (--i >= 0) {
uint64_t p = mul2(*a, factor) + hi;
*a++ = p;
hi = p >> 32;
}
*a = p;
}
For what it's worth, I tweaked the assembler by hand to avoid local stack
variables:
.globl _mul32
.def _mul32; .scl 2; .type 32; .endef
_mul32:
pushl %ebp
pushl %esi
xorl %esi, %esi
pushl %ebx
movl 20(%esp), %ecx
movl 16(%esp), %ebp
testl %ecx, %ecx
movl 24(%esp), %ebx
je L6
.p2align 4,,7
L7:
movl (%ebp), %eax
mull %ebx
addl %esi, %eax
movl %eax, (%ebp)
adcl $0, %edx
decl %ecx
leal 4(%ebp), %ebp
movl %edx, %esi
jne L7
L6:
movl %esi, (%ebp)
popl %ebx
popl %esi
popl %ebp
ret
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37233
