On Mon, Feb 16, 2009 at 11:19 AM, Narasimha Datta <datt...@yahoo.com> wrote:
> Hello,
>
> Here's a simple memory copy macro:
>
> #define MYMEMCOPY(dp, sp, len) \
> do { \
> long __len = len; \
> while (--__len >= 0) \
> (dp)[__len] = (sp)[__len]; \
> } while (0)
>
> void foo(unsigned char *dp, const unsigned char *sp, unsigned long size) {
> MYMEMCOPY(dp, sp, size);
> }
>
> void bar(unsigned char *dp, const unsigned char *sp) {
> MYMEMCOPY(dp, sp, 128);
> }
>
> The code fragments generated for the foo and bar functions with -O and -O2
> optimizations respectively is as follows:
>
> /* ===== With -O switch ===== */
> /* function foo */
> .L4:
> movzbl -1(%rcx), %eax
> movb %al, -1(%rdx)
> subq $1, %rcx
> subq $1, %rdx
> subq $1, %r8
> jns .L4
>
> /* function bar */
> movl $126, %edx
> .L8:
> .LBB3:
> .loc 1 13 0
> movzbl 1(%rdx,%rsi), %eax
> movb %al, 1(%rdx,%rdi)
> subq $1, %rdx
> cmpq $-2, %rdx
> jne .L8
>
> /* ===== With -O2 switch =====*/
> /* function foo */
> .L4:
> movzbl -1(%rsi), %eax
> addq $1, %rdi
> subq $1, %rsi
> movb %al, -1(%rcx)
> subq $1, %rcx
> cmpq %rdx, %rdi
> jne .L4
>
> /* function bar */
> movl $126, %edx
> .L9:
> .LBB3:
> .loc 1 13 0
> movzbl 1(%rdx,%rsi), %eax
> movb %al, 1(%rdx,%rdi)
> subq $1, %rdx
> cmpq $-2, %rdx
> jne .L9
>
> Now my questions are:
> (i) Why does the compiler generate an addq, cmpq and jne for the foo function
> with -O2? Isn't subq/jns more efficient, as seen from the output from -O?
> (ii) For function bar, why is the "cmpq $-2, %rdx" instruction generated?
> Won't it be better to count down from 128 to 0 instead of 126 to -2?
>
> Here's my OS and compiler version (I'm running a 64-bit FreeBSD):
> $ uname -a
> FreeBSD xxx 7.0-RELEASE FreeBSD 7.0-RELEASE #0: Wed Nov 12 18:54:21 PST 2008
> r...@wc7:/usr/obj/usr/src/sys/SMKERNEL amd64
> $ cc --version
> cc (GCC) 4.2.1 20070719 [FreeBSD]
> Copyright (C) 2007 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
> And these are the commands I used to compile the program:
> cc -S -O -g test.c
> cc -S -O2 -g test.c
>
> Any pointers would be appreciated. Thanks!
1) Try a more recent GCC
2) Use memcpy. It is properly inlined/optimized.
Richard.
