Hi Daniel Thanks for running the test for me.
The option "-mregnames" exists only on gcc for PPC. Results are as before for x86. crc32do_while is the winner followed by crc32do_while_dec. Gcc should be able generate the same code for crc32org and crc32do_while, it's a simple optimization. crc32do_while_dec is possibly only useful on PPC. On PPC I expect crc32do_while_dec to be the winner. Do you have a gcc 3.2 which will generate PPC assembly? Jocke PS. You don't have to be on the list to post to it. I will CC the list for now. > > > Jocke, > > The option "-mregnames" no longer exists in version 3.2 of gcc. I > couldn't find anything equivalent. I ran it without that option (gcc -S > -O2 testcode.c) and produced the following on a i686 RedHat 7.3 box > using gcc 3.2 (gcc 3.2.1 is the latest release I believe) > > I am not on the list, hence I cannot CC the list. This message was > forwarded to me from someone else. > > Dan Eisenhut > GE Medical Systems - Information Technologies > daniel.eisenhut at med.ge.com > 414-362-3151 > > > .file "testcode.c" > .text > .align 2 > .p2align 4,,15 > .globl crc32org > .type crc32org, at function > crc32org: > pushl %ebp > movl %esp, %ebp > pushl %esi > movl 16(%ebp), %edx > pushl %ebx > movl 8(%ebp), %ecx > movl 12(%ebp), %ebx > decl %edx > cmpl $-1, %edx > je .L7 > movl $crc32_table, %esi > .p2align 4,,15 > .L5: > movzbl (%ebx), %eax > decl %edx > incl %ebx > xorb %cl, %al > shrl $8, %ecx > movzbl %al, %eax > xorl (%esi,%eax,4), %ecx > cmpl $-1, %edx > jne .L5 > .L7: > popl %ebx > movl %ecx, %eax > popl %esi > popl %ebp > ret > .Lfe1: > .size crc32org,.Lfe1-crc32org > .align 2 > .p2align 4,,15 > .globl crc32do_while > .type crc32do_while, at function > crc32do_while: > pushl %ebp > movl %esp, %ebp > pushl %esi > movl 8(%ebp), %edx > pushl %ebx > movl 16(%ebp), %ebx > movl 12(%ebp), %ecx > testl %ebx, %ebx > je .L9 > movl $crc32_table, %esi > .p2align 4,,15 > .L10: > movzbl (%ecx), %eax > incl %ecx > xorb %dl, %al > shrl $8, %edx > movzbl %al, %eax > xorl (%esi,%eax,4), %edx > decl %ebx > jne .L10 > .L9: > popl %ebx > movl %edx, %eax > popl %esi > popl %ebp > ret > .Lfe2: > .size crc32do_while,.Lfe2-crc32do_while > .align 2 > .p2align 4,,15 > .globl crc32do_while_dec > .type crc32do_while_dec, at function > crc32do_while_dec: > pushl %ebp > movl %esp, %ebp > pushl %esi > movl 8(%ebp), %edx > pushl %ebx > movl 16(%ebp), %ebx > movl 12(%ebp), %ecx > testl %ebx, %ebx > je .L15 > decl %ecx > movl $crc32_table, %esi > .p2align 4,,15 > .L16: > incl %ecx > movzbl (%ecx), %eax > xorb %dl, %al > shrl $8, %edx > movzbl %al, %eax > xorl (%esi,%eax,4), %edx > decl %ebx > jne .L16 > .L15: > popl %ebx > movl %edx, %eax > popl %esi > popl %ebp > ret > .Lfe3: > .size crc32do_while_dec,.Lfe3-crc32do_while_dec > .ident "GCC: (GNU) 3.2" > > > -----Original Message----- > From: Joakim Tjernlund [mailto:Joakim.Tjernlund at lumentis.se] > Sent: Wednesday, January 01, 2003 8:45 AM > To: linuxppc-embedded at lists.linuxppc.org > Subject: gcc optimizes loops badly. > > > > I have spent some time to optimize the crc32 function since JFFS2 uses > it heavily. I found that > gcc 2.95.3 optimizes loops badly, even gcc 2.96 RH produces better code > for x86 in some cases. > > So I optimized the C code a bit and got much better results. > Now I wounder how recent(>= 3.2) gcc performs. Could somebody run gcc -S > -O2 -mregnames on > functions below and mail me the results? > > Jocke > > These are different version of the same crc32 function: > #include <linux/types.h> > > extern const __u32 crc32_table[256]; > > /* Return a 32-bit CRC of the contents of the buffer. */ > > __u32 crc32org(__u32 val, const void *ss, unsigned int len) > { > const unsigned char *s = ss; > > while (len--){ > val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8); > } > return val; > } > __u32 crc32do_while(__u32 val, const void *ss, unsigned int len) > { > const unsigned char *s = ss; > > if(len){ > do { > val = crc32_table[(val ^ *s++) & 0xff] ^ (val >> 8); > } while (--len); > } > return val; > } > __u32 crc32do_while_dec(__u32 val, const void *ss, unsigned int len) > { > const unsigned char *s = ss; > > if(len){ > --s; > do { > val = crc32_table[(val ^ *(++s)) & 0xff] ^ (val >> 8); > } while (--len); > } > return val; > } > > and the resulting assembly: > .file "crc32.c" > gcc2_compiled.: > .section ".text" > .align 2 > .globl crc32org > .type crc32org, at function > crc32org: > cmpwi %cr0,%r5,0 > addi %r5,%r5,-1 > bclr 12,2 > lis %r9,crc32_table at ha > la %r10,crc32_table at l(%r9) > .L18: > lbz %r0,0(%r4) > cmpwi %cr0,%r5,0 > xor %r0,%r3,%r0 > rlwinm %r0,%r0,2,22,29 > lwzx %r11,%r10,%r0 > srwi %r9,%r3,8 > xor %r3,%r11,%r9 > addi %r4,%r4,1 > addi %r5,%r5,-1 > bc 4,2,.L18 > blr > .Lfe1: > .size crc32org,.Lfe1-crc32org > .align 2 > .globl crc32do_while > .type crc32do_while, at function > crc32do_while: > mr. %r0,%r5 > mtctr %r0 > bclr 12,2 > lis %r9,crc32_table at ha > la %r10,crc32_table at l(%r9) > .L25: > lbz %r0,0(%r4) > srwi %r11,%r3,8 > xor %r0,%r3,%r0 > rlwinm %r0,%r0,2,22,29 > lwzx %r9,%r10,%r0 > addi %r4,%r4,1 > xor %r3,%r9,%r11 > bdnz .L25 > blr > .Lfe2: > .size crc32do_while,.Lfe2-crc32do_while > .align 2 > .globl crc32do_while_dec > .type crc32do_while_dec, at function > crc32do_while_dec: > mr. %r0,%r5 > mtctr %r0 > bclr 12,2 > lis %r9,crc32_table at ha > la %r10,crc32_table at l(%r9) > addi %r4,%r4,-1 > .L31: > lbzu %r0,1(4) > srwi %r11,%r3,8 > xor %r0,%r3,%r0 > rlwinm %r0,%r0,2,22,29 > lwzx %r9,%r10,%r0 > xor %r3,%r9,%r11 > bdnz .L31 > blr > > ** Sent via the linuxppc-embedded mail list. 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