From df7958a9606a342e3c3ac5a40fc41f3a79669d62 Mon Sep 17 00:00:00 2001 From: Denis Vlasenko Date: Tue, 15 Apr 2008 08:27:24 +0000 Subject: amd64 string ops: use alignment more carefully, and comment it. By capping max padding to not be bigger than three next insns, we avoid having ridiculously big NOPs like this one: 53:66 66 66 66 2e 0f 1f nopw %cs:0x0(%rax,%rax,1) 5a:84 00 00 00 00 00 which was bigger than next three insns combined! Size changes: text data bss dec hex filename 102 0 0 102 66 x86_64/memcpy.o 102 0 0 102 66 x86_64.old/memcpy.o 90 0 0 90 5a x86_64/mempcpy.o 102 0 0 102 66 x86_64.old/mempcpy.o 210 0 0 210 d2 x86_64/memset.o 242 0 0 242 f2 x86_64.old/memset.o 213 0 0 213 d5 x86_64/stpcpy.o 220 0 0 220 dc x86_64.old/stpcpy.o 428 0 0 428 1ac x86_64/strcat.o 444 0 0 444 1bc x86_64.old/strcat.o 417 0 0 417 1a1 x86_64/strchr.o 418 0 0 418 1a2 x86_64.old/strchr.o 33 0 0 33 21 x86_64/strcmp.o 33 0 0 33 21 x86_64.old/strcmp.o 213 0 0 213 d5 x86_64/strcpy.o 220 0 0 220 dc x86_64.old/strcpy.o 135 0 0 135 87 x86_64/strcspn.o 151 0 0 151 97 x86_64.old/strcspn.o 225 0 0 225 e1 x86_64/strlen.o 233 0 0 233 e9 x86_64.old/strlen.o 140 0 0 140 8c x86_64/strpbrk.o 156 0 0 156 9c x86_64.old/strpbrk.o 135 0 0 135 87 x86_64/strspn.o 151 0 0 151 97 x86_64.old/strspn.o Also, a few files got their .text alignment relaxed from 16 to 8 bytes, which reduces padding at link time. --- libc/string/x86_64/strspn.S | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) (limited to 'libc/string/x86_64/strspn.S') diff --git a/libc/string/x86_64/strspn.S b/libc/string/x86_64/strspn.S index 416424565..41cff0490 100644 --- a/libc/string/x86_64/strspn.S +++ b/libc/string/x86_64/strspn.S @@ -50,8 +50,10 @@ ENTRY (strspn) Although all the following instruction only modify %cl we always have a correct zero-extended 64-bit value in %rcx. */ - .p2align 4 -L(2): movb (%rax), %cl /* get byte from stopset */ + /* Next 3 insns are 6 bytes total, make sure we decode them in one go */ + .p2align 3,,6 +L(2): + movb (%rax), %cl /* get byte from stopset */ testb %cl, %cl /* is NUL char? */ jz L(1) /* yes => start compare loop */ movb %cl, (%rsp,%rcx) /* set corresponding byte in stopset table */ @@ -83,8 +85,14 @@ L(1): leaq -4(%rdx), %rax /* prepare loop */ value in the table. But the value of NUL is NUL so the loop terminates for NUL in every case. */ - .p2align 4 -L(3): addq $4, %rax /* adjust pointer for full loop round */ + /* Next 3 insns are 9 bytes total. */ + /* .p2align 4,,9 would make sure we decode them in one go, */ + /* but it will also align entire function to 16 bytes, */ + /* potentially creating largish padding at link time. */ + /* We are aligning to 8 bytes instead: */ + .p2align 3,,8 +L(3): + addq $4, %rax /* adjust pointer for full loop round */ movb (%rax), %cl /* get byte from string */ testb %cl, (%rsp,%rcx) /* is it contained in skipset? */ -- cgit v1.2.3