git: af366d353b84 - main - amd64: implement strlen in assembly
Mateusz Guzik
mjg at FreeBSD.org
Mon Feb 8 19:15:34 UTC 2021
The branch main has been updated by mjg:
URL: https://cgit.FreeBSD.org/src/commit/?id=af366d353b84bdc4e730f0fc563853abc338271c
commit af366d353b84bdc4e730f0fc563853abc338271c
Author: Mateusz Guzik <mjg at FreeBSD.org>
AuthorDate: 2021-02-08 17:01:48 +0000
Commit: Mateusz Guzik <mjg at FreeBSD.org>
CommitDate: 2021-02-08 19:15:21 +0000
amd64: implement strlen in assembly
The C variant in libkern performs excessive branching to find the
non-zero byte instead of using the bsfq instruction. The same code
patched to use it is still slower than the routine implemented here
as the compiler keeps neglecting to perform certain optimizations
(like using leaq).
On top of that the routine can is a starting point for copyinstr
which operates on words instead of bytes.
Tested with glibc test suite.
Sample results (calls/s):
Haswell:
$(perl -e "print 'A' x 3"):
stock: 211198039
patched:338626619
asm: 465609618
$(perl -e "print 'A' x 100"):
stock: 83151997
patched: 98285919
asm: 120719888
AMD EPYC 7R32:
$(perl -e "print 'A' x 3"):
stock: 282523617
asm: 491498172
$(perl -e "print 'A' x 100"):
stock: 114857172
asm: 112082057
---
sys/amd64/amd64/support.S | 66 +++++++++++++++++++++++++++++++++++++++++++++++
sys/conf/files | 1 -
sys/conf/files.arm | 1 +
sys/conf/files.arm64 | 1 +
sys/conf/files.i386 | 1 +
sys/conf/files.mips | 1 +
sys/conf/files.powerpc | 1 +
sys/conf/files.riscv | 1 +
8 files changed, 72 insertions(+), 1 deletion(-)
diff --git a/sys/amd64/amd64/support.S b/sys/amd64/amd64/support.S
index b623fba277db..994c5f15e245 100644
--- a/sys/amd64/amd64/support.S
+++ b/sys/amd64/amd64/support.S
@@ -697,6 +697,72 @@ ENTRY(fillw)
ret
END(fillw)
+/*
+ * strlen(string)
+ * %rdi
+ *
+ * Uses the ((x - 0x01....01) & ~x & 0x80....80) trick.
+ *
+ * 0x80....80 is replaced with 0 - 0x80....80 so that it can be added
+ * with leaq.
+ *
+ * For a description see either:
+ * - "Hacker's Delight" by Henry S. Warren, Jr.
+ * - "Optimizing subroutines in assembly language: An optimization guide for x86 platforms"
+ * by Agner Fog
+ *
+ * The latter contains a 32-bit variant of the same algorithm coded in assembly for i386.
+ */
+ENTRY(strlen)
+ PUSH_FRAME_POINTER
+ movabsq $0xfefefefefefefeff,%r8
+ movabsq $0x8080808080808080,%r9
+
+ movq %rdi,%r10
+ movq %rdi,%rcx
+ testb $7,%dil
+ jz 2f
+
+ /*
+ * Handle misaligned reads: align to 8 and fill
+ * the spurious bytes.
+ */
+ andq $~7,%rdi
+ movq (%rdi),%r11
+ shlq $3,%rcx
+ movq $-1,%rdx
+ shlq %cl,%rdx
+ notq %rdx
+ orq %rdx,%r11
+
+ leaq (%r11,%r8),%rcx
+ notq %r11
+ andq %r11,%rcx
+ andq %r9,%rcx
+ jnz 3f
+
+ /*
+ * Main loop.
+ */
+ ALIGN_TEXT
+1:
+ leaq 8(%rdi),%rdi
+2:
+ movq (%rdi),%r11
+ leaq (%r11,%r8),%rcx
+ notq %r11
+ andq %rcx,%r11
+ andq %r9,%rcx
+ jz 1b
+3:
+ bsfq %rcx,%rcx
+ shrq $3,%rcx
+ leaq (%rcx,%rdi),%rax
+ subq %r10,%rax
+ POP_FRAME_POINTER
+ ret
+END(strlen)
+
/*****************************************************************************/
/* copyout and fubyte family */
/*****************************************************************************/
diff --git a/sys/conf/files b/sys/conf/files
index edca1003e904..1abfadb1e8d8 100644
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -4085,7 +4085,6 @@ libkern/strdup.c standard
libkern/strndup.c standard
libkern/strlcat.c standard
libkern/strlcpy.c standard
-libkern/strlen.c standard
libkern/strncat.c standard
libkern/strncmp.c standard
libkern/strncpy.c standard
diff --git a/sys/conf/files.arm b/sys/conf/files.arm
index eb3a23b5fc21..69986585bdf6 100644
--- a/sys/conf/files.arm
+++ b/sys/conf/files.arm
@@ -127,6 +127,7 @@ libkern/lshrdi3.c standard
libkern/memcmp.c standard
libkern/moddi3.c standard
libkern/qdivrem.c standard
+libkern/strlen.c standard
libkern/ucmpdi2.c standard
libkern/udivdi3.c standard
libkern/umoddi3.c standard
diff --git a/sys/conf/files.arm64 b/sys/conf/files.arm64
index f7003b1048c8..42ec3b2787b1 100644
--- a/sys/conf/files.arm64
+++ b/sys/conf/files.arm64
@@ -432,6 +432,7 @@ libkern/memcmp.c standard \
compile-with "${NORMAL_C:N-fsanitize*}"
libkern/memset.c standard \
compile-with "${NORMAL_C:N-fsanitize*}"
+libkern/strlen.c standard
libkern/arm64/crc32c_armv8.S standard
cddl/dev/dtrace/aarch64/dtrace_asm.S optional dtrace compile-with "${DTRACE_S}"
cddl/dev/dtrace/aarch64/dtrace_subr.c optional dtrace compile-with "${DTRACE_C}"
diff --git a/sys/conf/files.i386 b/sys/conf/files.i386
index 6560ab217d96..1e2ab5f8c52a 100644
--- a/sys/conf/files.i386
+++ b/sys/conf/files.i386
@@ -218,6 +218,7 @@ libkern/memcmp.c standard
libkern/memset.c standard
libkern/moddi3.c standard
libkern/qdivrem.c standard
+libkern/strlen.c standard
libkern/ucmpdi2.c standard
libkern/udivdi3.c standard
libkern/umoddi3.c standard
diff --git a/sys/conf/files.mips b/sys/conf/files.mips
index c18f0a5c69be..7ee5b0019bd7 100644
--- a/sys/conf/files.mips
+++ b/sys/conf/files.mips
@@ -66,6 +66,7 @@ libkern/ucmpdi2.c optional mips | mipshf | mipsel | mipselhf
libkern/ashldi3.c standard
libkern/ashrdi3.c standard
libkern/memcmp.c standard
+libkern/strlen.c standard
# cfe support
dev/cfe/cfe_api.c optional cfe
diff --git a/sys/conf/files.powerpc b/sys/conf/files.powerpc
index 3022fd6f6e39..347abee153d2 100644
--- a/sys/conf/files.powerpc
+++ b/sys/conf/files.powerpc
@@ -129,6 +129,7 @@ libkern/memcmp.c standard
libkern/memset.c standard
libkern/moddi3.c optional powerpc | powerpcspe
libkern/qdivrem.c optional powerpc | powerpcspe
+libkern/strlen.c standard
libkern/ucmpdi2.c optional powerpc | powerpcspe
libkern/udivdi3.c optional powerpc | powerpcspe
libkern/umoddi3.c optional powerpc | powerpcspe
diff --git a/sys/conf/files.riscv b/sys/conf/files.riscv
index 3969528db07e..7ecea016b9a3 100644
--- a/sys/conf/files.riscv
+++ b/sys/conf/files.riscv
@@ -29,6 +29,7 @@ libkern/flsl.c standard
libkern/flsll.c standard
libkern/memcmp.c standard
libkern/memset.c standard
+libkern/strlen.c standard
riscv/riscv/autoconf.c standard
riscv/riscv/bus_machdep.c standard
riscv/riscv/bus_space_asm.S standard
More information about the dev-commits-src-main
mailing list