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Subject[PATCH v-1] x86_64: new and improved memset() + question
Current memset() implementation does silly things:
* multiplication to get wide constant:
waste of cycles if filler is known at compile time,

* REP STOSQ followed by REP STOSB:
this code is used when REP STOSB is slow but still it is used
for small length (< 8) when setup overhead is relatively big,

* suboptimal calling convention:
REP STOSB/STOSQ favours (rdi, rcx)

* memset_orig():
it is hard to even look at it :^)

New implementation is based on the following observations:
* c == 0 is the most common form,
filler can be done with "xor eax, eax" and pushed into memset()
saving 2 bytes per call and multiplication

* len divisible by 8 is the most common form:
all it takes is one pointer or unsigned long inside structure,
dispatch at compile time to code without those ugly "lets fill
at most 7 bytes" tails,

* multiplication to get wider filler value can be done at compile time
for "c != 0" with 1 insn/10 bytes at most saving multiplication.

* those leaner forms of memset can be done withing 3/4 registers (RDI,
RCX, RAX, [RSI]) saving the rest from clobbering.

Note: "memset0" name is chosen because "bzero" is officially deprecated.
Note: memset(,0,) form is interleaved into memset(,c,) form to save
space.

QUESTION: is it possible to tell gcc "this function is semantically
equivalent to memset(3) so make high level optimizations but call it
when it is necessary"? I suspect the answer is "no" :-\

TODO:
CONFIG_FORTIFY_SOURCE is enabled by distros
benchmarks
testing
more comments
check with memset_io() so that no surprises pop up

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

lightly boot tested by you get the idea where it is going...

arch/x86/boot/compressed/Makefile | 1
arch/x86/include/asm/string_64.h | 101 ++++++++++++++++++++++++++++++++++++++
arch/x86/lib/Makefile | 1
arch/x86/lib/memset0_64.S | 86 ++++++++++++++++++++++++++++++++
4 files changed, 189 insertions(+)

--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -38,6 +38,7 @@ KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
KBUILD_CFLAGS += -Wno-pointer-sign
+KBUILD_CFLAGS += -D_ARCH_X86_BOOT

KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
GCOV_PROFILE := n
--- a/arch/x86/include/asm/string_64.h
+++ b/arch/x86/include/asm/string_64.h
@@ -30,7 +30,108 @@ extern void *__memcpy(void *to, const void *from, size_t len);
#endif /* !CONFIG_FORTIFY_SOURCE */

#define __HAVE_ARCH_MEMSET
+#if defined(_ARCH_X86_BOOT) || defined(CONFIG_FORTIFY_SOURCE)
void *memset(void *s, int c, size_t n);
+#else
+#include <asm/alternative.h>
+#include <asm/cpufeatures.h>
+
+/* Internal, do not use. */
+static __always_inline void memset0(void *s, size_t n)
+{
+ /* Internal, do not use. */
+ void _memset0_mov(void);
+ void _memset0_rep_stosq(void);
+ void memset0_mov(void);
+ void memset0_rep_stosq(void);
+ void memset0_rep_stosb(void);
+
+ if (__builtin_constant_p(n) && n == 0) {
+ } else if (__builtin_constant_p(n) && n == 1) {
+ *(uint8_t *)s = 0;
+ } else if (__builtin_constant_p(n) && n == 2) {
+ *(uint16_t *)s = 0;
+ } else if (__builtin_constant_p(n) && n == 4) {
+ *(uint32_t *)s = 0;
+ } else if (__builtin_constant_p(n) && n == 8) {
+ *(uint64_t *)s = 0;
+ } else if (__builtin_constant_p(n) && (n & 7) == 0) {
+ alternative_call_2(
+ _memset0_mov,
+ _memset0_rep_stosq, X86_FEATURE_REP_GOOD,
+ memset0_rep_stosb, X86_FEATURE_ERMS,
+ ASM_OUTPUT2("=D" (s), "=c" (n)),
+ "0" (s), "1" (n)
+ : "rax", "cc", "memory"
+ );
+ } else {
+ alternative_call_2(
+ memset0_mov,
+ memset0_rep_stosq, X86_FEATURE_REP_GOOD,
+ memset0_rep_stosb, X86_FEATURE_ERMS,
+ ASM_OUTPUT2("=D" (s), "=c" (n)),
+ "0" (s), "1" (n)
+ : "rax", "rsi", "cc", "memory"
+ );
+ }
+}
+
+/* Internal, do not use. */
+static __always_inline void memsetx(void *s, int c, size_t n)
+{
+ /* Internal, do not use. */
+ void _memsetx_mov(void);
+ void _memsetx_rep_stosq(void);
+ void memsetx_mov(void);
+ void memsetx_rep_stosq(void);
+ void memsetx_rep_stosb(void);
+
+ const uint64_t ccc = (uint8_t)c * 0x0101010101010101ULL;
+
+ if (__builtin_constant_p(n) && n == 0) {
+ } else if (__builtin_constant_p(n) && n == 1) {
+ *(uint8_t *)s = ccc;
+ } else if (__builtin_constant_p(n) && n == 2) {
+ *(uint16_t *)s = ccc;
+ } else if (__builtin_constant_p(n) && n == 4) {
+ *(uint32_t *)s = ccc;
+ } else if (__builtin_constant_p(n) && n == 8) {
+ *(uint64_t *)s = ccc;
+ } else if (__builtin_constant_p(n) && (n & 7) == 0) {
+ alternative_call_2(
+ _memsetx_mov,
+ _memsetx_rep_stosq, X86_FEATURE_REP_GOOD,
+ memsetx_rep_stosb, X86_FEATURE_ERMS,
+ ASM_OUTPUT2("=D" (s), "=c" (n)),
+ "0" (s), "1" (n), "a" (ccc)
+ : "cc", "memory"
+ );
+ } else {
+ alternative_call_2(
+ memsetx_mov,
+ memsetx_rep_stosq, X86_FEATURE_REP_GOOD,
+ memsetx_rep_stosb, X86_FEATURE_ERMS,
+ ASM_OUTPUT2("=D" (s), "=c" (n)),
+ "0" (s), "1" (n), "a" (ccc)
+ : "rsi", "cc", "memory"
+ );
+ }
+}
+
+static __always_inline void *memset(void *s, int c, size_t n)
+{
+ if (__builtin_constant_p(c)) {
+ if (c == 0) {
+ memset0(s, n);
+ } else {
+ memsetx(s, c, n);
+ }
+ return s;
+ } else {
+ return __builtin_memset(s, c, n);
+ }
+}
+#endif
void *__memset(void *s, int c, size_t n);

#define __HAVE_ARCH_MEMSET16
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -47,6 +47,7 @@ else
lib-y += csum-partial_64.o csum-copy_64.o csum-wrappers_64.o
lib-y += clear_page_64.o copy_page_64.o
lib-y += memmove_64.o memset_64.o
+ lib-y += memset0_64.o
lib-y += copy_user_64.o
lib-y += cmpxchg16b_emu.o
endif
new file mode 100644
--- /dev/null
+++ b/arch/x86/lib/memset0_64.S
@@ -0,0 +1,86 @@
+#include <linux/linkage.h>
+#include <asm/export.h>
+
+.intel_syntax noprefix
+
+ENTRY(memset0_rep_stosb)
+ xor eax, eax
+.globl memsetx_rep_stosb
+memsetx_rep_stosb:
+ rep stosb
+ ret
+ENDPROC(memset0_rep_stosb)
+ENDPROC(memsetx_rep_stosb)
+EXPORT_SYMBOL(memset0_rep_stosb)
+EXPORT_SYMBOL(memsetx_rep_stosb)
+
+ENTRY(_memset0_rep_stosq)
+ xor eax, eax
+.globl _memsetx_rep_stosq
+_memsetx_rep_stosq:
+ shr rcx, 3
+ rep stosq
+ ret
+ENDPROC(_memset0_rep_stosq)
+ENDPROC(_memsetx_rep_stosq)
+EXPORT_SYMBOL(_memset0_rep_stosq)
+EXPORT_SYMBOL(_memsetx_rep_stosq)
+
+ENTRY(memset0_rep_stosq)
+ xor eax, eax
+.globl memsetx_rep_stosq
+memsetx_rep_stosq:
+ lea rsi, [rdi + rcx]
+ shr rcx, 3
+ rep stosq
+ cmp rdi, rsi
+ je 1f
+2:
+ mov [rdi], al
+ add rdi, 1
+ cmp rdi, rsi
+ jne 2b
+1:
+ ret
+ENDPROC(memset0_rep_stosq)
+ENDPROC(memsetx_rep_stosq)
+EXPORT_SYMBOL(memset0_rep_stosq)
+EXPORT_SYMBOL(memsetx_rep_stosq)
+
+ENTRY(_memset0_mov)
+ xor eax, eax
+.globl _memsetx_mov
+_memsetx_mov:
+ add rcx, rdi
+ cmp rdi, rcx
+ je 1f
+2:
+ mov [rdi], rax
+ add rdi, 8
+ cmp rdi, rcx
+ jne 2b
+1:
+ ret
+ENDPROC(_memset0_mov)
+ENDPROC(_memsetx_mov)
+EXPORT_SYMBOL(_memset0_mov)
+EXPORT_SYMBOL(_memsetx_mov)
+
+ENTRY(memset0_mov)
+ xor eax, eax
+.globl memsetx_mov
+memsetx_mov:
+ lea rsi, [rdi + rcx]
+ cmp rdi, rsi
+ je 1f
+2:
+ mov [rdi], al
+ add rdi, 1
+ cmp rdi, rsi
+ jne 2b
+1:
+ ret
+ENDPROC(memset0_mov)
+ENDPROC(memsetx_mov)
+EXPORT_SYMBOL(memset0_mov)
+EXPORT_SYMBOL(memsetx_mov)
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