[PATCH v-1] x86_64: new and improved memset() + question

[Alexey Dobriyan]

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@xxxxxxxxx>
---

	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)