x86, mem: memset_64.S: Optimize memset by enhanced REP MOVSB/STOSB

Support memset() with enhanced rep stosb. On processors supporting enhanced
REP MOVSB/STOSB, the alternative memset_c_e function using enhanced rep stosb
overrides the fast string alternative memset_c and the original function.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1305671358-14478-10-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
index 09d3442..79bd454 100644 (file)
--- a/arch/x86/lib/memset_64.S
+++ b/arch/x86/lib/memset_64.S
@@ -2,9 +2,13 @@
 
 #include <linux/linkage.h>
 #include <asm/dwarf2.h>
+#include <asm/cpufeature.h>
+#include <asm/alternative-asm.h>
 
 /*
- * ISO C memset - set a memory block to a byte value.
+ * ISO C memset - set a memory block to a byte value. This function uses fast
+ * string to get better performance than the original function. The code is
+ * simpler and shorter than the orignal function as well.
  *     
  * rdi   destination
  * rsi   value (char) 
@@ -31,6 +35,28 @@
 .Lmemset_e:
        .previous
 
+/*
+ * ISO C memset - set a memory block to a byte value. This function uses
+ * enhanced rep stosb to override the fast string function.
+ * The code is simpler and shorter than the fast string function as well.
+ *
+ * rdi   destination
+ * rsi   value (char)
+ * rdx   count (bytes)
+ *
+ * rax   original destination
+ */
+       .section .altinstr_replacement, "ax", @progbits
+.Lmemset_c_e:
+       movq %rdi,%r9
+       movb %sil,%al
+       movl %edx,%ecx
+       rep stosb
+       movq %r9,%rax
+       ret
+.Lmemset_e_e:
+       .previous
+
 ENTRY(memset)
 ENTRY(__memset)
        CFI_STARTPROC
@@ -112,16 +138,20 @@ ENTRY(__memset)
 ENDPROC(memset)
 ENDPROC(__memset)
 
-       /* Some CPUs run faster using the string instructions.
-          It is also a lot simpler. Use this when possible */
-
-#include <asm/cpufeature.h>
-
+       /* Some CPUs support enhanced REP MOVSB/STOSB feature.
+        * It is recommended to use this when possible.
+        *
+        * If enhanced REP MOVSB/STOSB feature is not available, use fast string
+        * instructions.
+        *
+        * Otherwise, use original memset function.
+        *
+        * In .altinstructions section, ERMS feature is placed after REG_GOOD
+         * feature to implement the right patch order.
+        */
        .section .altinstructions,"a"
-       .align 8
-       .quad memset
-       .quad .Lmemset_c
-       .word X86_FEATURE_REP_GOOD
-       .byte .Lfinal - memset
-       .byte .Lmemset_e - .Lmemset_c
+       altinstruction_entry memset,.Lmemset_c,X86_FEATURE_REP_GOOD,\
+                            .Lfinal-memset,.Lmemset_e-.Lmemset_c
+       altinstruction_entry memset,.Lmemset_c_e,X86_FEATURE_ERMS, \
+                            .Lfinal-memset,.Lmemset_e_e-.Lmemset_c_e
        .previous