[PATCH] Remove some divide instructions
From: Zachary Amsden
Date: Wed Oct 27 2004 - 11:29:48 EST
I noticed several 64-bit divides for HZ/USER_HZ, and also the fact that
HZ == USER_HZ on many architectures (or if you play with scaling it ;).
Since do_div is macroized to optimized assembler on many platforms, we
emit long divides for divide by one. This could be extended further to
recognize other power of two divides, but I don't think the complexity
of the macros would be justified. I also didn't feel it was worthwhile
to optimize this for non-constant divides; if you feel otherwise, please
extend.
Cheers,
Zachary Amsden
zach@xxxxxxxxxx
diff -ru linux-2.6.10-rc1-nsz/include/asm-arm/div64.h linux-2.6.10-rc1/include/asm-arm/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-arm/div64.h 2004-10-25 10:53:12.000000000 -0700
+++ linux-2.6.10-rc1/include/asm-arm/div64.h 2004-10-27 08:29:36.000000000 -0700
@@ -28,6 +28,7 @@
#endif
#define do_div(n,base) \
+((__builtin_constant_p(base) && ((base) == 1)) ? 0 : \
({ \
register unsigned int __base asm("r4") = base; \
register unsigned long long __n asm("r0") = n; \
@@ -43,6 +44,6 @@
: "ip", "lr", "cc"); \
n = __res; \
__rem; \
-})
+}))
#endif
diff -ru linux-2.6.10-rc1-nsz/include/asm-generic/div64.h linux-2.6.10-rc1/include/asm-generic/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-generic/div64.h 2003-12-17 18:59:30.000000000 -0800
+++ linux-2.6.10-rc1/include/asm-generic/div64.h 2004-10-27 08:26:01.000000000 -0700
@@ -22,13 +22,14 @@
#if BITS_PER_LONG == 64
-# define do_div(n,base) ({ \
- uint32_t __base = (base); \
+# define do_div(n,base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
uint32_t __rem; \
+ uint32_t __base = (base); \
__rem = ((uint64_t)(n)) % __base; \
(n) = ((uint64_t)(n)) / __base; \
__rem; \
- })
+ }))
#elif BITS_PER_LONG == 32
@@ -37,17 +38,18 @@
/* The unnecessary pointer compare is there
* to check for type safety (n must be 64bit)
*/
-# define do_div(n,base) ({ \
- uint32_t __base = (base); \
- uint32_t __rem; \
- (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \
- if (likely(((n) >> 32) == 0)) { \
- __rem = (uint32_t)(n) % __base; \
- (n) = (uint32_t)(n) / __base; \
- } else \
- __rem = __div64_32(&(n), __base); \
- __rem; \
- })
+# define do_div(n,base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
+ uint32_t __base = (base); \
+ uint32_t __rem; \
+ (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \
+ if (likely(((n) >> 32) == 0)) { \
+ __rem = (uint32_t)(n) % __base; \
+ (n) = (uint32_t)(n) / __base; \
+ } else \
+ __rem = __div64_32(&(n), __base); \
+ __rem; \
+ }))
#else /* BITS_PER_LONG == ?? */
diff -ru linux-2.6.10-rc1-nsz/include/asm-i386/div64.h linux-2.6.10-rc1/include/asm-i386/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-i386/div64.h 2003-12-17 18:57:59.000000000 -0800
+++ linux-2.6.10-rc1/include/asm-i386/div64.h 2004-10-27 08:31:45.000000000 -0700
@@ -13,7 +13,8 @@
* This ends up being the most efficient "calling
* convention" on x86.
*/
-#define do_div(n,base) ({ \
+#define do_div(n,base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
unsigned long __upper, __low, __high, __mod, __base; \
__base = (base); \
asm("":"=a" (__low), "=d" (__high):"A" (n)); \
@@ -25,7 +26,7 @@
asm("divl %2":"=a" (__low), "=d" (__mod):"rm" (__base), "0" (__low), "1" (__upper)); \
asm("":"=A" (n):"a" (__low),"d" (__high)); \
__mod; \
-})
+}))
/*
* (long)X = ((long long)divs) / (long)div
diff -ru linux-2.6.10-rc1-nsz/include/asm-m32r/div64.h linux-2.6.10-rc1/include/asm-m32r/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-m32r/div64.h 2004-10-25 11:15:58.000000000 -0700
+++ linux-2.6.10-rc1/include/asm-m32r/div64.h 2004-10-27 08:21:53.000000000 -0700
@@ -12,6 +12,7 @@
* return value = n % base;
*/
#define do_div(n, base) \
+((__builtin_constant_p(base) && ((base) == 1)) ? 0 : \
({ \
unsigned long _res, _high, _mid, _low; \
\
@@ -33,6 +34,6 @@
n = (_low / (unsigned long)(base)); \
} \
_res; \
-})
+}))
#endif /* _ASM_M32R_DIV64 */
diff -ru linux-2.6.10-rc1-nsz/include/asm-m68k/div64.h linux-2.6.10-rc1/include/asm-m68k/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-m68k/div64.h 2003-12-17 18:59:37.000000000 -0800
+++ linux-2.6.10-rc1/include/asm-m68k/div64.h 2004-10-27 08:30:55.000000000 -0700
@@ -3,7 +3,8 @@
/* n = n / base; return rem; */
-#define do_div(n, base) ({ \
+#define do_div(n, base) ( \
+(__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
union { \
unsigned long n32[2]; \
unsigned long long n64; \
@@ -21,6 +22,6 @@
: "d" (base), "1" (__upper), "0" (__n.n32[1])); \
(n) = __n.n64; \
__rem; \
-})
+}))
#endif /* _M68K_DIV64_H */
diff -ru linux-2.6.10-rc1-nsz/include/asm-mips/div64.h linux-2.6.10-rc1/include/asm-mips/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-mips/div64.h 2003-12-17 18:59:06.000000000 -0800
+++ linux-2.6.10-rc1/include/asm-mips/div64.h 2004-10-27 08:25:29.000000000 -0700
@@ -51,7 +51,8 @@
(res) = __quot; \
__mod; })
-#define do_div(n, base) ({ \
+#define do_div(n, base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
unsigned long long __quot; \
unsigned long __mod; \
unsigned long long __div; \
@@ -74,7 +75,7 @@
__quot = __high; \
__quot = __quot << 32 | __low; \
(n) = __quot; \
- __mod; })
+ __mod; }))
#endif /* (_MIPS_SZLONG == 32) */
#if (_MIPS_SZLONG == 64)
@@ -104,7 +105,8 @@
* Hey, we're already 64-bit, no
* need to play games..
*/
-#define do_div(n, base) ({ \
+#define do_div(n, base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
unsigned long __quot; \
unsigned int __mod; \
unsigned long __div; \
@@ -117,7 +119,7 @@
__quot = __div / __base; \
\
(n) = __quot; \
- __mod; })
+ __mod; }))
#endif /* (_MIPS_SZLONG == 64) */
diff -ru linux-2.6.10-rc1-nsz/include/asm-s390/div64.h linux-2.6.10-rc1/include/asm-s390/div64.h
--- linux-2.6.10-rc1-nsz/include/asm-s390/div64.h 2004-10-25 10:50:43.000000000 -0700
+++ linux-2.6.10-rc1/include/asm-s390/div64.h 2004-10-27 08:27:34.000000000 -0700
@@ -4,7 +4,8 @@
#ifndef __s390x__
/* for do_div "base" needs to be smaller than 2^31-1 */
-#define do_div(n, base) ({ \
+#define do_div(n, base) ( \
+ (__builtin_constant_p(base) && ((base) == 1)) ? 0 : ({ \
unsigned long long __n = (n); \
unsigned long __r; \
\
@@ -40,7 +41,7 @@
: "d" (base), "m" (__n) : "0", "1", "2", "cc" ); \
(n) = (__n); \
__r; \
-})
+}))
#else /* __s390x__ */
#include <asm-generic/div64.h>
div64.patch :
Get rid of 64-bit constant divide by one. This appears to be a common case
for HZ == USER_HZ. I tested the patch on older 2.6 kernels and was able to
produce some harmless warnings (statement has no effect), but it builds clean
for i386 with a 2.6.10 kernel. I tested the generic asm inline by extracting
to gcc, but I have not tested any other platforms.
Doubtful, but if this breaks your build for some other platform, send me mail.
Zach Amsden
zach@xxxxxxxxxx