It appears that gcc is better at optimising a double call to min
and max rather than open coded min3 and max3. This can be observed
here:
$ cat min-max.c
#define min(x, y) ({ \
typeof(x) _min1 = (x); \
typeof(y) _min2 = (y); \
(void) (&_min1 == &_min2); \
_min1 < _min2 ? _min1 : _min2; })
#define min3(x, y, z) ({ \
typeof(x) _min1 = (x); \
typeof(y) _min2 = (y); \
typeof(z) _min3 = (z); \
(void) (&_min1 == &_min2); \
(void) (&_min1 == &_min3); \
_min1 < _min2 ? (_min1 < _min3 ? _min1 : _min3) : \
(_min2 < _min3 ? _min2 : _min3); })
int fmin3(int x, int y, int z) { return min3(x, y, z); }
int fmin2(int x, int y, int z) { return min(min(x, y), z); }
$ gcc -O2 -o min-max.s -S min-max.c; cat min-max.s
.file "min-max.c"
.text
.p2align 4,,15
.globl fmin3
.type fmin3, @function
fmin3:
.LFB0:
.cfi_startproc
cmpl %esi, %edi
jl .L5
cmpl %esi, %edx
movl %esi, %eax
cmovle %edx, %eax
ret
.p2align 4,,10
.p2align 3
.L5:
cmpl %edi, %edx
movl %edi, %eax
cmovle %edx, %eax
ret
.cfi_endproc
.LFE0:
.size fmin3, .-fmin3
.p2align 4,,15
.globl fmin2
.type fmin2, @function
fmin2:
.LFB1:
.cfi_startproc
cmpl %edi, %esi
movl %edx, %eax
cmovle %esi, %edi
cmpl %edx, %edi
cmovle %edi, %eax
ret
.cfi_endproc
.LFE1:
.size fmin2, .-fmin2
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits
fmin3 function, which uses open-coded min3 macro, is compiled into
total of ten instructions including a conditional branch, whereas fmin2
function, which uses two calls to min2 macro, is compiled into six
instructions with no branches.
Similarly, open-coded clamp produces the same code as clamp using min
and max macros, but the latter is much shorter:
$ cat clamp.c
#define clamp(val, min, max) ({ \
typeof(val) __val = (val); \
typeof(min) __min = (min); \
typeof(max) __max = (max); \
(void) (&__val == &__min); \
(void) (&__val == &__max); \
__val = __val < __min ? __min: __val; \
__val > __max ? __max: __val; })
#define min(x, y) ({ \
typeof(x) _min1 = (x); \
typeof(y) _min2 = (y); \
(void) (&_min1 == &_min2); \
_min1 < _min2 ? _min1 : _min2; })
#define max(x, y) ({ \
typeof(x) _max1 = (x); \
typeof(y) _max2 = (y); \
(void) (&_max1 == &_max2); \
_max1 > _max2 ? _max1 : _max2; })
int fclamp(int v, int min, int max) { return clamp(v, min, max); }
int fclampmm(int v, int min, int max) { return min(max(v, min), max); }
$ gcc -O2 -o clamp.s -S clamp.c; cat clamp.s
.file "clamp.c"
.text
.p2align 4,,15
.globl fclamp
.type fclamp, @function
fclamp:
.LFB0:
.cfi_startproc
cmpl %edi, %esi
movl %edx, %eax
cmovge %esi, %edi
cmpl %edx, %edi
cmovle %edi, %eax
ret
.cfi_endproc
.LFE0:
.size fclamp, .-fclamp
.p2align 4,,15
.globl fclampmm
.type fclampmm, @function
fclampmm:
.LFB1:
.cfi_startproc
cmpl %edi, %esi
cmovge %esi, %edi
cmpl %edi, %edx
movl %edi, %eax
cmovle %edx, %eax
ret
.cfi_endproc
.LFE1:
.size fclampmm, .-fclampmm
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits
Furthermore, after “make allmodconfig && make bzImage modules” this is the
comparison of image and modules sizes:
# Without this patch applied
$ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print size}'
350715800
# With this patch applied
$ ls -l arch/x86/boot/bzImage **/*.ko |awk '{size += $5} END {print size}'
349856528
The above builds were done on:
$ uname -a; gcc --version
Linux mpn-glaptop 3.13.0-29-generic #53~precise1-Ubuntu SMP Wed Jun 4
22:06:25 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Signed-off-by: Michal Nazarewicz <min...@mina86.com>
---
include/linux/kernel.h | 32 +++++---------------------------
1 file changed, 5 insertions(+), 27 deletions(-)
Interestingly commit [f27c85c56b: “add {min,max}3 macros”] claims
thatthe open-coded min3/max3 “will save some cycles as well as some
bytes on the stack”, but as far as I can see that statement is false.
But maybe there's something Hagen knows that I missed?
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 4c52907..44649e0 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -719,23 +719,8 @@ static inline void ftrace_dump(enum ftrace_dump_mode
oops_dump_mode) { }
(void) (&_max1 == &_max2); \
_max1 > _max2 ? _max1 : _max2; })
-#define min3(x, y, z) ({ \
- typeof(x) _min1 = (x); \
- typeof(y) _min2 = (y); \
- typeof(z) _min3 = (z); \
- (void) (&_min1 == &_min2); \
- (void) (&_min1 == &_min3); \
- _min1 < _min2 ? (_min1 < _min3 ? _min1 : _min3) : \
- (_min2 < _min3 ? _min2 : _min3); })
-
-#define max3(x, y, z) ({ \
- typeof(x) _max1 = (x); \
- typeof(y) _max2 = (y); \
- typeof(z) _max3 = (z); \
- (void) (&_max1 == &_max2); \
- (void) (&_max1 == &_max3); \
- _max1 > _max2 ? (_max1 > _max3 ? _max1 : _max3) : \
- (_max2 > _max3 ? _max2 : _max3); })
+#define min3(x, y, z) min(min(x, y), z)
+#define max3(x, y, z) max(max(x, y), z)
/**
* min_not_zero - return the minimum that is _not_ zero, unless both are zero
@@ -750,20 +735,13 @@ static inline void ftrace_dump(enum ftrace_dump_mode
oops_dump_mode) { }
/**
* clamp - return a value clamped to a given range with strict typechecking
* @val: current value
- * @min: minimum allowable value
- * @max: maximum allowable value
+ * @lo: lowest allowable value
+ * @hi: highest allowable value
*
* This macro does strict typechecking of min/max to make sure they are of the
* same type as val. See the unnecessary pointer comparisons.
*/
-#define clamp(val, min, max) ({ \
- typeof(val) __val = (val); \
- typeof(min) __min = (min); \
- typeof(max) __max = (max); \
- (void) (&__val == &__min); \
- (void) (&__val == &__max); \
- __val = __val < __min ? __min: __val; \
- __val > __max ? __max: __val; })
+#define clamp(val, lo, hi) min(max(val, lo), hi)
/*
* ..and if you can't take the strict
--
2.0.0.526.g5318336
--
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