On Thu, 1 Nov 2007, Christoph Mallon wrote:
Andrey Chernov wrote:
On Tue, Oct 30, 2007 at 10:03:31AM -1000, Juli Mallett wrote:
* "Andrey A. Chernov" <[EMAIL PROTECTED]> [ 2007-10-27 ]
[ cvs commit: src/include _ctype.h ]
ache 2007-10-27 22:32:28 UTC
FreeBSD src repository
Modified files:
include _ctype.h Log:
Micro-optimization of prev. commit, change
(_c < 0 || _c >= 128) to (_c & ~0x7F)
Isn't that a non-optimization in code and a minor pessimization of
readability?
...
For ones who doubts there two tests compiled with -O2. As you may see the
result is almost identical (andl vs cmpl):
We never doubted that it was a small negative or non-optimization :-).
Look closer and you will see that the andl version takes 2 extra
instructions, since both versions are smart enough to avoid a branch,
and for this they need the result of the condition code generated by
the andl or cmpl, and the cmpl generates the desired condition code
directly while 2 more instructions are needed after the andl.
-------------------- a.c --------------------
main () {
int c;
return (c & ~0x7f) ? 0 : c * 2;
}
This example has many flaws as pointed out by Cristoph:
- c is uninitialized
- the result depends on c in a way that is quite different than the table
lookup for ctype. The above expression happens to be more optimizable.
-------------------- a.s --------------------
.file "a.c"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
movl %eax, %edx <--- extra instruction since andl
clobbers a register. Normally,
testl should be used to avoid
this clobber.
andl $-128, %edx <--- this sets %edx to something
and also sets the condition
codes, but not like we want
addl %eax, %eax <--- c * 2
cmpl $1, %edx <--- this sets the condition codes
like we want
sbbl %edx, %edx <--- turn condition codes into a
mask in %edx: mask = 0xffffffff
if the result should be c *2
and mask = 0 if the result should
be 0
pushl %ebp
andl %edx, %eax <--- result = (c * 2) & mask
movl %esp, %ebp <--- why is it bothering to set up
a frame this late?
pushl %ecx
popl %ecx
popl %ebp
leal -4(%ecx), %esp
ret
.size main, .-main
.ident "GCC: (GNU) 4.2.1 20070719 [FreeBSD]"
-------------------- a1.c --------------------
main () {
int c;
return (c < 0 || c >= 128) ? 0 : c * 2;
}
-------------------- a1.s --------------------
.file "a1.c"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
addl %eax, %eax
cmpl $128, %eax <--- cmpl puts result in condition
codes directly where we want it
sbbl %edx, %edx <--- same masking stuff ...
andl %edx, %eax
pushl %ebp
movl %esp, %ebp
pushl %ecx
popl %ecx
popl %ebp
leal -4(%ecx), %esp
ret
.size main, .-main
.ident "GCC: (GNU) 4.2.1 20070719 [FreeBSD]"
Your example is invalid. The value of c is undefined in this function and you
see random garbage as result (for example in the code snippet you see the c *
2 (addl %eax, %eax) and after that is the cmpl, which uses %eax, too). In
fact it would be perfectly legal for the compiler to always return 0, call
abort(), or let demons fly out of your nose.
However, the uninitialized c = %eax seems to be transformed correctly in
both cases. The first case even preserves %eax from the andl.
Also the example is still unrealistic: You usually don't multiply chars by
two. Lets try something more realistic: an ASCII filter
Indeed.
Bruce
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