On Fri, 25 Mar 2005, linux-os wrote:
> On Fri, 25 Mar 2005, Jesper Juhl wrote:
>
> > (please keep me on CC)
> >
> >
> > checking for NULL before calling kfree() is redundant and needlessly
> > enlarges the kernel image, let's get rid of those checks.
> >
>
> Hardly. ORing a value with itself and jumping on condition is
> real cheap compared with pushing a value into the stack and
> calling a function. This is NOT a good change if you want
> performance. You really should reconsider this activity. It
> is quite counter-productive.
>
>
Let's have a look - at fs/nfsd/export.o for example.
Size first.
Without my patch :
$ ls -l fs/nfsd/export.o
-rw-r--r-- 1 juhl users 97144 2005-03-25 23:58 fs/nfsd/export.o
With my patch :
$ ls -l fs/nfsd/export.o
-rw-r--r-- 1 juhl users 97092 2005-03-25 23:59 fs/nfsd/export.o
That's our first bennefit - 52 bytes saved - not much, but still some.
Now let's take a look at the code gcc generates for one of the functions -
expkey_parse for example. Here's a diff -u of an objdump -D of export.o
cut down to just the bit for expkey_parse :
--- func-without-patch 2005-03-26 00:02:47.000000000 +0100
+++ func-with-patch 2005-03-26 00:03:26.000000000 +0100
@@ -8,7 +8,7 @@
13b: 81 ec d0 00 00 00 sub $0xd0,%esp
141: 89 95 40 ff ff ff mov %edx,0xffffff40(%ebp)
147: 80 7c 0a ff 0a cmpb $0xa,0xffffffff(%edx,%ecx,1)
- 14c: 0f 85 2b 02 00 00 jne 37d <expkey_parse+0x24d>
+ 14c: 0f 85 27 02 00 00 jne 379 <expkey_parse+0x249>
152: c6 44 0a ff 00 movb $0x0,0xffffffff(%edx,%ecx,1)
157: a1 64 00 00 00 mov 0x64,%eax
15c: ba d0 00 00 00 mov $0xd0,%edx
@@ -17,7 +17,7 @@
168: 89 c3 mov %eax,%ebx
16a: c7 85 30 ff ff ff f4 movl $0xfffffff4,0xffffff30(%ebp)
171: ff ff ff
- 174: 0f 84 fd 01 00 00 je 377 <expkey_parse+0x247>
+ 174: 0f 84 f2 01 00 00 je 36c <expkey_parse+0x23c>
17a: 89 c2 mov %eax,%edx
17c: 8d 85 40 ff ff ff lea 0xffffff40(%ebp),%eax
182: b9 00 10 00 00 mov $0x1000,%ecx
@@ -52,7 +52,7 @@
208: 80 38 00 cmpb $0x0,(%eax)
20b: 0f 85 46 01 00 00 jne 357 <expkey_parse+0x227>
211: f6 05 00 00 00 00 04 testb $0x4,0x0
- 218: 0f 85 6a 01 00 00 jne 388 <expkey_parse+0x258>
+ 218: 0f 85 66 01 00 00 jne 384 <expkey_parse+0x254>
21e: 83 ff 02 cmp $0x2,%edi
221: 0f 8f 30 01 00 00 jg 357 <expkey_parse+0x227>
227: 8d 85 40 ff ff ff lea 0xffffff40(%ebp),%eax
@@ -134,22 +134,20 @@
35f: 74 0b je 36c <expkey_parse+0x23c>
361: 8b 85 34 ff ff ff mov 0xffffff34(%ebp),%eax
367: e8 fc ff ff ff call 368 <expkey_parse+0x238>
- 36c: 85 db test %ebx,%ebx
- 36e: 74 07 je 377 <expkey_parse+0x247>
- 370: 89 d8 mov %ebx,%eax
- 372: e8 fc ff ff ff call 373 <expkey_parse+0x243>
- 377: 8b 85 30 ff ff ff mov 0xffffff30(%ebp),%eax
- 37d: 81 c4 d0 00 00 00 add $0xd0,%esp
- 383: 5b pop %ebx
- 384: 5e pop %esi
- 385: 5f pop %edi
- 386: c9 leave
- 387: c3 ret
- 388: 89 7c 24 04 mov %edi,0x4(%esp)
- 38c: c7 04 24 03 00 00 00 movl $0x3,(%esp)
- 393: e8 fc ff ff ff call 394 <expkey_parse+0x264>
- 398: e9 81 fe ff ff jmp 21e <expkey_parse+0xee>
- 39d: 8d 76 00 lea 0x0(%esi),%esi
+ 36c: 89 d8 mov %ebx,%eax
+ 36e: e8 fc ff ff ff call 36f <expkey_parse+0x23f>
+ 373: 8b 85 30 ff ff ff mov 0xffffff30(%ebp),%eax
+ 379: 81 c4 d0 00 00 00 add $0xd0,%esp
+ 37f: 5b pop %ebx
+ 380: 5e pop %esi
+ 381: 5f pop %edi
+ 382: c9 leave
+ 383: c3 ret
+ 384: 89 7c 24 04 mov %edi,0x4(%esp)
+ 388: c7 04 24 03 00 00 00 movl $0x3,(%esp)
+ 38f: e8 fc ff ff ff call 390 <expkey_parse+0x260>
+ 394: e9 85 fe ff ff jmp 21e <expkey_parse+0xee>
+ 399: 8d b4 26 00 00 00 00 lea 0x0(%esi),%esi
3a0: 89 5c 24 04 mov %ebx,0x4(%esp)
3a4: c7 04 24 16 00 00 00 movl $0x16,(%esp)
3ab: e8 fc ff ff ff call 3ac <expkey_parse+0x27c>
This is not too bad, but I've seen a lot worse, see this one for a gross
example : http://www.ussg.iu.edu/hypermail/linux/kernel/0503.2/1050.html
--
Jesper Juhl
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