Re: [PATCH v3] skip_prefix: rewrite so that prefix is scanned once

Duy Nguyen [off-list ref] writes:

On Tue, Mar 04, 2014 at 01:09:39AM +0100, David Kastrup wrote:

quoted

Duy Nguyen [off-list ref] writes:

quoted

On Tue, Mar 4, 2014 at 5:43 AM, Junio C Hamano [off-list ref] wrote:

quoted

diff --git a/git-compat-util.h b/git-compat-util.h
index cbd86c3..68ffaef 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h

@@ -357,8 +357,14 @@ extern int suffixcmp(const char *str, const char *suffix);

 static inline const char *skip_prefix(const char *str, const char *prefix)
 {
-       size_t len = strlen(prefix);
-       return strncmp(str, prefix, len) ? NULL : str + len;

Just a note. gcc does optimize strlen("abcdef") to 6, and with that
information at compile time built-in strncmp might do better.

Indeed, most (but not all) of the calls have a constant string as
prefix.  However, strncmp in each iteration checks for both *str as well
as *prefix to be different from '\0' independently (and it appears
unlikely to me that the optimizer will figure out that it's unnecessary
for either) _and_ compares them for equality so it's not likely to be
faster than the open-coded loop.

One could, however, use memcmp instead of strncmp.  I'm just not sure
whether memcmp is guaranteed not to peek beyond the first mismatching
byte even if the count would allow for more.  It could lead to undefined
behavior if the first mismatching byte would be the ending NUL byte of
str.

It turns out gcc does not generate a call to strncmp either. It
inlines repz cmpsb instead.

Oh wow.  So it _does_ know that it's not necessary to check for a NUL
byte when the length of one argument is already known.  I am seriously
impressed.

I recall we had a discussion long ago about the inefficiency of repz
and and open-coded loop is preferred,

I think that this mostly applies for Pentium I, possibly also the
dead-ended Pentium Pro architecture (that sort-of translated the x86
opcodes into RISC instructions).  I think that later processor variants
(and AMD anyway) got those instructions back to usable shape.

One thing where there was a _lot_ of performance difference between
open-coding and builtin was using memcpy (repz movb) for copying
well-aligned data bytewise rather than copying, say, integer arrays
element-wise.

But since that was egg-on-face material, the hardware got better at it.

And anyway, GCC should know what to pick here.  So with GCC being as
smart as that (using the equivalent of memcmp on its own initiative
instead of the full strncmp), I don't think we have a reasonable chance
to beat its performance with an open-coded variant.

produces this assembly with gcc -O2 (on x86, apparently)

-- 8< --
00000000 <main>:
   0:   55                      push   %ebp
   1:   b9 03 00 00 00          mov    $0x3,%ecx
   6:   89 e5                   mov    %esp,%ebp
   8:   57                      push   %edi
   9:   bf 00 00 00 00          mov    $0x0,%edi
   e:   56                      push   %esi
   f:   53                      push   %ebx
  10:   83 e4 f0                and    $0xfffffff0,%esp
  13:   83 ec 10                sub    $0x10,%esp
  16:   8b 45 0c                mov    0xc(%ebp),%eax
  19:   8b 40 04                mov    0x4(%eax),%eax
  1c:   89 c6                   mov    %eax,%esi
  1e:   f3 a6                   repz cmpsb %es:(%edi),%ds:(%esi)
  20:   0f 97 c3                seta   %bl
  23:   0f 92 c1                setb   %cl
  26:   83 c0 03                add    $0x3,%eax
  29:   31 d2                   xor    %edx,%edx
  2b:   38 cb                   cmp    %cl,%bl
  2d:   0f 44 d0                cmove  %eax,%edx

More like i686 than x86 here.

  30:   89 14 24                mov    %edx,(%esp)
  33:   e8 fc ff ff ff          call   34 <main+0x34>
  38:   8d 65 f4                lea    -0xc(%ebp),%esp
  3b:   31 c0                   xor    %eax,%eax
  3d:   5b                      pop    %ebx
  3e:   5e                      pop    %esi
  3f:   5f                      pop    %edi
  40:   5d                      pop    %ebp
  41:   c3                      ret

Well, we won't beat this here.

-- 
David Kastrup