Hi Eric,
> I've got an even more efficient implementation, inspired by
> http://www.cl.cam.ac.uk/~am21/progtricks.html
Very nice! And it has no "false positive" case, like the current memchr.c
implementation.
> Also, is anyone interested in making gnulib's memchr and strchrnul more
> efficient by copying the optimizations learned in memchr2?
That will definitely be useful, yes. But first, some polishing of the code.
Here is a proposed patch to fix a few things on the surface:
- The naming of variables. A variable 'magic_bits' tells the reader
"hey I'm clever, you can stay dumb". 'charmask1' is also a misnomer,
since the term "mask" designates values which operate bit by bit, i.e.
are only used with and/or/xor/not.
- Reduce the scope of the variables. Too many variables, declared at the
top of a function, reduce the legibility.
- The comment. "For all byte values except 0x00 and 0x80, subtracting 1
from the byte will leave the most significant bit unchanged." made me
think that values of c1+0x80 or c2+0x80 would lead to false positives.
- The comments. What is the effect of the carries induced by the subtraction
on the following & operations? This was completely unclear.
- Missing comment about why the last longword1, longword2 cannot be used
to determine the position of the first match.
- In the 'while' loop at the end, there is no need to decrement n when it's
already 0.
OK to apply?
2008-04-25 Bruno Haible <[EMAIL PROTECTED]>
* lib/memchr2.c (memchr2): Rename local variables. Add explanatory
comments.
*** lib/memchr2.c.orig 2008-04-26 04:55:50.000000000 +0200
--- lib/memchr2.c 2008-04-26 04:54:50.000000000 +0200
***************
*** 46,59 ****
const unsigned char *char_ptr;
const longword *longword_ptr;
! longword longword1;
! longword longword2;
! longword magic_bits;
! longword charmask1;
! longword charmask2;
unsigned char c1;
unsigned char c2;
- int i;
c1 = (unsigned char) c1_in;
c2 = (unsigned char) c2_in;
--- 46,56 ----
const unsigned char *char_ptr;
const longword *longword_ptr;
! longword repeated_one;
! longword repeated_c1;
! longword repeated_c2;
unsigned char c1;
unsigned char c2;
c1 = (unsigned char) c1_in;
c2 = (unsigned char) c2_in;
***************
*** 61,128 ****
if (c1 == c2)
return memchr (s, c1, n);
! /* Handle the first few characters by reading one character at a time.
Do this until CHAR_PTR is aligned on a longword boundary. */
for (char_ptr = (const unsigned char *) s;
! n > 0 && (size_t) char_ptr % sizeof longword1 != 0;
--n, ++char_ptr)
if (*char_ptr == c1 || *char_ptr == c2)
return (void *) char_ptr;
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to any size longwords. */
! longword_ptr = (const longword *) char_ptr;
! magic_bits = 0x01010101;
! charmask1 = c1 | (c1 << 8);
! charmask2 = c2 | (c2 << 8);
! charmask1 |= charmask1 << 16;
! charmask2 |= charmask2 << 16;
if (0xffffffffU < TYPE_MAXIMUM (longword))
{
! magic_bits |= magic_bits << 31 << 1;
! charmask1 |= charmask1 << 31 << 1;
! charmask2 |= charmask2 << 31 << 1;
! if (8 < sizeof longword1)
! for (i = 64; i < sizeof longword1 * 8; i *= 2)
! {
! magic_bits |= magic_bits << i;
! charmask1 |= charmask1 << i;
! charmask2 |= charmask2 << i;
! }
}
! /* Instead of the traditional loop which tests each character,
! we will test a longword at a time. The tricky part is testing
! if *any of the four* bytes in the longword in question are zero.
!
! We first use an xor to convert target bytes into a NUL byte,
! since the test for a zero byte is more efficient. For all byte
! values except 0x00 and 0x80, subtracting 1 from the byte will
! leave the most significant bit unchanged. So detecting 0 is
! simply a matter of subtracting from all bytes in parallel, and
! checking for a most significant bit that changed to 1. */
! while (n >= sizeof longword1)
{
! longword1 = *longword_ptr ^ charmask1;
! longword2 = *longword_ptr ^ charmask2;
! if (((((longword1 - magic_bits) & ~longword1)
! | ((longword2 - magic_bits) & ~longword2))
! & (magic_bits << 7)) != 0)
break;
longword_ptr++;
! n -= sizeof longword1;
}
char_ptr = (const unsigned char *) longword_ptr;
! while (n-- > 0)
{
if (*char_ptr == c1 || *char_ptr == c2)
return (void *) char_ptr;
- ++char_ptr;
}
return NULL;
--- 58,165 ----
if (c1 == c2)
return memchr (s, c1, n);
! /* Handle the first few bytes by reading one byte at a time.
Do this until CHAR_PTR is aligned on a longword boundary. */
for (char_ptr = (const unsigned char *) s;
! n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
--n, ++char_ptr)
if (*char_ptr == c1 || *char_ptr == c2)
return (void *) char_ptr;
+ longword_ptr = (const longword *) char_ptr;
+
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to any size longwords. */
! /* Compute auxiliary longword values:
! repeated_one is a value which has a 1 in every byte.
! repeated_c1 has a c1 in every byte.
! repeated_c2 has a c2 in every byte. */
! repeated_one = 0x01010101;
! repeated_c1 = c1 | (c1 << 8);
! repeated_c2 = c2 | (c2 << 8);
! repeated_c1 |= repeated_c1 << 16;
! repeated_c2 |= repeated_c2 << 16;
if (0xffffffffU < TYPE_MAXIMUM (longword))
{
! repeated_one |= repeated_one << 31 << 1;
! repeated_c1 |= repeated_c1 << 31 << 1;
! repeated_c2 |= repeated_c2 << 31 << 1;
! if (8 < sizeof (longword))
! {
! int i;
!
! for (i = 64; i < sizeof (longword) * 8; i *= 2)
! {
! repeated_one |= repeated_one << i;
! repeated_c1 |= repeated_c1 << i;
! repeated_c2 |= repeated_c2 << i;
! }
! }
}
! /* Instead of the traditional loop which tests each byte, we will test a
! longword at a time. The tricky part is testing if *any of the four*
! bytes in the longword in question are equal to c1 or c2. We first use
! an xor with repeated_c1 and repeated_c2, respectively. This reduces
! the task to testing whether *any of the four* bytes in longword1 or
! longword2 is zero.
!
! Let's consider longword1. We compute tmp1 =
! ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
! That is, we perform the following operations:
! 1. Subtract repeated_one.
! 2. & ~longword1.
! 3. & a mask consisting of 0x80 in every byte.
! Consider what happens in each byte:
! - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
! and step 3 transforms it into 0x80. A carry can also be propagated
! to more significant bytes.
! - If a byte of longword1 is nonzero, let its lowest 1 bit be at
! position k (0 <= k <= 7); so the lowest k bits are 0. After step 1,
! the byte ends in a single bit of value 0 and k bits of value 1.
! After step 2, the result is just k bits of value 1: 2^k - 1. After
! step 3, the result is 0. And no carry is produced.
! So, if longword1 has only non-zero bytes, tmp1 is zero.
! Whereas if longword1 has a zero byte, call j the position of the least
! significant zero byte. Then the result has a zero at positions 0, ...,
! j-1 and a 0x80 at position j. We cannot predict the result at the more
! significant bytes (positions j+1..3), but it does not matter since we
! already have a non-zero bit at position 8*j+7.
!
! Similary, we compute tmp2 =
! ((longword2 - repeated_one) & ~longword2) & (repeated_one << 7).
!
! The test whether any byte in longword1 or longword2 is zero is equivalent
! to testing whether tmp1 is nonzero or tmp2 is nonzero. We can combine
! this into a single test, whether (tmp1 | tmp2) is nonzero. */
! while (n >= sizeof (longword))
{
! longword longword1 = *longword_ptr ^ repeated_c1;
! longword longword2 = *longword_ptr ^ repeated_c2;
! if (((((longword1 - repeated_one) & ~longword1)
! | ((longword2 - repeated_one) & ~longword2))
! & (repeated_one << 7)) != 0)
break;
longword_ptr++;
! n -= sizeof (longword);
}
char_ptr = (const unsigned char *) longword_ptr;
! /* At this point, we know that either n < sizeof (longword), or one of the
! sizeof (longword) bytes starting at char_ptr is == c1 or == c2. On
! little-endian machines, we could determine the first such byte without
! any further memory accesses, just by looking at the (tmp1 | tmp2) result
! from the last loop iteration. But this does not work on big-endian
! machines. Choose a code that works in both cases. */
!
! for (; n > 0; char_ptr++, n--)
{
if (*char_ptr == c1 || *char_ptr == c2)
return (void *) char_ptr;
}
return NULL;
