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[PATCH] rawmemchr: modernize and simplify
From: |
Paul Eggert |
Subject: |
[PATCH] rawmemchr: modernize and simplify |
Date: |
Fri, 20 Aug 2021 19:26:16 -0700 |
* lib/rawmemchr.c (HAVE_RAWMEMCHR): Assume it’s not defined;
otherwise this file would not be compiled. Include limits.h,
stdalign.h, stdint.h, verify.h.
(rawmemchr): Prefer uintptr_t to unsigned long and to size_t when
it’s the better type. Verify that longword lacks padding. Use
alignof rather than sizeof when checking alignment. Simplify by
assuming C99 decl-after-statement, and by using multiplication
rather than repeated shifting and OR (modern compilers can
optimize the multiplication if needed). Avoid unnecessary casts.
Don’t assume CHAR_WIDTH is 8. Convert back and forth between void *
to suppress bogus GCC warnings about alignment. Omit a
duplicate assignment to char_ptr.
* modules/rawmemchr (Depends-on): Add stdalign, stdint, verify.
---
ChangeLog | 17 ++++++++++
lib/rawmemchr.c | 79 +++++++++++++++++------------------------------
modules/rawmemchr | 3 ++
3 files changed, 49 insertions(+), 50 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index d42551ce1..bf6c9d66b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,20 @@
+2021-08-20 Paul Eggert <eggert@cs.ucla.edu>
+
+ rawmemchr: modernize and simplify
+ * lib/rawmemchr.c (HAVE_RAWMEMCHR): Assume it’s not defined;
+ otherwise this file would not be compiled. Include limits.h,
+ stdalign.h, stdint.h, verify.h.
+ (rawmemchr): Prefer uintptr_t to unsigned long and to size_t when
+ it’s the better type. Verify that longword lacks padding. Use
+ alignof rather than sizeof when checking alignment. Simplify by
+ assuming C99 decl-after-statement, and by using multiplication
+ rather than repeated shifting and OR (modern compilers can
+ optimize the multiplication if needed). Avoid unnecessary casts.
+ Don’t assume CHAR_WIDTH is 8. Convert back and forth between void *
+ to suppress bogus GCC warnings about alignment. Omit a
+ duplicate assignment to char_ptr.
+ * modules/rawmemchr (Depends-on): Add stdalign, stdint, verify.
+
2021-08-17 Paul Eggert <eggert@cs.ucla.edu>
c-stack: fix libsigsegv dependency
diff --git a/lib/rawmemchr.c b/lib/rawmemchr.c
index 896d435af..469bfa3cd 100644
--- a/lib/rawmemchr.c
+++ b/lib/rawmemchr.c
@@ -19,71 +19,54 @@
/* Specification. */
#include <string.h>
-/* A function definition is only needed if HAVE_RAWMEMCHR is not defined. */
-#if !HAVE_RAWMEMCHR
+#include <limits.h>
+#include <stdalign.h>
+#include <stdint.h>
+
+#include "verify.h"
/* Find the first occurrence of C in S. */
void *
rawmemchr (const void *s, int c_in)
{
- /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
- long instead of a 64-bit uintmax_t tends to give better
- performance. On 64-bit hardware, unsigned long is generally 64
- bits already. Change this typedef to experiment with
- performance. */
- typedef unsigned long int longword;
+ /* Change this typedef to experiment with performance. */
+ typedef uintptr_t longword;
+ /* If you change the "uintptr_t", you should change UINTPTR_WIDTH to match.
+ This verifies that the type does not have padding bits. */
+ verify (UINTPTR_WIDTH == UCHAR_WIDTH * sizeof (longword));
const unsigned char *char_ptr;
- const longword *longword_ptr;
- longword repeated_one;
- longword repeated_c;
- unsigned char c;
-
- c = (unsigned char) c_in;
+ unsigned char c = c_in;
/* 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;
- (size_t) char_ptr % sizeof (longword) != 0;
+ (uintptr_t) char_ptr % alignof (longword) != 0;
++char_ptr)
if (*char_ptr == c)
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. */
+ longword const *longword_ptr = s = char_ptr;
/* Compute auxiliary longword values:
repeated_one is a value which has a 1 in every byte.
repeated_c has c in every byte. */
- repeated_one = 0x01010101;
- repeated_c = c | (c << 8);
- repeated_c |= repeated_c << 16;
- if (0xffffffffU < (longword) -1)
- {
- repeated_one |= repeated_one << 31 << 1;
- repeated_c |= repeated_c << 31 << 1;
- if (8 < sizeof (longword))
- {
- size_t i;
-
- for (i = 64; i < sizeof (longword) * 8; i *= 2)
- {
- repeated_one |= repeated_one << i;
- repeated_c |= repeated_c << i;
- }
- }
- }
+ longword repeated_one = (longword) -1 / UCHAR_MAX;
+ longword repeated_c = repeated_one * c;
+ longword repeated_hibit = repeated_one * (UCHAR_MAX / 2 + 1);
/* 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 NUL or
+ test a longword at a time. The tricky part is testing if any of
+ the bytes in the longword in question are equal to
c. We first use an xor with repeated_c. This reduces the task
- to testing whether *any of the four* bytes in longword1 is zero.
+ to testing whether any of the bytes in longword1 is zero.
+
+ (The following comments assume 8-bit bytes, as POSIX requires;
+ the code's use of UCHAR_MAX should work even if bytes have more
+ than 8 bits.)
We compute tmp =
- ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
+ ((longword1 - repeated_one) & ~longword1) & (repeated_one * 0x80).
That is, we perform the following operations:
1. Subtract repeated_one.
2. & ~longword1.
@@ -117,25 +100,21 @@ rawmemchr (const void *s, int c_in)
{
longword longword1 = *longword_ptr ^ repeated_c;
- if ((((longword1 - repeated_one) & ~longword1)
- & (repeated_one << 7)) != 0)
+ if ((((longword1 - repeated_one) & ~longword1) & repeated_hibit) != 0)
break;
longword_ptr++;
}
- char_ptr = (const unsigned char *) longword_ptr;
+ char_ptr = s = longword_ptr;
/* At this point, we know that one of the sizeof (longword) bytes
- starting at char_ptr is == c. On little-endian machines, we
+ starting at char_ptr is == c. If we knew endianness, we
could determine the first such byte without any further memory
accesses, just by looking at the tmp result from the last loop
- iteration. But this does not work on big-endian machines.
- Choose code that works in both cases. */
+ iteration. However, the following simple and portable code does
+ not attempt this potential optimization. */
- char_ptr = (unsigned char *) longword_ptr;
while (*char_ptr != c)
char_ptr++;
return (void *) char_ptr;
}
-
-#endif
diff --git a/modules/rawmemchr b/modules/rawmemchr
index 32320568c..d30065e74 100644
--- a/modules/rawmemchr
+++ b/modules/rawmemchr
@@ -8,7 +8,10 @@ m4/rawmemchr.m4
Depends-on:
extensions
+stdalign
+stdint
string
+verify
configure.ac:
gl_FUNC_RAWMEMCHR
--
2.30.2
- [PATCH] rawmemchr: modernize and simplify,
Paul Eggert <=