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util/bufferiszero: Improve scalar variant 

Split less-than and greater-than 256 cases.
Use unaligned accesses for head and tail.
Avoid using out-of-bounds pointers in loop boundary conditions.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2024-04-06 14:40:32 -10:00
parent f28e0bbefa
commit 7ae6399a85
1 changed files with 51 additions and 34 deletions
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85
util/bufferiszero.c
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@ -28,40 +28,57 @@
static bool (*buffer_is_zero_accel)(const void *, size_t);
static bool buffer_is_zero_integer(const void *buf, size_t len)
static bool buffer_is_zero_int_lt256(const void *buf, size_t len)
{
if (unlikely(len < 8)) {
/* For a very small buffer, simply accumulate all the bytes. */
const unsigned char *p = buf;
const unsigned char *e = buf + len;
unsigned char t = 0;
uint64_t t;
const uint64_t *p, *e;
do {
t |= *p++;
} while (p < e);
return t == 0;
} else {
/* Otherwise, use the unaligned memory access functions to
handle the beginning and end of the buffer, with a couple
of loops handling the middle aligned section. */
uint64_t t = ldq_he_p(buf);
const uint64_t *p = (uint64_t *)(((uintptr_t)buf + 8) & -8);
const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8);
for (; p + 8 <= e; p += 8) {
if (t) {
return false;
}
t = p[0] | p[1] | p[2] | p[3] | p[4] | p[5] | p[6] | p[7];
}
while (p < e) {
t |= *p++;
}
t |= ldq_he_p(buf + len - 8);
return t == 0;
/*
* Use unaligned memory access functions to handle
* the beginning and end of the buffer.
*/
if (unlikely(len <= 8)) {
return (ldl_he_p(buf) | ldl_he_p(buf + len - 4)) == 0;
}
t = ldq_he_p(buf) | ldq_he_p(buf + len - 8);
p = QEMU_ALIGN_PTR_DOWN(buf + 8, 8);
e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 8);
/* Read 0 to 31 aligned words from the middle. */
while (p < e) {
t |= *p++;
}
return t == 0;
}
static bool buffer_is_zero_int_ge256(const void *buf, size_t len)
{
/*
* Use unaligned memory access functions to handle
* the beginning and end of the buffer.
*/
uint64_t t = ldq_he_p(buf) | ldq_he_p(buf + len - 8);
const uint64_t *p = QEMU_ALIGN_PTR_DOWN(buf + 8, 8);
const uint64_t *e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 8);
/* Collect a partial block at the tail end. */
t |= e[-7] | e[-6] | e[-5] | e[-4] | e[-3] | e[-2] | e[-1];
/*
* Loop over 64 byte blocks.
* With the head and tail removed, e - p >= 30,
* so the loop must iterate at least 3 times.
*/
do {
if (t) {
return false;
}
t = p[0] | p[1] | p[2] | p[3] | p[4] | p[5] | p[6] | p[7];
p += 8;
} while (p < e - 7);
return t == 0;
}
#if defined(CONFIG_AVX2_OPT) || defined(__SSE2__)
@ -173,7 +190,7 @@ select_accel_cpuinfo(unsigned info)
{ CPUINFO_AVX2, buffer_zero_avx2 },
#endif
{ CPUINFO_SSE2, buffer_zero_sse2 },
{ CPUINFO_ALWAYS, buffer_is_zero_integer },
{ CPUINFO_ALWAYS, buffer_is_zero_int_ge256 },
};
for (unsigned i = 0; i < ARRAY_SIZE(all); ++i) {
@ -211,7 +228,7 @@ bool test_buffer_is_zero_next_accel(void)
return false;
}
#define INIT_ACCEL buffer_is_zero_integer
#define INIT_ACCEL buffer_is_zero_int_ge256
#endif
static bool (*buffer_is_zero_accel)(const void *, size_t) = INIT_ACCEL;
@ -232,7 +249,7 @@ bool buffer_is_zero_ool(const void *buf, size_t len)
if (likely(len >= 256)) {
return buffer_is_zero_accel(buf, len);
}
return buffer_is_zero_integer(buf, len);
return buffer_is_zero_int_lt256(buf, len);
}
bool buffer_is_zero_ge256(const void *buf, size_t len)