From 8a917b99d5394d34ffcd851c8b287ced6eb48133 Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Tue, 6 Feb 2024 23:48:04 +0300 Subject: [PATCH 01/10] util/bufferiszero: Remove SSE4.1 variant The SSE4.1 variant is virtually identical to the SSE2 variant, except for using 'PTEST+JNZ' in place of 'PCMPEQB+PMOVMSKB+CMP+JNE' for testing if an SSE register is all zeroes. The PTEST instruction decodes to two uops, so it can be handled only by the complex decoder, and since CMP+JNE are macro-fused, both sequences decode to three uops. The uops comprising the PTEST instruction dispatch to p0 and p5 on Intel CPUs, so PCMPEQB+PMOVMSKB is comparatively more flexible from dispatch standpoint. Hence, the use of PTEST brings no benefit from throughput standpoint. Its latency is not important, since it feeds only a conditional jump, which terminates the dependency chain. I never observed PTEST variants to be faster on real hardware. Signed-off-by: Alexander Monakov Signed-off-by: Mikhail Romanov Reviewed-by: Richard Henderson Message-Id: <20240206204809.9859-2-amonakov@ispras.ru> --- util/bufferiszero.c | 29 ----------------------------- 1 file changed, 29 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 3e6a5dfd63..f5a3634f9a 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -100,34 +100,6 @@ buffer_zero_sse2(const void *buf, size_t len) } #ifdef CONFIG_AVX2_OPT -static bool __attribute__((target("sse4"))) -buffer_zero_sse4(const void *buf, size_t len) -{ - __m128i t = _mm_loadu_si128(buf); - __m128i *p = (__m128i *)(((uintptr_t)buf + 5 * 16) & -16); - __m128i *e = (__m128i *)(((uintptr_t)buf + len) & -16); - - /* Loop over 16-byte aligned blocks of 64. */ - while (likely(p <= e)) { - __builtin_prefetch(p); - if (unlikely(!_mm_testz_si128(t, t))) { - return false; - } - t = p[-4] | p[-3] | p[-2] | p[-1]; - p += 4; - } - - /* Finish the aligned tail. */ - t |= e[-3]; - t |= e[-2]; - t |= e[-1]; - - /* Finish the unaligned tail. */ - t |= _mm_loadu_si128(buf + len - 16); - - return _mm_testz_si128(t, t); -} - static bool __attribute__((target("avx2"))) buffer_zero_avx2(const void *buf, size_t len) { @@ -221,7 +193,6 @@ select_accel_cpuinfo(unsigned info) #endif #ifdef CONFIG_AVX2_OPT { CPUINFO_AVX2, 128, buffer_zero_avx2 }, - { CPUINFO_SSE4, 64, buffer_zero_sse4 }, #endif { CPUINFO_SSE2, 64, buffer_zero_sse2 }, { CPUINFO_ALWAYS, 0, buffer_zero_int }, From d018425c324704949c7f65230def9586e71f07f5 Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Wed, 14 Feb 2024 10:34:24 -1000 Subject: [PATCH 02/10] util/bufferiszero: Remove AVX512 variant Thanks to early checks in the inline buffer_is_zero wrapper, the SIMD routines are invoked much more rarely in normal use when most buffers are non-zero. This makes use of AVX512 unprofitable, as it incurs extra frequency and voltage transition periods during which the CPU operates at reduced performance, as described in https://travisdowns.github.io/blog/2020/01/17/avxfreq1.html Signed-off-by: Mikhail Romanov Signed-off-by: Alexander Monakov Reviewed-by: Richard Henderson Message-Id: <20240206204809.9859-4-amonakov@ispras.ru> Signed-off-by: Richard Henderson --- util/bufferiszero.c | 38 +++----------------------------------- 1 file changed, 3 insertions(+), 35 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index f5a3634f9a..641d5f9b9e 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -64,7 +64,7 @@ buffer_zero_int(const void *buf, size_t len) } } -#if defined(CONFIG_AVX512F_OPT) || defined(CONFIG_AVX2_OPT) || defined(__SSE2__) +#if defined(CONFIG_AVX2_OPT) || defined(__SSE2__) #include /* Note that each of these vectorized functions require len >= 64. */ @@ -128,41 +128,12 @@ buffer_zero_avx2(const void *buf, size_t len) } #endif /* CONFIG_AVX2_OPT */ -#ifdef CONFIG_AVX512F_OPT -static bool __attribute__((target("avx512f"))) -buffer_zero_avx512(const void *buf, size_t len) -{ - /* Begin with an unaligned head of 64 bytes. */ - __m512i t = _mm512_loadu_si512(buf); - __m512i *p = (__m512i *)(((uintptr_t)buf + 5 * 64) & -64); - __m512i *e = (__m512i *)(((uintptr_t)buf + len) & -64); - - /* Loop over 64-byte aligned blocks of 256. */ - while (p <= e) { - __builtin_prefetch(p); - if (unlikely(_mm512_test_epi64_mask(t, t))) { - return false; - } - t = p[-4] | p[-3] | p[-2] | p[-1]; - p += 4; - } - - t |= _mm512_loadu_si512(buf + len - 4 * 64); - t |= _mm512_loadu_si512(buf + len - 3 * 64); - t |= _mm512_loadu_si512(buf + len - 2 * 64); - t |= _mm512_loadu_si512(buf + len - 1 * 64); - - return !_mm512_test_epi64_mask(t, t); - -} -#endif /* CONFIG_AVX512F_OPT */ - /* * Make sure that these variables are appropriately initialized when * SSE2 is enabled on the compiler command-line, but the compiler is * too old to support CONFIG_AVX2_OPT. */ -#if defined(CONFIG_AVX512F_OPT) || defined(CONFIG_AVX2_OPT) +#if defined(CONFIG_AVX2_OPT) # define INIT_USED 0 # define INIT_LENGTH 0 # define INIT_ACCEL buffer_zero_int @@ -188,9 +159,6 @@ select_accel_cpuinfo(unsigned info) unsigned len; bool (*fn)(const void *, size_t); } all[] = { -#ifdef CONFIG_AVX512F_OPT - { CPUINFO_AVX512F, 256, buffer_zero_avx512 }, -#endif #ifdef CONFIG_AVX2_OPT { CPUINFO_AVX2, 128, buffer_zero_avx2 }, #endif @@ -208,7 +176,7 @@ select_accel_cpuinfo(unsigned info) return 0; } -#if defined(CONFIG_AVX512F_OPT) || defined(CONFIG_AVX2_OPT) +#if defined(CONFIG_AVX2_OPT) static void __attribute__((constructor)) init_accel(void) { used_accel = select_accel_cpuinfo(cpuinfo_init()); From cbe3d5264631aa193fd2705820cbde6c5a602abb Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Tue, 6 Feb 2024 23:48:05 +0300 Subject: [PATCH 03/10] util/bufferiszero: Reorganize for early test for acceleration Test for length >= 256 inline, where is is often a constant. Before calling into the accelerated routine, sample three bytes from the buffer, which handles most non-zero buffers. Signed-off-by: Alexander Monakov Signed-off-by: Mikhail Romanov Message-Id: <20240206204809.9859-3-amonakov@ispras.ru> [rth: Use __builtin_constant_p; move the indirect call out of line.] Signed-off-by: Richard Henderson --- include/qemu/cutils.h | 32 ++++++++++++++++- util/bufferiszero.c | 84 +++++++++++++++++-------------------------- 2 files changed, 63 insertions(+), 53 deletions(-) diff --git a/include/qemu/cutils.h b/include/qemu/cutils.h index 92c927a6a3..741dade7cf 100644 --- a/include/qemu/cutils.h +++ b/include/qemu/cutils.h @@ -187,9 +187,39 @@ char *freq_to_str(uint64_t freq_hz); /* used to print char* safely */ #define STR_OR_NULL(str) ((str) ? (str) : "null") -bool buffer_is_zero(const void *buf, size_t len); +/* + * Check if a buffer is all zeroes. + */ + +bool buffer_is_zero_ool(const void *vbuf, size_t len); +bool buffer_is_zero_ge256(const void *vbuf, size_t len); bool test_buffer_is_zero_next_accel(void); +static inline bool buffer_is_zero_sample3(const char *buf, size_t len) +{ + /* + * For any reasonably sized buffer, these three samples come from + * three different cachelines. In qemu-img usage, we find that + * each byte eliminates more than half of all buffer testing. + * It is therefore critical to performance that the byte tests + * short-circuit, so that we do not pull in additional cache lines. + * Do not "optimize" this to !(a | b | c). + */ + return !buf[0] && !buf[len - 1] && !buf[len / 2]; +} + +#ifdef __OPTIMIZE__ +static inline bool buffer_is_zero(const void *buf, size_t len) +{ + return (__builtin_constant_p(len) && len >= 256 + ? buffer_is_zero_sample3(buf, len) && + buffer_is_zero_ge256(buf, len) + : buffer_is_zero_ool(buf, len)); +} +#else +#define buffer_is_zero buffer_is_zero_ool +#endif + /* * Implementation of ULEB128 (http://en.wikipedia.org/wiki/LEB128) * Input is limited to 14-bit numbers diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 641d5f9b9e..972f394cbd 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -26,8 +26,9 @@ #include "qemu/bswap.h" #include "host/cpuinfo.h" -static bool -buffer_zero_int(const void *buf, size_t len) +static bool (*buffer_is_zero_accel)(const void *, size_t); + +static bool buffer_is_zero_integer(const void *buf, size_t len) { if (unlikely(len < 8)) { /* For a very small buffer, simply accumulate all the bytes. */ @@ -128,60 +129,38 @@ buffer_zero_avx2(const void *buf, size_t len) } #endif /* CONFIG_AVX2_OPT */ -/* - * Make sure that these variables are appropriately initialized when - * SSE2 is enabled on the compiler command-line, but the compiler is - * too old to support CONFIG_AVX2_OPT. - */ -#if defined(CONFIG_AVX2_OPT) -# define INIT_USED 0 -# define INIT_LENGTH 0 -# define INIT_ACCEL buffer_zero_int -#else -# ifndef __SSE2__ -# error "ISA selection confusion" -# endif -# define INIT_USED CPUINFO_SSE2 -# define INIT_LENGTH 64 -# define INIT_ACCEL buffer_zero_sse2 -#endif - -static unsigned used_accel = INIT_USED; -static unsigned length_to_accel = INIT_LENGTH; -static bool (*buffer_accel)(const void *, size_t) = INIT_ACCEL; - static unsigned __attribute__((noinline)) select_accel_cpuinfo(unsigned info) { /* Array is sorted in order of algorithm preference. */ static const struct { unsigned bit; - unsigned len; bool (*fn)(const void *, size_t); } all[] = { #ifdef CONFIG_AVX2_OPT - { CPUINFO_AVX2, 128, buffer_zero_avx2 }, + { CPUINFO_AVX2, buffer_zero_avx2 }, #endif - { CPUINFO_SSE2, 64, buffer_zero_sse2 }, - { CPUINFO_ALWAYS, 0, buffer_zero_int }, + { CPUINFO_SSE2, buffer_zero_sse2 }, + { CPUINFO_ALWAYS, buffer_is_zero_integer }, }; for (unsigned i = 0; i < ARRAY_SIZE(all); ++i) { if (info & all[i].bit) { - length_to_accel = all[i].len; - buffer_accel = all[i].fn; + buffer_is_zero_accel = all[i].fn; return all[i].bit; } } return 0; } -#if defined(CONFIG_AVX2_OPT) +static unsigned used_accel; + static void __attribute__((constructor)) init_accel(void) { used_accel = select_accel_cpuinfo(cpuinfo_init()); } -#endif /* CONFIG_AVX2_OPT */ + +#define INIT_ACCEL NULL bool test_buffer_is_zero_next_accel(void) { @@ -194,36 +173,37 @@ bool test_buffer_is_zero_next_accel(void) used_accel |= used; return used; } - -static bool select_accel_fn(const void *buf, size_t len) -{ - if (likely(len >= length_to_accel)) { - return buffer_accel(buf, len); - } - return buffer_zero_int(buf, len); -} - #else -#define select_accel_fn buffer_zero_int bool test_buffer_is_zero_next_accel(void) { return false; } + +#define INIT_ACCEL buffer_is_zero_integer #endif -/* - * Checks if a buffer is all zeroes - */ -bool buffer_is_zero(const void *buf, size_t len) +static bool (*buffer_is_zero_accel)(const void *, size_t) = INIT_ACCEL; + +bool buffer_is_zero_ool(const void *buf, size_t len) { if (unlikely(len == 0)) { return true; } + if (!buffer_is_zero_sample3(buf, len)) { + return false; + } + /* All bytes are covered for any len <= 3. */ + if (unlikely(len <= 3)) { + return true; + } - /* Fetch the beginning of the buffer while we select the accelerator. */ - __builtin_prefetch(buf); - - /* Use an optimized zero check if possible. Note that this also - includes a check for an unrolled loop over 64-bit integers. */ - return select_accel_fn(buf, len); + if (likely(len >= 256)) { + return buffer_is_zero_accel(buf, len); + } + return buffer_is_zero_integer(buf, len); +} + +bool buffer_is_zero_ge256(const void *buf, size_t len) +{ + return buffer_is_zero_accel(buf, len); } From 93a6085618f16fb2cd316d1e84f1a638b7e2d8ff Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Tue, 6 Feb 2024 23:48:07 +0300 Subject: [PATCH 04/10] util/bufferiszero: Remove useless prefetches Use of prefetching in bufferiszero.c is quite questionable: - prefetches are issued just a few CPU cycles before the corresponding line would be hit by demand loads; - they are done for simple access patterns, i.e. where hardware prefetchers can perform better; - they compete for load ports in loops that should be limited by load port throughput rather than ALU throughput. Signed-off-by: Alexander Monakov Signed-off-by: Mikhail Romanov Reviewed-by: Richard Henderson Message-Id: <20240206204809.9859-5-amonakov@ispras.ru> --- util/bufferiszero.c | 3 --- 1 file changed, 3 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 972f394cbd..00118d649e 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -50,7 +50,6 @@ static bool buffer_is_zero_integer(const void *buf, size_t len) const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8); for (; p + 8 <= e; p += 8) { - __builtin_prefetch(p + 8); if (t) { return false; } @@ -80,7 +79,6 @@ buffer_zero_sse2(const void *buf, size_t len) /* Loop over 16-byte aligned blocks of 64. */ while (likely(p <= e)) { - __builtin_prefetch(p); t = _mm_cmpeq_epi8(t, zero); if (unlikely(_mm_movemask_epi8(t) != 0xFFFF)) { return false; @@ -111,7 +109,6 @@ buffer_zero_avx2(const void *buf, size_t len) /* Loop over 32-byte aligned blocks of 128. */ while (p <= e) { - __builtin_prefetch(p); if (unlikely(!_mm256_testz_si256(t, t))) { return false; } From f28e0bbefa41fe643cce2f107e868abff312ced9 Mon Sep 17 00:00:00 2001 From: Alexander Monakov Date: Tue, 6 Feb 2024 23:48:08 +0300 Subject: [PATCH 05/10] util/bufferiszero: Optimize SSE2 and AVX2 variants Increase unroll factor in SIMD loops from 4x to 8x in order to move their bottlenecks from ALU port contention to load issue rate (two loads per cycle on popular x86 implementations). Avoid using out-of-bounds pointers in loop boundary conditions. Follow SSE2 implementation strategy in the AVX2 variant. Avoid use of PTEST, which is not profitable there (like in the removed SSE4 variant). Signed-off-by: Alexander Monakov Signed-off-by: Mikhail Romanov Reviewed-by: Richard Henderson Message-Id: <20240206204809.9859-6-amonakov@ispras.ru> --- util/bufferiszero.c | 111 +++++++++++++++++++++++++++++--------------- 1 file changed, 73 insertions(+), 38 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 00118d649e..02df82b4ff 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -67,62 +67,97 @@ static bool buffer_is_zero_integer(const void *buf, size_t len) #if defined(CONFIG_AVX2_OPT) || defined(__SSE2__) #include -/* Note that each of these vectorized functions require len >= 64. */ +/* Helper for preventing the compiler from reassociating + chains of binary vector operations. */ +#define SSE_REASSOC_BARRIER(vec0, vec1) asm("" : "+x"(vec0), "+x"(vec1)) + +/* Note that these vectorized functions may assume len >= 256. */ static bool __attribute__((target("sse2"))) buffer_zero_sse2(const void *buf, size_t len) { - __m128i t = _mm_loadu_si128(buf); - __m128i *p = (__m128i *)(((uintptr_t)buf + 5 * 16) & -16); - __m128i *e = (__m128i *)(((uintptr_t)buf + len) & -16); - __m128i zero = _mm_setzero_si128(); + /* Unaligned loads at head/tail. */ + __m128i v = *(__m128i_u *)(buf); + __m128i w = *(__m128i_u *)(buf + len - 16); + /* Align head/tail to 16-byte boundaries. */ + const __m128i *p = QEMU_ALIGN_PTR_DOWN(buf + 16, 16); + const __m128i *e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 16); + __m128i zero = { 0 }; - /* Loop over 16-byte aligned blocks of 64. */ - while (likely(p <= e)) { - t = _mm_cmpeq_epi8(t, zero); - if (unlikely(_mm_movemask_epi8(t) != 0xFFFF)) { + /* Collect a partial block at tail end. */ + v |= e[-1]; w |= e[-2]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-3]; w |= e[-4]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-5]; w |= e[-6]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-7]; v |= w; + + /* + * Loop over complete 128-byte blocks. + * With the head and tail removed, e - p >= 14, so the loop + * must iterate at least once. + */ + do { + v = _mm_cmpeq_epi8(v, zero); + if (unlikely(_mm_movemask_epi8(v) != 0xFFFF)) { return false; } - t = p[-4] | p[-3] | p[-2] | p[-1]; - p += 4; - } + v = p[0]; w = p[1]; + SSE_REASSOC_BARRIER(v, w); + v |= p[2]; w |= p[3]; + SSE_REASSOC_BARRIER(v, w); + v |= p[4]; w |= p[5]; + SSE_REASSOC_BARRIER(v, w); + v |= p[6]; w |= p[7]; + SSE_REASSOC_BARRIER(v, w); + v |= w; + p += 8; + } while (p < e - 7); - /* Finish the aligned tail. */ - t |= e[-3]; - t |= e[-2]; - t |= e[-1]; - - /* Finish the unaligned tail. */ - t |= _mm_loadu_si128(buf + len - 16); - - return _mm_movemask_epi8(_mm_cmpeq_epi8(t, zero)) == 0xFFFF; + return _mm_movemask_epi8(_mm_cmpeq_epi8(v, zero)) == 0xFFFF; } #ifdef CONFIG_AVX2_OPT static bool __attribute__((target("avx2"))) buffer_zero_avx2(const void *buf, size_t len) { - /* Begin with an unaligned head of 32 bytes. */ - __m256i t = _mm256_loadu_si256(buf); - __m256i *p = (__m256i *)(((uintptr_t)buf + 5 * 32) & -32); - __m256i *e = (__m256i *)(((uintptr_t)buf + len) & -32); + /* Unaligned loads at head/tail. */ + __m256i v = *(__m256i_u *)(buf); + __m256i w = *(__m256i_u *)(buf + len - 32); + /* Align head/tail to 32-byte boundaries. */ + const __m256i *p = QEMU_ALIGN_PTR_DOWN(buf + 32, 32); + const __m256i *e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 32); + __m256i zero = { 0 }; - /* Loop over 32-byte aligned blocks of 128. */ - while (p <= e) { - if (unlikely(!_mm256_testz_si256(t, t))) { + /* Collect a partial block at tail end. */ + v |= e[-1]; w |= e[-2]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-3]; w |= e[-4]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-5]; w |= e[-6]; + SSE_REASSOC_BARRIER(v, w); + v |= e[-7]; v |= w; + + /* Loop over complete 256-byte blocks. */ + for (; p < e - 7; p += 8) { + /* PTEST is not profitable here. */ + v = _mm256_cmpeq_epi8(v, zero); + if (unlikely(_mm256_movemask_epi8(v) != 0xFFFFFFFF)) { return false; } - t = p[-4] | p[-3] | p[-2] | p[-1]; - p += 4; - } ; + v = p[0]; w = p[1]; + SSE_REASSOC_BARRIER(v, w); + v |= p[2]; w |= p[3]; + SSE_REASSOC_BARRIER(v, w); + v |= p[4]; w |= p[5]; + SSE_REASSOC_BARRIER(v, w); + v |= p[6]; w |= p[7]; + SSE_REASSOC_BARRIER(v, w); + v |= w; + } - /* Finish the last block of 128 unaligned. */ - t |= _mm256_loadu_si256(buf + len - 4 * 32); - t |= _mm256_loadu_si256(buf + len - 3 * 32); - t |= _mm256_loadu_si256(buf + len - 2 * 32); - t |= _mm256_loadu_si256(buf + len - 1 * 32); - - return _mm256_testz_si256(t, t); + return _mm256_movemask_epi8(_mm256_cmpeq_epi8(v, zero)) == 0xFFFFFFFF; } #endif /* CONFIG_AVX2_OPT */ From 7ae6399a85f6a0818a532d9f3c6e200691f6ef68 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Sat, 6 Apr 2024 14:40:32 -1000 Subject: [PATCH 06/10] util/bufferiszero: Improve scalar variant MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Richard Henderson --- util/bufferiszero.c | 85 +++++++++++++++++++++++++++------------------ 1 file changed, 51 insertions(+), 34 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 02df82b4ff..c9a7ded016 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -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) From 0100ce2b49725e6ba2fbe8301855978d5d3dc790 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Wed, 14 Feb 2024 20:56:52 -1000 Subject: [PATCH 07/10] util/bufferiszero: Introduce biz_accel_fn typedef MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Reviewed-by: Philippe Mathieu-Daudé Signed-off-by: Richard Henderson --- util/bufferiszero.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index c9a7ded016..f9af7841ba 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -26,7 +26,8 @@ #include "qemu/bswap.h" #include "host/cpuinfo.h" -static bool (*buffer_is_zero_accel)(const void *, size_t); +typedef bool (*biz_accel_fn)(const void *, size_t); +static biz_accel_fn buffer_is_zero_accel; static bool buffer_is_zero_int_lt256(const void *buf, size_t len) { @@ -184,7 +185,7 @@ select_accel_cpuinfo(unsigned info) /* Array is sorted in order of algorithm preference. */ static const struct { unsigned bit; - bool (*fn)(const void *, size_t); + biz_accel_fn fn; } all[] = { #ifdef CONFIG_AVX2_OPT { CPUINFO_AVX2, buffer_zero_avx2 }, @@ -231,7 +232,7 @@ bool test_buffer_is_zero_next_accel(void) #define INIT_ACCEL buffer_is_zero_int_ge256 #endif -static bool (*buffer_is_zero_accel)(const void *, size_t) = INIT_ACCEL; +static biz_accel_fn buffer_is_zero_accel = INIT_ACCEL; bool buffer_is_zero_ool(const void *buf, size_t len) { From bf67aa3dd2d8b28d7618d8ec62cd9f6055366751 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Wed, 14 Feb 2024 21:10:56 -1000 Subject: [PATCH 08/10] util/bufferiszero: Simplify test_buffer_is_zero_next_accel MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Because the three alternatives are monotonic, we don't need to keep a couple of bitmasks, just identify the strongest alternative at startup. Generalize test_buffer_is_zero_next_accel and init_accel by always defining an accel_table array. Reviewed-by: Philippe Mathieu-Daudé Signed-off-by: Richard Henderson --- util/bufferiszero.c | 81 ++++++++++++++++++++------------------------- 1 file changed, 35 insertions(+), 46 deletions(-) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index f9af7841ba..7218154a13 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -27,7 +27,6 @@ #include "host/cpuinfo.h" typedef bool (*biz_accel_fn)(const void *, size_t); -static biz_accel_fn buffer_is_zero_accel; static bool buffer_is_zero_int_lt256(const void *buf, size_t len) { @@ -179,60 +178,35 @@ buffer_zero_avx2(const void *buf, size_t len) } #endif /* CONFIG_AVX2_OPT */ -static unsigned __attribute__((noinline)) -select_accel_cpuinfo(unsigned info) -{ - /* Array is sorted in order of algorithm preference. */ - static const struct { - unsigned bit; - biz_accel_fn fn; - } all[] = { +static biz_accel_fn const accel_table[] = { + buffer_is_zero_int_ge256, + buffer_zero_sse2, #ifdef CONFIG_AVX2_OPT - { CPUINFO_AVX2, buffer_zero_avx2 }, + buffer_zero_avx2, #endif - { CPUINFO_SSE2, buffer_zero_sse2 }, - { CPUINFO_ALWAYS, buffer_is_zero_int_ge256 }, - }; +}; - for (unsigned i = 0; i < ARRAY_SIZE(all); ++i) { - if (info & all[i].bit) { - buffer_is_zero_accel = all[i].fn; - return all[i].bit; - } +static unsigned best_accel(void) +{ + unsigned info = cpuinfo_init(); + +#ifdef CONFIG_AVX2_OPT + if (info & CPUINFO_AVX2) { + return 2; } - return 0; +#endif + return info & CPUINFO_SSE2 ? 1 : 0; } -static unsigned used_accel; - -static void __attribute__((constructor)) init_accel(void) -{ - used_accel = select_accel_cpuinfo(cpuinfo_init()); -} - -#define INIT_ACCEL NULL - -bool test_buffer_is_zero_next_accel(void) -{ - /* - * Accumulate the accelerators that we've already tested, and - * remove them from the set to test this round. We'll get back - * a zero from select_accel_cpuinfo when there are no more. - */ - unsigned used = select_accel_cpuinfo(cpuinfo & ~used_accel); - used_accel |= used; - return used; -} #else -bool test_buffer_is_zero_next_accel(void) -{ - return false; -} - -#define INIT_ACCEL buffer_is_zero_int_ge256 +#define best_accel() 0 +static biz_accel_fn const accel_table[1] = { + buffer_is_zero_int_ge256 +}; #endif -static biz_accel_fn buffer_is_zero_accel = INIT_ACCEL; +static biz_accel_fn buffer_is_zero_accel; +static unsigned accel_index; bool buffer_is_zero_ool(const void *buf, size_t len) { @@ -257,3 +231,18 @@ bool buffer_is_zero_ge256(const void *buf, size_t len) { return buffer_is_zero_accel(buf, len); } + +bool test_buffer_is_zero_next_accel(void) +{ + if (accel_index != 0) { + buffer_is_zero_accel = accel_table[--accel_index]; + return true; + } + return false; +} + +static void __attribute__((constructor)) init_accel(void) +{ + accel_index = best_accel(); + buffer_is_zero_accel = accel_table[accel_index]; +} From 22437b4de94c37e6104d90d46b31d80cf14358d4 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Sat, 10 Feb 2024 00:02:17 +0000 Subject: [PATCH 09/10] util/bufferiszero: Add simd acceleration for aarch64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Because non-embedded aarch64 is expected to have AdvSIMD enabled, merely double-check with the compiler flags for __ARM_NEON and don't bother with a runtime check. Otherwise, model the loop after the x86 SSE2 function. Use UMAXV for the vector reduction. This is 3 cycles on cortex-a76 and 2 cycles on neoverse-n1. Reviewed-by: Philippe Mathieu-Daudé Signed-off-by: Richard Henderson --- util/bufferiszero.c | 67 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) diff --git a/util/bufferiszero.c b/util/bufferiszero.c index 7218154a13..74864f7b78 100644 --- a/util/bufferiszero.c +++ b/util/bufferiszero.c @@ -198,6 +198,73 @@ static unsigned best_accel(void) return info & CPUINFO_SSE2 ? 1 : 0; } +#elif defined(__aarch64__) && defined(__ARM_NEON) +#include + +/* + * Helper for preventing the compiler from reassociating + * chains of binary vector operations. + */ +#define REASSOC_BARRIER(vec0, vec1) asm("" : "+w"(vec0), "+w"(vec1)) + +static bool buffer_is_zero_simd(const void *buf, size_t len) +{ + uint32x4_t t0, t1, t2, t3; + + /* Align head/tail to 16-byte boundaries. */ + const uint32x4_t *p = QEMU_ALIGN_PTR_DOWN(buf + 16, 16); + const uint32x4_t *e = QEMU_ALIGN_PTR_DOWN(buf + len - 1, 16); + + /* Unaligned loads at head/tail. */ + t0 = vld1q_u32(buf) | vld1q_u32(buf + len - 16); + + /* Collect a partial block at tail end. */ + t1 = e[-7] | e[-6]; + t2 = e[-5] | e[-4]; + t3 = e[-3] | e[-2]; + t0 |= e[-1]; + REASSOC_BARRIER(t0, t1); + REASSOC_BARRIER(t2, t3); + t0 |= t1; + t2 |= t3; + REASSOC_BARRIER(t0, t2); + t0 |= t2; + + /* + * Loop over complete 128-byte blocks. + * With the head and tail removed, e - p >= 14, so the loop + * must iterate at least once. + */ + do { + /* + * Reduce via UMAXV. Whatever the actual result, + * it will only be zero if all input bytes are zero. + */ + if (unlikely(vmaxvq_u32(t0) != 0)) { + return false; + } + + t0 = p[0] | p[1]; + t1 = p[2] | p[3]; + t2 = p[4] | p[5]; + t3 = p[6] | p[7]; + REASSOC_BARRIER(t0, t1); + REASSOC_BARRIER(t2, t3); + t0 |= t1; + t2 |= t3; + REASSOC_BARRIER(t0, t2); + t0 |= t2; + p += 8; + } while (p < e - 7); + + return vmaxvq_u32(t0) == 0; +} + +#define best_accel() 1 +static biz_accel_fn const accel_table[] = { + buffer_is_zero_int_ge256, + buffer_is_zero_simd, +}; #else #define best_accel() 0 static biz_accel_fn const accel_table[1] = { From a06d9eddb015a9f5895161b0a3958a2e4be21579 Mon Sep 17 00:00:00 2001 From: Richard Henderson Date: Fri, 16 Feb 2024 13:59:36 -1000 Subject: [PATCH 10/10] tests/bench: Add bufferiszero-bench MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Benchmark each acceleration function vs an aligned buffer of zeros. Reviewed-by: Philippe Mathieu-Daudé Signed-off-by: Richard Henderson --- tests/bench/bufferiszero-bench.c | 47 ++++++++++++++++++++++++++++++++ tests/bench/meson.build | 1 + 2 files changed, 48 insertions(+) create mode 100644 tests/bench/bufferiszero-bench.c diff --git a/tests/bench/bufferiszero-bench.c b/tests/bench/bufferiszero-bench.c new file mode 100644 index 0000000000..222695c1fa --- /dev/null +++ b/tests/bench/bufferiszero-bench.c @@ -0,0 +1,47 @@ +/* + * QEMU buffer_is_zero speed benchmark + * + * This work is licensed under the terms of the GNU GPL, version 2 or + * (at your option) any later version. See the COPYING file in the + * top-level directory. + */ +#include "qemu/osdep.h" +#include "qemu/cutils.h" +#include "qemu/units.h" + +static void test(const void *opaque) +{ + size_t max = 64 * KiB; + void *buf = g_malloc0(max); + int accel_index = 0; + + do { + if (accel_index != 0) { + g_test_message("%s", ""); /* gnu_printf Werror for simple "" */ + } + for (size_t len = 1 * KiB; len <= max; len *= 4) { + double total = 0.0; + + g_test_timer_start(); + do { + buffer_is_zero_ge256(buf, len); + total += len; + } while (g_test_timer_elapsed() < 0.5); + + total /= MiB; + g_test_message("buffer_is_zero #%d: %2zuKB %8.0f MB/sec", + accel_index, len / (size_t)KiB, + total / g_test_timer_last()); + } + accel_index++; + } while (test_buffer_is_zero_next_accel()); + + g_free(buf); +} + +int main(int argc, char **argv) +{ + g_test_init(&argc, &argv, NULL); + g_test_add_data_func("/cutils/bufferiszero/speed", NULL, test); + return g_test_run(); +} diff --git a/tests/bench/meson.build b/tests/bench/meson.build index 7e76338a52..4cd7a2f6b5 100644 --- a/tests/bench/meson.build +++ b/tests/bench/meson.build @@ -21,6 +21,7 @@ benchs = {} if have_block benchs += { + 'bufferiszero-bench': [], 'benchmark-crypto-hash': [crypto], 'benchmark-crypto-hmac': [crypto], 'benchmark-crypto-cipher': [crypto],