On Thu, Mar 21, 2024 at 11:30:30AM +0700, John Naylor wrote:
> I'm much happier about v5-0001. With a small tweak it would match what
> I had in mind:
>
> + if (nelem < nelem_per_iteration)
> + goto one_by_one;
>
> If this were "<=" then the for long arrays we could assume there is
> always more than one block, and wouldn't need to check if any elements
> remain -- first block, then a single loop and it's done.
>
> The loop could also then be a "do while" since it doesn't have to
> check the exit condition up front.
Good idea. That causes us to re-check all of the tail elements when the
number of elements is evenly divisible by nelem_per_iteration, but that
might be worth the trade-off.
> Yes, that spike is weird, because it seems super-linear. However, the
> more interesting question for me is: AVX2 isn't really buying much for
> the numbers covered in this test. Between 32 and 48 elements, and
> between 64 and 80, it's indistinguishable from SSE2. The jumps to the
> next shelf are postponed, but the jumps are just as high. From earlier
> system benchmarks, I recall it eventually wins out with hundreds of
> elements, right? Is that still true?
It does still eventually win, although not nearly to the same extent as
before. I extended the benchmark a bit to show this. I wouldn't be
devastated if we only got 0001 committed for v17, given these results.
> Further, now that the algorithm is more SIMD-appropriate, I wonder
> what doing 4 registers at a time is actually buying us for either SSE2
> or AVX2. It might just be a matter of scale, but that would be good to
> understand.
I'll follow up with these numbers shortly.
--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com
>From 5d4d91d169b973838c99e8d4fdadcb09df36a6ea Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nat...@postgresql.org>
Date: Wed, 20 Mar 2024 14:20:24 -0500
Subject: [PATCH v6 1/2] pg_lfind32(): add "overlap" code for remaining
elements
---
src/include/port/pg_lfind.h | 103 ++++++++++++++++++++++++------------
1 file changed, 70 insertions(+), 33 deletions(-)
diff --git a/src/include/port/pg_lfind.h b/src/include/port/pg_lfind.h
index b8dfa66eef..22a3711ab5 100644
--- a/src/include/port/pg_lfind.h
+++ b/src/include/port/pg_lfind.h
@@ -80,6 +80,49 @@ pg_lfind8_le(uint8 key, uint8 *base, uint32 nelem)
return false;
}
+/*
+ * pg_lfind32_helper
+ *
+ * Searches one 4-register-block of integers. The caller is responsible for
+ * ensuring that there are at least 4-registers-worth of integers remaining.
+ */
+static inline bool
+pg_lfind32_helper(const Vector32 keys, uint32 *base)
+{
+ const uint32 nelem_per_vector = sizeof(Vector32) / sizeof(uint32);
+ Vector32 vals1,
+ vals2,
+ vals3,
+ vals4,
+ result1,
+ result2,
+ result3,
+ result4,
+ tmp1,
+ tmp2,
+ result;
+
+ /* load the next block into 4 registers */
+ vector32_load(&vals1, base);
+ vector32_load(&vals2, &base[nelem_per_vector]);
+ vector32_load(&vals3, &base[nelem_per_vector * 2]);
+ vector32_load(&vals4, &base[nelem_per_vector * 3]);
+
+ /* compare each value to the key */
+ result1 = vector32_eq(keys, vals1);
+ result2 = vector32_eq(keys, vals2);
+ result3 = vector32_eq(keys, vals3);
+ result4 = vector32_eq(keys, vals4);
+
+ /* combine the results into a single variable */
+ tmp1 = vector32_or(result1, result2);
+ tmp2 = vector32_or(result3, result4);
+ result = vector32_or(tmp1, tmp2);
+
+ /* return whether there was a match */
+ return vector32_is_highbit_set(result);
+}
+
/*
* pg_lfind32
*
@@ -119,46 +162,40 @@ pg_lfind32(uint32 key, uint32 *base, uint32 nelem)
}
#endif
- for (i = 0; i < tail_idx; i += nelem_per_iteration)
+ /*
+ * If there aren't enough elements for the SIMD code, jump to the standard
+ * one-by-one linear search code.
+ */
+ if (nelem <= nelem_per_iteration)
+ goto one_by_one;
+
+ /*
+ * Process as many elements as possible with a block of 4 registers.
+ */
+ do
{
- Vector32 vals1,
- vals2,
- vals3,
- vals4,
- result1,
- result2,
- result3,
- result4,
- tmp1,
- tmp2,
- result;
-
- /* load the next block into 4 registers */
- vector32_load(&vals1, &base[i]);
- vector32_load(&vals2, &base[i + nelem_per_vector]);
- vector32_load(&vals3, &base[i + nelem_per_vector * 2]);
- vector32_load(&vals4, &base[i + nelem_per_vector * 3]);
-
- /* compare each value to the key */
- result1 = vector32_eq(keys, vals1);
- result2 = vector32_eq(keys, vals2);
- result3 = vector32_eq(keys, vals3);
- result4 = vector32_eq(keys, vals4);
-
- /* combine the results into a single variable */
- tmp1 = vector32_or(result1, result2);
- tmp2 = vector32_or(result3, result4);
- result = vector32_or(tmp1, tmp2);
-
- /* see if there was a match */
- if (vector32_is_highbit_set(result))
+ if (pg_lfind32_helper(keys, &base[i]))
{
Assert(assert_result == true);
return true;
}
- }
+
+ i += nelem_per_iteration;
+
+ } while (i < tail_idx);
+
+ /*
+ * Process the last 'nelem_per_iteration' elements in the array with a
+ * 4-register block. This will cause us to check some of the elements
+ * more than once, but that won't affect correctness, and testing has
+ * demonstrated that this helps more cases than it harms.
+ */
+ Assert(assert_result == pg_lfind32_helper(keys, &base[nelem - nelem_per_iteration]));
+ return pg_lfind32_helper(keys, &base[nelem - nelem_per_iteration]);
+
#endif /* ! USE_NO_SIMD */
+one_by_one:
/* Process the remaining elements one at a time. */
for (; i < nelem; i++)
{
--
2.25.1
>From 7e7781454646992218a990cf75f0654c67ce2dab Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nat...@postgresql.org>
Date: Mon, 18 Mar 2024 11:02:05 -0500
Subject: [PATCH v6 2/2] Add support for AVX2 in simd.h.
Discussion: https://postgr.es/m/20231129171526.GA857928%40nathanxps13
---
src/include/port/simd.h | 61 ++++++++++++++++++++++++++++++++---------
1 file changed, 48 insertions(+), 13 deletions(-)
diff --git a/src/include/port/simd.h b/src/include/port/simd.h
index 597496f2fb..f06b21876b 100644
--- a/src/include/port/simd.h
+++ b/src/include/port/simd.h
@@ -18,7 +18,18 @@
#ifndef SIMD_H
#define SIMD_H
-#if (defined(__x86_64__) || defined(_M_AMD64))
+#if defined(__AVX2__)
+
+/*
+ * XXX: Need to add a big comment here.
+ */
+#include <immintrin.h>
+#define USE_AVX2
+typedef __m256i Vector8;
+typedef __m256i Vector32;
+
+#elif (defined(__x86_64__) || defined(_M_AMD64))
+
/*
* SSE2 instructions are part of the spec for the 64-bit x86 ISA. We assume
* that compilers targeting this architecture understand SSE2 intrinsics.
@@ -107,7 +118,9 @@ static inline Vector32 vector32_eq(const Vector32 v1, const Vector32 v2);
static inline void
vector8_load(Vector8 *v, const uint8 *s)
{
-#if defined(USE_SSE2)
+#if defined(USE_AVX2)
+ *v = _mm256_loadu_si256((const __m256i *) s);
+#elif defined(USE_SSE2)
*v = _mm_loadu_si128((const __m128i *) s);
#elif defined(USE_NEON)
*v = vld1q_u8(s);
@@ -120,7 +133,9 @@ vector8_load(Vector8 *v, const uint8 *s)
static inline void
vector32_load(Vector32 *v, const uint32 *s)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ *v = _mm256_loadu_si256((const __m256i *) s);
+#elif defined(USE_SSE2)
*v = _mm_loadu_si128((const __m128i *) s);
#elif defined(USE_NEON)
*v = vld1q_u32(s);
@@ -134,7 +149,9 @@ vector32_load(Vector32 *v, const uint32 *s)
static inline Vector8
vector8_broadcast(const uint8 c)
{
-#if defined(USE_SSE2)
+#if defined(USE_AVX2)
+ return _mm256_set1_epi8(c);
+#elif defined(USE_SSE2)
return _mm_set1_epi8(c);
#elif defined(USE_NEON)
return vdupq_n_u8(c);
@@ -147,7 +164,9 @@ vector8_broadcast(const uint8 c)
static inline Vector32
vector32_broadcast(const uint32 c)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_set1_epi32(c);
+#elif defined(USE_SSE2)
return _mm_set1_epi32(c);
#elif defined(USE_NEON)
return vdupq_n_u32(c);
@@ -270,7 +289,9 @@ vector8_has_le(const Vector8 v, const uint8 c)
static inline bool
vector8_is_highbit_set(const Vector8 v)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_movemask_epi8(v) != 0;
+#elif defined(USE_SSE2)
return _mm_movemask_epi8(v) != 0;
#elif defined(USE_NEON)
return vmaxvq_u8(v) > 0x7F;
@@ -308,7 +329,9 @@ vector32_is_highbit_set(const Vector32 v)
static inline uint32
vector8_highbit_mask(const Vector8 v)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return (uint32) _mm256_movemask_epi8(v);
+#elif defined(USE_SSE2)
return (uint32) _mm_movemask_epi8(v);
#elif defined(USE_NEON)
/*
@@ -337,7 +360,9 @@ vector8_highbit_mask(const Vector8 v)
static inline Vector8
vector8_or(const Vector8 v1, const Vector8 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_or_si256(v1, v2);
+#elif defined(USE_SSE2)
return _mm_or_si128(v1, v2);
#elif defined(USE_NEON)
return vorrq_u8(v1, v2);
@@ -350,7 +375,9 @@ vector8_or(const Vector8 v1, const Vector8 v2)
static inline Vector32
vector32_or(const Vector32 v1, const Vector32 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_or_si256(v1, v2);
+#elif defined(USE_SSE2)
return _mm_or_si128(v1, v2);
#elif defined(USE_NEON)
return vorrq_u32(v1, v2);
@@ -368,7 +395,9 @@ vector32_or(const Vector32 v1, const Vector32 v2)
static inline Vector8
vector8_ssub(const Vector8 v1, const Vector8 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_subs_epu8(v1, v2);
+#elif defined(USE_SSE2)
return _mm_subs_epu8(v1, v2);
#elif defined(USE_NEON)
return vqsubq_u8(v1, v2);
@@ -384,7 +413,9 @@ vector8_ssub(const Vector8 v1, const Vector8 v2)
static inline Vector8
vector8_eq(const Vector8 v1, const Vector8 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_cmpeq_epi8(v1, v2);
+#elif defined(USE_SSE2)
return _mm_cmpeq_epi8(v1, v2);
#elif defined(USE_NEON)
return vceqq_u8(v1, v2);
@@ -396,7 +427,9 @@ vector8_eq(const Vector8 v1, const Vector8 v2)
static inline Vector32
vector32_eq(const Vector32 v1, const Vector32 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_cmpeq_epi32(v1, v2);
+#elif defined(USE_SSE2)
return _mm_cmpeq_epi32(v1, v2);
#elif defined(USE_NEON)
return vceqq_u32(v1, v2);
@@ -411,7 +444,9 @@ vector32_eq(const Vector32 v1, const Vector32 v2)
static inline Vector8
vector8_min(const Vector8 v1, const Vector8 v2)
{
-#ifdef USE_SSE2
+#if defined(USE_AVX2)
+ return _mm256_min_epu8(v1, v2);
+#elif defined(USE_SSE2)
return _mm_min_epu8(v1, v2);
#elif defined(USE_NEON)
return vminq_u8(v1, v2);
--
2.25.1
