Date: Thu, 18 Apr 2024 16:59:02 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: "Devulapalli, Raghuveer" <raghuveer.devulapalli@intel.com>
Cc: "Shankaran, Akash" <akash.shankaran@intel.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>,
	Ants Aasma <ants.aasma@cybertec.at>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	"Amonson, Paul D" <paul.d.amonson@intel.com>,
	Andres Freund <andres@anarazel.de>, Noah Misch <noah@leadboat.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Message-ID: <20240418215902.GA3830246@nathanxps13>
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On Thu, Apr 18, 2024 at 09:29:55PM +0000, Devulapalli, Raghuveer wrote:
> (1) Shouldn't it be: return (_xgetbv(0) & 0xe6) == 0xe6; ? Otherwise
> zmm_regs_available() will return false..

Yes, that's a mistake.  I fixed that in v3.

> (2) Nitpick: avx512_popcnt_available and avx512_bw_available() run the
> same cpuid leaf. You could combine them into one to avoid running cpuid
> twice. My apologies, I should have mentioned this before..

Good call.  The byte-and-word instructions were a late addition to the
patch, so I missed this originally.

On that note, is it necessary to also check for avx512f?  At the moment, we
are assuming that's supported if the other AVX-512 instructions are
available.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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From e04c348eb389c6aa1597ac35d57b5e7ae7075381 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Thu, 18 Apr 2024 15:57:56 -0500
Subject: [PATCH v3 1/1] osxsave

---
 src/port/pg_popcount_avx512_choose.c | 80 ++++++++++++++++------------
 1 file changed, 47 insertions(+), 33 deletions(-)

diff --git a/src/port/pg_popcount_avx512_choose.c b/src/port/pg_popcount_avx512_choose.c
index ae3fa3d306..b37107803a 100644
--- a/src/port/pg_popcount_avx512_choose.c
+++ b/src/port/pg_popcount_avx512_choose.c
@@ -34,39 +34,13 @@
 #ifdef TRY_POPCNT_FAST
 
 /*
- * Returns true if the CPU supports the instructions required for the AVX-512
- * pg_popcount() implementation.
+ * Does CPUID say there's support for XSAVE instructions?
  */
-bool
-pg_popcount_avx512_available(void)
+static inline bool
+xsave_available(void)
 {
 	unsigned int exx[4] = {0, 0, 0, 0};
 
-	/* Does CPUID say there's support for AVX-512 popcount instructions? */
-#if defined(HAVE__GET_CPUID_COUNT)
-	__get_cpuid_count(7, 0, &exx[0], &exx[1], &exx[2], &exx[3]);
-#elif defined(HAVE__CPUIDEX)
-	__cpuidex(exx, 7, 0);
-#else
-#error cpuid instruction not available
-#endif
-	if ((exx[2] & (1 << 14)) == 0)	/* avx512-vpopcntdq */
-		return false;
-
-	/* Does CPUID say there's support for AVX-512 byte and word instructions? */
-	memset(exx, 0, sizeof(exx));
-#if defined(HAVE__GET_CPUID_COUNT)
-	__get_cpuid_count(7, 0, &exx[0], &exx[1], &exx[2], &exx[3]);
-#elif defined(HAVE__CPUIDEX)
-	__cpuidex(exx, 7, 0);
-#else
-#error cpuid instruction not available
-#endif
-	if ((exx[1] & (1 << 30)) == 0)	/* avx512-bw */
-		return false;
-
-	/* Does CPUID say there's support for XSAVE instructions? */
-	memset(exx, 0, sizeof(exx));
 #if defined(HAVE__GET_CPUID)
 	__get_cpuid(1, &exx[0], &exx[1], &exx[2], &exx[3]);
 #elif defined(HAVE__CPUID)
@@ -74,15 +48,55 @@ pg_popcount_avx512_available(void)
 #else
 #error cpuid instruction not available
 #endif
-	if ((exx[2] & (1 << 26)) == 0)	/* xsave */
-		return false;
+	return (exx[2] & (1 << 27)) != 0;	/* osxsave */
+}
 
-	/* Does XGETBV say the ZMM registers are enabled? */
+/*
+ * Does XGETBV say the ZMM registers are enabled?
+ *
+ * NB: Caller is responsible for verifying that xsave_available() returns true
+ * before calling this.
+ */
+static inline bool
+zmm_regs_available(void)
+{
 #ifdef HAVE_XSAVE_INTRINSICS
-	return (_xgetbv(0) & 0xe0) != 0;
+	return (_xgetbv(0) & 0xe6) == 0xe6;
 #else
 	return false;
 #endif
 }
 
+/*
+ * Does CPUID say there's support for AVX-512 popcount and byte-and-word
+ * instructions?
+ */
+static inline bool
+avx512_popcnt_available(void)
+{
+	unsigned int exx[4] = {0, 0, 0, 0};
+
+#if defined(HAVE__GET_CPUID_COUNT)
+	__get_cpuid_count(7, 0, &exx[0], &exx[1], &exx[2], &exx[3]);
+#elif defined(HAVE__CPUIDEX)
+	__cpuidex(exx, 7, 0);
+#else
+#error cpuid instruction not available
+#endif
+	return (exx[2] & (1 << 14)) != 0 && /* avx512-vpopcntdq */
+		(exx[1] & (1 << 30)) != 0;	/* avx512-bw */
+}
+
+/*
+ * Returns true if the CPU supports the instructions required for the AVX-512
+ * pg_popcount() implementation.
+ */
+bool
+pg_popcount_avx512_available(void)
+{
+	return xsave_available() &&
+		zmm_regs_available() &&
+		avx512_popcnt_available();
+}
+
 #endif							/* TRY_POPCNT_FAST */
-- 
2.25.1


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