From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: David Rowley <david.rowley@2ndquadrant.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
Date: Wed, 13 Feb 2019 19:17:19 -0300
Message-ID: <20190213221719.GA15976@alvherre.pgsql> (raw)
In-Reply-To: <20190213205904.GA13925@alvherre.pgsql>
On 2019-Feb-13, Alvaro Herrera wrote:
> It definitely is ... plans have changed from using IndexOnly scans to
> Seqscans, which is likely fallout from the visibilitymap_count() change.
I think the problem here is that "unsigned long" is 32 bits in this
machine:
checking whether long int is 64 bits... no
and we have defined pg_popcount64() like this:
static int
pg_popcount64_sse42(uint64 word)
{
return __builtin_popcountl(word);
}
so it's counting bits in the lower half of the uint64.
If that's correct, then I think we need something like this patch. But
it makes me wonder whether we need a configure test for
__builtin_popcountll() and friends. I wonder if there's any compiler
that implements __builtin_popcountl() but not __builtin_popcountll() ...
and if not, then the test for __builtin_popcountl() should be removed,
and have everything rely on the one for __builtin_popcount().
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Attachments:
[text/x-diff] uint64-is-longlong.patch (1.2K, ../20190213221719.GA15976@alvherre.pgsql/2-uint64-is-longlong.patch)
download | inline diff:diff --git a/src/port/pg_bitutils.c b/src/port/pg_bitutils.cindex 97422e05040..fe6afb9fba5 100644--- a/src/port/pg_bitutils.c+++ b/src/port/pg_bitutils.c@@ -299,7 +299,14 @@ pg_popcount64_choose(uint64 word)
static int
pg_popcount64_sse42(uint64 word)
{
+#if defined(HAVE_LONG_INT_64)
return __builtin_popcountl(word);
+#elif defined(HAVE_LONG_LONG_INT_64)+ return __builtin_popcountll(word);+#else+ /* shouldn't happen */+#error must have a working 64-bit integer datatype+#endif
}
#endif
@@ -407,7 +414,14 @@ pg_rightmost_one64_choose(uint64 word)
static int
pg_rightmost_one64_abm(uint64 word)
{
+#if defined(HAVE_LONG_INT_64)
return __builtin_ctzl(word);
+#elif defined(HAVE_LONG_LONG_INT_64)+ return __builtin_ctzll(word);+#else+ /* shouldn't happen */+#error must have a working 64-bit integer datatype+#endif
}
#endif
@@ -493,7 +507,15 @@ pg_leftmost_one64_choose(uint64 word)
static int
pg_leftmost_one64_abm(uint64 word)
{
+#if defined(HAVE_LONG_INT_64)
return 63 - __builtin_clzl(word);
+#elif defined(HAVE_LONG_LONG_INT_64)+ return 63 - __builtin_clzll(word);+#else+ /* shouldn't happen */+#error must have a working 64-bit integer datatype+#endif+
}
#endif
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To: pgsql-hackers@postgresql.org
Cc: alvherre@2ndquadrant.com, andrew@tao11.riddles.org.uk, david.rowley@2ndquadrant.com, pgsql-hackers@lists.postgresql.org, thomas.munro@enterprisedb.com
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
In-Reply-To: <20190213221719.GA15976@alvherre.pgsql>
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