From e38654507d2efad4b5ad75548e0c388c3db9cfe5 Mon Sep 17 00:00:00 2001
From: John Naylor <john.naylor@postgresql.org>
Date: Fri, 28 Feb 2025 18:27:40 +0700
Subject: [PATCH v12 5/6] Use runtime check even when we have SSE 4.2 at
 compile time

This allows us to use PCLMUL for longer inputs. Short inputs are
inlined to avoid the indirection through a function pointer.
---
 configure                         |  2 +-
 configure.ac                      |  2 +-
 src/include/port/pg_crc32c.h      | 15 +++++++++++----
 src/port/meson.build              |  1 +
 src/port/pg_crc32c_sse42_choose.c |  2 ++
 5 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/configure b/configure
index 93fddd69981..91c0ffc8272 100755
--- a/configure
+++ b/configure
@@ -17684,7 +17684,7 @@ if test x"$USE_SSE42_CRC32C" = x"1"; then
 
 $as_echo "#define USE_SSE42_CRC32C 1" >>confdefs.h
 
-  PG_CRC32C_OBJS="pg_crc32c_sse42.o"
+  PG_CRC32C_OBJS="pg_crc32c_sse42.o pg_crc32c_sse42_choose.o"
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: SSE 4.2" >&5
 $as_echo "SSE 4.2" >&6; }
 else
diff --git a/configure.ac b/configure.ac
index b6d02f5ecc7..a85bdbd4ff6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2151,7 +2151,7 @@ fi
 AC_MSG_CHECKING([which CRC-32C implementation to use])
 if test x"$USE_SSE42_CRC32C" = x"1"; then
   AC_DEFINE(USE_SSE42_CRC32C, 1, [Define to 1 use Intel SSE 4.2 CRC instructions.])
-  PG_CRC32C_OBJS="pg_crc32c_sse42.o"
+  PG_CRC32C_OBJS="pg_crc32c_sse42.o pg_crc32c_sse42_choose.o"
   AC_MSG_RESULT(SSE 4.2)
 else
   if test x"$USE_SSE42_CRC32C_WITH_RUNTIME_CHECK" = x"1"; then
diff --git a/src/include/port/pg_crc32c.h b/src/include/port/pg_crc32c.h
index fe0e1b6b275..26b676dddc9 100644
--- a/src/include/port/pg_crc32c.h
+++ b/src/include/port/pg_crc32c.h
@@ -55,22 +55,29 @@ typedef uint32 pg_crc32c;
 	((crc) = pg_comp_crc32c_dispatch((crc), (data), (len)))
 #define FIN_CRC32C(crc) ((crc) ^= 0xFFFFFFFF)
 
+extern pg_crc32c pg_comp_crc32c_sb8(pg_crc32c crc, const void *data, size_t len);
+extern pg_crc32c (*pg_comp_crc32c) (pg_crc32c crc, const void *data, size_t len);
 extern pg_crc32c pg_comp_crc32c_sse42(pg_crc32c crc, const void *data, size_t len);
+#ifdef USE_PCLMUL_WITH_RUNTIME_CHECK
+extern pg_crc32c pg_comp_crc32c_pclmul(pg_crc32c crc, const void *data, size_t len);
+#endif
 
 static inline
 pg_crc32c
 pg_comp_crc32c_dispatch(pg_crc32c crc, const void *data, size_t len)
 {
-	if (__builtin_constant_p(len) && len < 64)
+	if (len < 64)
 	{
 		/*
-		 * For small constant inputs, inline the computation. This allows the
-		 * compiler to unroll loops.
+		 * For small inputs, inline the computation to avoid the runtime
+		 * check. This also allows the compiler to unroll loops for constant
+		 * input.
 		 */
 		return pg_comp_crc32c_sse42_inline(crc, data, len);
 	}
 	else
-		return pg_comp_crc32c_sse42(crc, data, len);
+		/* For larger inputs, use a runtime check for PCLMUL instructions. */
+		return pg_comp_crc32c(crc, data, len);
 }
 
 #elif defined(USE_SSE42_CRC32C_WITH_RUNTIME_CHECK)
diff --git a/src/port/meson.build b/src/port/meson.build
index 7fcfa728d43..8d70a4d510e 100644
--- a/src/port/meson.build
+++ b/src/port/meson.build
@@ -83,6 +83,7 @@ replace_funcs_pos = [
   # x86/x64
   ['pg_crc32c_sse42', 'USE_SSE42_CRC32C'],
   ['pg_crc32c_sse42', 'USE_SSE42_CRC32C_WITH_RUNTIME_CHECK'],
+  ['pg_crc32c_sse42_choose', 'USE_SSE42_CRC32C'],
   ['pg_crc32c_sse42_choose', 'USE_SSE42_CRC32C_WITH_RUNTIME_CHECK'],
   ['pg_crc32c_sb8', 'USE_SSE42_CRC32C_WITH_RUNTIME_CHECK'],
 
diff --git a/src/port/pg_crc32c_sse42_choose.c b/src/port/pg_crc32c_sse42_choose.c
index abea0f90eb3..89a48c76894 100644
--- a/src/port/pg_crc32c_sse42_choose.c
+++ b/src/port/pg_crc32c_sse42_choose.c
@@ -55,8 +55,10 @@ pg_comp_crc32c_choose(pg_crc32c crc, const void *data, size_t len)
 			pg_comp_crc32c = pg_comp_crc32c_pclmul;
 #endif
 	}
+#ifdef USE_SSE42_CRC32C_WITH_RUNTIME_CHECK
 	else
 		pg_comp_crc32c = pg_comp_crc32c_sb8;
+#endif
 
 	return pg_comp_crc32c(crc, data, len);
 }
-- 
2.48.1