Improve our method for probing the availability of ARM CRC instructions.

Instead of depending on glibc's getauxval() function, just try to execute
the CRC code, and trap SIGILL if that happens.

Thomas Munro

Discussion: https://postgr.es/m/HE1PR0801MB1323D171938EABC04FFE7FA9E3110@HE1PR0801MB1323.eurprd08.prod.outlook.com

Author: tglsfdc

configure

@@ -17344,46 +17344,6 @@ fi
 fi
 
 
-# In order to detect at runtime, if the ARM CRC Extension is available,
-# we will do "getauxval(AT_HWCAP) & HWCAP_CRC32". Check if we have
-# everything we need for that.
-for ac_func in getauxval
-do :
-  ac_fn_c_check_func "$LINENO" "getauxval" "ac_cv_func_getauxval"
-if test "x$ac_cv_func_getauxval" = xyes; then :
-  cat >>confdefs.h <<_ACEOF
-#define HAVE_GETAUXVAL 1
-_ACEOF
-
-fi
-done
-
-cat confdefs.h - <<_ACEOF >conftest.$ac_ext
-/* end confdefs.h.  */
-
-#include <sys/auxv.h>
-#include <asm/hwcap.h>
-
-int
-main ()
-{
-
-#ifndef AT_HWCAP
-#error AT_HWCAP not defined
-#endif
-#ifndef HWCAP_CRC32
-#error HWCAP_CRC32 not defined
-#endif
-
-  ;
-  return 0;
-}
-_ACEOF
-if ac_fn_c_try_compile "$LINENO"; then :
-  HAVE_HWCAP_CRC32=1
-fi
-rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-
 # Select CRC-32C implementation.
 #
 # If we are targeting a processor that has Intel SSE 4.2 instructions, we can
@@ -17414,9 +17374,8 @@ if test x"$USE_SLICING_BY_8_CRC32C" = x"" && test x"$USE_SSE42_CRC32C" = x"" &&
       if test x"$pgac_armv8_crc32c_intrinsics" = x"yes" && test x"$CFLAGS_ARMV8_CRC32C" = x""; then
         USE_ARMV8_CRC32C=1
       else
-        # ARM CRC Extension, with runtime check? The getauxval() function and
-        # HWCAP_CRC32 are needed for the runtime check.
-        if test x"$pgac_armv8_crc32c_intrinsics" = x"yes" && test x"$ac_cv_func_getauxval" = x"yes" && test x"$HAVE_HWCAP_CRC32" = x"1"; then
+        # ARM CRC Extension, with runtime check?
+        if test x"$pgac_armv8_crc32c_intrinsics" = x"yes"; then
           USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK=1
         else
           # fall back to slicing-by-8 algorithm, which doesn't require any

configure.in

@@ -2014,22 +2014,6 @@ if test x"$pgac_armv8_crc32c_intrinsics" != x"yes"; then
 fi
 AC_SUBST(CFLAGS_ARMV8_CRC32C)
 
-# In order to detect at runtime, if the ARM CRC Extension is available,
-# we will do "getauxval(AT_HWCAP) & HWCAP_CRC32". Check if we have
-# everything we need for that.
-AC_CHECK_FUNCS([getauxval])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([
-#include <sys/auxv.h>
-#include <asm/hwcap.h>
-], [
-#ifndef AT_HWCAP
-#error AT_HWCAP not defined
-#endif
-#ifndef HWCAP_CRC32
-#error HWCAP_CRC32 not defined
-#endif
-])], [HAVE_HWCAP_CRC32=1])
-
 # Select CRC-32C implementation.
 #
 # If we are targeting a processor that has Intel SSE 4.2 instructions, we can
@@ -2060,9 +2044,8 @@ if test x"$USE_SLICING_BY_8_CRC32C" = x"" && test x"$USE_SSE42_CRC32C" = x"" &&
       if test x"$pgac_armv8_crc32c_intrinsics" = x"yes" && test x"$CFLAGS_ARMV8_CRC32C" = x""; then
         USE_ARMV8_CRC32C=1
       else
-        # ARM CRC Extension, with runtime check? The getauxval() function and
-        # HWCAP_CRC32 are needed for the runtime check.
-        if test x"$pgac_armv8_crc32c_intrinsics" = x"yes" && test x"$ac_cv_func_getauxval" = x"yes" && test x"$HAVE_HWCAP_CRC32" = x"1"; then
+        # ARM CRC Extension, with runtime check?
+        if test x"$pgac_armv8_crc32c_intrinsics" = x"yes"; then
           USE_ARMV8_CRC32C_WITH_RUNTIME_CHECK=1
         else
           # fall back to slicing-by-8 algorithm, which doesn't require any

src/include/pg_config.h.in

@@ -239,9 +239,6 @@
 /* Define to 1 if you have the `getaddrinfo' function. */
 #undef HAVE_GETADDRINFO
 
-/* Define to 1 if you have the `getauxval' function. */
-#undef HAVE_GETAUXVAL
-
 /* Define to 1 if you have the `gethostbyname_r' function. */
 #undef HAVE_GETHOSTBYNAME_R
 

src/port/pg_crc32c_armv8_choose.c

@@ -8,10 +8,6 @@
  * computation. Otherwise, fall back to the pure software implementation
  * (slicing-by-8).
  *
- * XXX: The glibc-specific getauxval() function, with the HWCAP_CRC32
- * flag, is used to determine if the CRC Extension is available on the
- * current platform. Is there a more portable way to determine that?
- *
  * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
@@ -24,17 +20,38 @@
 
 #include "c.h"
 
-#include <sys/auxv.h>
-#include <asm/hwcap.h>
+#include <setjmp.h>
 
+#include "libpq/pqsignal.h"
 #include "port/pg_crc32c.h"
 
+
+static sigjmp_buf illegal_instruction_jump;
+
+/*
+ * Probe by trying to execute pg_comp_crc32c_armv8().  If the instruction
+ * isn't available, we expect to get SIGILL, which we can trap.
+ */
+static void
+illegal_instruction_handler(int signo)
+{
+	siglongjmp(illegal_instruction_jump, 1);
+}
+
 static bool
 pg_crc32c_armv8_available(void)
 {
-	unsigned long auxv = getauxval(AT_HWCAP);
+	uint64		data = 42;
+	bool		result;
+
+	pqsignal(SIGILL, illegal_instruction_handler);
+	if (sigsetjmp(illegal_instruction_jump, 1) == 0)
+		result = (pg_comp_crc32c_armv8(0, &data, sizeof(data)) == 0xdd439b0d);
+	else
+		result = false;
+	pqsignal(SIGILL, SIG_DFL);
 
-	return (auxv & HWCAP_CRC32) != 0;
+	return result;
 }
 
 /*