Error out for clang on x86-32 without SSE2 support, no -fexcess-precision.

anarazel · anarazel · commit bd1463e348fc · 2018-09-20T17:39:40.000-07:00
As clang currently doesn't support -fexcess-precision=standard, compiling x86-32 code with SSE2 disabled, can lead to problems with floating point overflow checks and the like. This issue was noticed because clang, on at least some BSDs, defaults to i386 compatibility, whereas it defaults to pentium4 on Linux. Our forced usage of __builtin_isinf() lead to some overflow checks not triggering when compiling for i386, e.g. when the result of the calculation didn't overflow in 80bit registers, but did so in 64bit. While we could just fall back to a non-builtin isinf, it seems likely that the use of 80bit registers leads to other problems (which is why we force the flag for GCC already). Therefore error out when detecting clang in that situation. Reported-By: Victor Wagner Analyzed-By: Andrew Gierth and Andres Freund Author: Andres Freund Discussion: https://postgr.es/m/20180905005130.ewk4xcs5dgyzcy45@alap3.anarazel.de Backpatch: 9.3-, all supported versions are affected
diff --git a/configure b/configure
@@ -7023,6 +7023,39 @@ fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 fi
 
+# Defend against clang being used on x86-32 without SSE2 enabled.  As current
+# versions of clang do not understand -fexcess-precision=standard, the use of
+# x87 floating point operations leads to problems like isinf possibly returning
+# false for a value that is infinite when converted from the 80bit register to
+# the 8byte memory representation.
+#
+# Only perform the test if the compiler doesn't understand
+# -fexcess-precision=standard, that way a potentially fixed compiler will work
+# automatically.
+if test "$pgac_cv_prog_CC_cflags__fexcess_precision_standard" = no; then
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+#if defined(__clang__) && defined(__i386__) && !defined(__SSE2_MATH__)
+choke me
+#endif
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+else
+  as_fn_error $? "Compiling PostgreSQL with clang, on 32bit x86, requires SSE2 support. Use -msse2 or use gcc." "$LINENO" 5
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
diff --git a/configure.in b/configure.in
@@ -624,6 +624,24 @@ choke me
 @%:@endif])], [], [AC_MSG_ERROR([do not put -ffast-math in CFLAGS])])
 fi
 
+# Defend against clang being used on x86-32 without SSE2 enabled.  As current
+# versions of clang do not understand -fexcess-precision=standard, the use of
+# x87 floating point operations leads to problems like isinf possibly returning
+# false for a value that is infinite when converted from the 80bit register to
+# the 8byte memory representation.
+#
+# Only perform the test if the compiler doesn't understand
+# -fexcess-precision=standard, that way a potentially fixed compiler will work
+# automatically.
+if test "$pgac_cv_prog_CC_cflags__fexcess_precision_standard" = no; then
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [
+@%:@if defined(__clang__) && defined(__i386__) && !defined(__SSE2_MATH__)
+choke me
+@%:@endif
+])], [],
+[AC_MSG_ERROR([Compiling PostgreSQL with clang, on 32bit x86, requires SSE2 support. Use -msse2 or use gcc.])])
+fi
+
 AC_PROG_CPP
 AC_SUBST(GCC)
 
