From 28205b203a4742c40080b4a2b4b2dcd800716edc Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Fri, 21 Apr 2017 11:24:34 +0200
Subject: [PATCH] bpo-30104: Use -fno-strict-aliasing on clang (#1221)

Python/dtoa.c is not compiled correctly with clang 4.0 and
optimization level -O2 or higher, because of an aliasing issue on
the double/ULong[2] union.

LLVM bug report:
https://bugs.llvm.org//show_bug.cgi?id=31928
---
 configure    | 22 ++++++++++++++++------
 configure.ac | 22 ++++++++++++++++------
 2 files changed, 32 insertions(+), 12 deletions(-)

diff --git a/configure b/configure
index b4dc5f13288..40e64adc8a8 100755
--- a/configure
+++ b/configure
@@ -6814,8 +6814,15 @@ then
 
         # Clang also needs -fwrapv
         case $CC in
-            *clang*) WRAP="-fwrapv"
-            ;;
+            *clang*)
+                WRAP="-fwrapv"
+                # bpo-30104: Python/dtoa.c requires to be build with
+                # -fno-strict-aliasing to fix compiler issue on the
+                # double/ULong[2] union using clang 4.0 and optimization level
+                # -O2 or higher
+                # https://bugs.llvm.org//show_bug.cgi?id=31928
+                ALIASING="-fno-strict-aliasing"
+                ;;
         esac
 
 	case $ac_cv_prog_cc_g in
@@ -6824,18 +6831,21 @@ then
 		# Optimization messes up debuggers, so turn it off for
 		# debug builds.
                 if "$CC" -v --help 2>/dev/null |grep -- -Og > /dev/null; then
-                    OPT="-g -Og -Wall $STRICT_PROTO"
+                    OPT="-g -Og -Wall"
                 else
-                    OPT="-g -O0 -Wall $STRICT_PROTO"
+                    OPT="-g -O0 -Wall"
                 fi
 	    else
-		OPT="-g $WRAP -O3 -Wall $STRICT_PROTO"
+		OPT="-g $WRAP -O3 -Wall"
 	    fi
 	    ;;
 	*)
-	    OPT="-O3 -Wall $STRICT_PROTO"
+	    OPT="-O3 -Wall"
 	    ;;
 	esac
+
+	OPT="$OPT $STRICT_PROTO $ALIASING"
+
 	case $ac_sys_system in
 	    SCO_SV*) OPT="$OPT -m486 -DSCO5"
 	    ;;
diff --git a/configure.ac b/configure.ac
index d7316b9fe85..cb79ea891f4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1454,8 +1454,15 @@ then
 
         # Clang also needs -fwrapv
         case $CC in
-            *clang*) WRAP="-fwrapv"
-            ;;
+            *clang*)
+                WRAP="-fwrapv"
+                # bpo-30104: Python/dtoa.c requires to be build with
+                # -fno-strict-aliasing to fix compiler issue on the
+                # double/ULong[2] union using clang 4.0 and optimization level
+                # -O2 or higher
+                # https://bugs.llvm.org//show_bug.cgi?id=31928
+                ALIASING="-fno-strict-aliasing"
+                ;;
         esac
 
 	case $ac_cv_prog_cc_g in
@@ -1464,18 +1471,21 @@ then
 		# Optimization messes up debuggers, so turn it off for
 		# debug builds.
                 if "$CC" -v --help 2>/dev/null |grep -- -Og > /dev/null; then
-                    OPT="-g -Og -Wall $STRICT_PROTO"
+                    OPT="-g -Og -Wall"
                 else
-                    OPT="-g -O0 -Wall $STRICT_PROTO"
+                    OPT="-g -O0 -Wall"
                 fi
 	    else
-		OPT="-g $WRAP -O3 -Wall $STRICT_PROTO"
+		OPT="-g $WRAP -O3 -Wall"
 	    fi
 	    ;;
 	*)
-	    OPT="-O3 -Wall $STRICT_PROTO"
+	    OPT="-O3 -Wall"
 	    ;;
 	esac
+
+	OPT="$OPT $STRICT_PROTO $ALIASING"
+
 	case $ac_sys_system in
 	    SCO_SV*) OPT="$OPT -m486 -DSCO5"
 	    ;;