From 826f83f1d562a7b878499bc3af2267cfdfe5f2f9 Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@gmail.com>
Date: Fri, 28 Apr 2017 15:07:10 +0200
Subject: bpo-30104: Only use -fno-strict-aliasing on dtoa.c (#1340)

On clang, only compile dtoa.c with -fno-strict-aliasing, use strict
aliasing to compile all other C files.
---
 Makefile.pre.in | 9 +++++++++
 1 file changed, 9 insertions(+)

(limited to 'Makefile.pre.in')

diff --git a/Makefile.pre.in b/Makefile.pre.in
index 4145634c03..e5141f11da 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -107,6 +107,8 @@ ARFLAGS=	@ARFLAGS@
 CFLAGSFORSHARED=@CFLAGSFORSHARED@
 # C flags used for building the interpreter object files
 PY_CORE_CFLAGS=	$(PY_CFLAGS) $(PY_CFLAGS_NODIST) $(PY_CPPFLAGS) $(CFLAGSFORSHARED) -DPy_BUILD_CORE
+# Strict or non-strict aliasing flags used to compile dtoa.c, see above
+CFLAGS_ALIASING=@CFLAGS_ALIASING@
 
 
 # Machine-dependent subdirectories
@@ -1536,6 +1538,13 @@ config.status:	$(srcdir)/configure
 .c.o:
 	$(CC) -c $(PY_CORE_CFLAGS) -o $@ $<
 
+# bpo-30104: dtoa.c uses union to cast double to unsigned long[2]. clang 4.0
+# with -O2 or higher and strict aliasing miscompiles the ratio() function
+# causing rounding issues. Compile dtoa.c using -fno-strict-aliasing on clang.
+# https://bugs.llvm.org//show_bug.cgi?id=31928
+Python/dtoa.o: Python/dtoa.c
+	$(CC) -c $(PY_CORE_CFLAGS) $(CFLAGS_ALIASING) -o $@ $<
+
 # Run reindent on the library
 reindent:
 	./$(BUILDPYTHON) $(srcdir)/Tools/scripts/reindent.py -r $(srcdir)/Lib
-- 
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