Avoid signed left shift undefined behaviour in sext_hwi

gcc/
	PR other/67042
	* hwint.h (sext_hwi): Switch to unsigned for the left shift, and
	conditionalize the whole on __GNUC__.  Add fallback code
	depending neither on undefined nor implementation-defined behaviour.

From-SVN: r227008

gcc/ChangeLog

+2015-08-19  Mikael Morin  <mikael@gcc.gnu.org>
+
+	PR other/67042
+	* hwint.h (sext_hwi): Switch to unsigned for the left shift, and
+	conditionalize the whole on __GNUC__.  Add fallback code
+	depending neither on undefined nor implementation-defined behaviour.
+
 2015-08-19  Jiong Wang  <jiong.wang@arm.com>
 
 	* config/aarch64/aarch64.c (aarch64_load_symref_appropriately): Replace
 	whitespaces with tab.
 
-2015-08-13  Florian Weimer  <fweimer@redhat.com>
+2015-08-19  Florian Weimer  <fweimer@redhat.com>
 
 	* prj.adb (For_Every_Project_Imported_Context.Recursive_Check_Context):
 	Move Name_Id_Set instantiation to the Prj package, to avoid trampolines.

gcc/hwint.h

@@ -244,11 +244,27 @@ sext_hwi (HOST_WIDE_INT src, unsigned int prec)
   if (prec == HOST_BITS_PER_WIDE_INT)
     return src;
   else
+#if defined (__GNUC__)
     {
+      /* Take the faster path if the implementation-defined bits it's relying
+	 on are implemented the way we expect them to be.  Namely, conversion
+	 from unsigned to signed preserves bit pattern, and right shift of
+	 a signed value propagates the sign bit.
+	 We have to convert from signed to unsigned and back, because when left
+	 shifting signed values, any overflow is undefined behaviour.  */
       gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT);
       int shift = HOST_BITS_PER_WIDE_INT - prec;
-      return (src << shift) >> shift;
+      return ((HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) src << shift)) >> shift;
     }
+#else
+    {
+      /* Fall back to the slower, well defined path otherwise.  */
+      gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT);
+      HOST_WIDE_INT sign_mask = HOST_WIDE_INT_1 << (prec - 1);
+      HOST_WIDE_INT value_mask = (HOST_WIDE_INT_1U << prec) - HOST_WIDE_INT_1U;
+      return (((src & value_mask) ^ sign_mask) - sign_mask);
+    }
+#endif
 }
 
 /* Zero extend SRC starting from PREC.  */