[Patch] sext_hwi: Avoid left shift of negative value undefined behaviour

[Mikael Morin]

Hello,

this avoids an error found with bootstrap-ubsan.
Regression tested on x86_64-unknown-linux-gnu.  OK for trunk?

Mikael


2015-08-05  Mikael Morin  <mik...@gcc.gnu.org>

        * hwint.h (sext_hwi): Rewrite without undefined behaviour on
        negative SRC.

diff --git a/gcc/hwint.h b/gcc/hwint.h
index 3793986..9c3eda0 100644
--- a/gcc/hwint.h
+++ b/gcc/hwint.h
@@ -246,8 +246,9 @@ sext_hwi (HOST_WIDE_INT src, unsigned int prec)
   else
     {
       gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT);
-      int shift = HOST_BITS_PER_WIDE_INT - prec;
-      return (src << shift) >> shift;
+      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);
     }
 }