[gcc r11-6823] c++: Avoid UB in signed shift [PR 98625]

[Nathan Sidwell]

https://gcc.gnu.org/g:911f797a9be2dc8ef59f5d5bd6d68baf650b8822

commit r11-6823-g911f797a9be2dc8ef59f5d5bd6d68baf650b8822
Author: Nathan Sidwell <nathan@acm.org>
Date:   Wed Jan 20 09:21:02 2021 -0800

    c++: Avoid UB in signed shift [PR 98625]
    
    I'd forgotten that left shifting a negative value is UB until C++20.
    Insert some casts to do unsigned shifts.
    
            PT c++/98625
            gcc/cp/
            * module.cc (bytes_in::i, bytes_in::wi): Avoid left shift of
            signed type.

Diff:
---
 gcc/cp/module.cc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 3b40c7ead05..7f88e3fea89 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -878,9 +878,12 @@ bytes_in::i ()
 	  v &= 0xf;
 	  if (v & 0x8)
 	    v |= -1 ^ 0x7;
+	  /* unsigned necessary due to left shifts of -ve values.  */
+	  unsigned uv = unsigned (v);
 	  if ((ptr = read (++bytes)))
 	    while (bytes--)
-	      v = (v << 8) | (*ptr++ & 0xff);
+	      uv = (uv << 8) | (*ptr++ & 0xff);
+	  v = int (uv);
 	}
       else if (v & 0x40)
 	v |= -1 ^ 0x3f;
@@ -969,9 +972,12 @@ bytes_in::wi ()
 	  v &= 0xf;
 	  if (v & 0x8)
 	    v |= -1 ^ 0x7;
+	  /* unsigned necessary due to left shifts of -ve values.  */
+	  unsigned HOST_WIDE_INT uv = (unsigned HOST_WIDE_INT) v;
 	  if ((ptr = read (++bytes)))
 	    while (bytes--)
-	      v = (v << 8) | (*ptr++ & 0xff);
+	      uv = (uv << 8) | (*ptr++ & 0xff);
+	  v = (HOST_WIDE_INT) uv;
 	}
       else if (v & 0x40)
 	v |= -1 ^ 0x3f;