[RFC PATCH] Do not sanitize left shifts for -fwrapv

Paolo Bonzini bonzini@gnu.org
Tue Nov 17 13:33:00 GMT 2015

Left shifts into the sign bit is a kind of overflow, and the
standard chooses to treat left shifts of negative values the
same way.

However, the -fwrapv option modifies the language to one where
integers are defined as two's complement---which also defines
entirely the behavior of shifts.  Disable sanitization of left
shifts when -fwrapv is in effect.

This needs test cases of course, but I wanted to be sure in advance
whether this is an acceptable change and whether it is considered
a bug (thus acceptable for stage 3).  The same change was proposed
for LLVM at https://llvm.org/bugs/show_bug.cgi?id=25552.


* c-family/c-ubsan.c (ubsan_instrument_shift): Disable sanitization
of left shifts for wrapping signed types as well.

Index: c-family/c-ubsan.c
--- c-family/c-ubsan.c	(revision 227511)
+++ c-family/c-ubsan.c	(working copy)
@@ -150,7 +150,7 @@
      (unsigned) x >> (uprecm1 - y)
      if non-zero, is undefined.  */
   if (code == LSHIFT_EXPR
-      && !TYPE_UNSIGNED (type0)
+      && !TYPE_OVERFLOW_WRAPS (type0)
       && flag_isoc99)
       tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1,
@@ -165,7 +165,7 @@
      x < 0 || ((unsigned) x >> (uprecm1 - y))
      if > 1, is undefined.  */
   if (code == LSHIFT_EXPR
-      && !TYPE_UNSIGNED (type0)
+      && !TYPE_OVERFLOW_WRAPS (type0)
       && (cxx_dialect >= cxx11))
       tree x = fold_build2 (MINUS_EXPR, op1_utype, uprecm1,

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