[PATCH] Do not sanitize left shifts for -fwrapv (PR68418)

[Paolo Bonzini]

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.  The same change was proposed
for LLVM at https://llvm.org/bugs/show_bug.cgi?id=25552.

Bootstrapped/regtested x86_64-pc-linux-gnu.  Ok for trunk, and for
GCC 5 branch after 5.3 is released?

Thanks,

Paolo

gcc:
	PR sanitizer/68418
	* c-family/c-ubsan.c (ubsan_instrument_shift): Disable
	sanitization of left shifts for wrapping signed types as well.

gcc/testsuite:
	PR sanitizer/68418
	* gcc.dg/ubsan/c99-wrapv-shift-1.c,
	gcc.dg/ubsan/c99-wrapv-shift-2.c: New testcases.

Index: c-family/c-ubsan.c
===================================================================
--- c-family/c-ubsan.c	(revision 230466)
+++ c-family/c-ubsan.c	(working copy)
@@ -128,7 +128,7 @@
      (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,
@@ -143,7 +143,7 @@
      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,
Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c
===================================================================
--- testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c	(revision 0)
+++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-1.c	(working copy)
@@ -0,0 +1,9 @@
+/* { dg-do run } */
+/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */
+
+int
+main (void)
+{
+  int a = -42;
+  a << 1;
+}
Index: testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c
===================================================================
--- testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c	(revision 0)
+++ testsuite/gcc.dg/ubsan/c99-wrapv-shift-2.c	(working copy)
@@ -0,0 +1,9 @@
+/* { dg-do run } */
+/* { dg-options "-fsanitize=shift -fwrapv -w -std=c99" } */
+
+int
+main (void)
+{
+  int a = 1;
+  a <<= 31;
+}