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[RFC PATCH] Do not sanitize left shifts for -fwrapv
- From: Paolo Bonzini <bonzini at gnu dot org>
- To: gcc-patches at gcc dot gnu dot org, joseph at codesourcery dot com, mpolacek at redhat dot com
- Date: Tue, 17 Nov 2015 14:32:50 +0100
- Subject: [RFC PATCH] Do not sanitize left shifts for -fwrapv
- Authentication-results: sourceware.org; auth=none
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.
Paolo
* 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,