On Sat, Apr 25, 2015 at 08:13:08PM +0000, Joseph Myers wrote:
> On Sat, 25 Apr 2015, Marek Polacek wrote:
>
> > + pedwarn (location, OPT_Wshift_negative_value,
> > + "left shift of negative value");
>
> Use of pedwarn is always suspect for something only undefined at runtime;
> it must not produce an error with -pedantic-errors in any context where a
> constant expression is not required.
Makes sense. So how about moving the warning into -Wextra? This way it won't
trigger by default. One change is that we reject programs that use shift with
undefined behavior in a context where a constant expression is required, thus
e.g. enum E { A = -1 << 0 };
But I hope that's reasonable.
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2015-04-27 Marek Polacek <pola...@redhat.com>
PR c/65179
* c-common.c (c_fully_fold_internal): Warn when left shifting a
negative value.
* c.opt (Wshift-negative-value): New option.
* c-typeck.c (build_binary_op): Warn when left shifting a negative
value.
* typeck.c (cp_build_binary_op): Warn when left shifting a negative
value.
* doc/invoke.texi: Document -Wshift-negative-value.
* c-c++-common/Wshift-negative-value-1.c: New test.
* c-c++-common/Wshift-negative-value-2.c: New test.
* c-c++-common/Wshift-negative-value-3.c: New test.
* c-c++-common/Wshift-negative-value-4.c: New test.
* gcc.dg/c99-const-expr-7.c: Add dg-error.
diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
index 9797e17..f2fe65e 100644
--- gcc/c-family/c-common.c
+++ gcc/c-family/c-common.c
@@ -1361,6 +1361,15 @@ c_fully_fold_internal (tree expr, bool in_init, bool
*maybe_const_operands,
&& !TREE_OVERFLOW_P (op0)
&& !TREE_OVERFLOW_P (op1))
overflow_warning (EXPR_LOCATION (expr), ret);
+ if (code == LSHIFT_EXPR
+ && TREE_CODE (orig_op0) != INTEGER_CST
+ && TREE_CODE (TREE_TYPE (orig_op0)) == INTEGER_TYPE
+ && TREE_CODE (op0) == INTEGER_CST
+ && c_inhibit_evaluation_warnings == 0
+ && tree_int_cst_sgn (op0) < 0
+ && flag_isoc99)
+ warning_at (loc, OPT_Wshift_negative_value,
+ "left shift of negative value");
if ((code == LSHIFT_EXPR || code == RSHIFT_EXPR)
&& TREE_CODE (orig_op1) != INTEGER_CST
&& TREE_CODE (op1) == INTEGER_CST
diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 983f4a8..c61dfb1 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -781,6 +781,10 @@ Wshift-count-overflow
C ObjC C++ ObjC++ Var(warn_shift_count_overflow) Init(1) Warning
Warn if shift count >= width of type
+Wshift-negative-value
+C ObjC C++ ObjC++ Var(warn_shift_negative_value) Warning EnabledBy(Wextra)
+Warn if left shifting a negative value
+
Wsign-compare
C ObjC C++ ObjC++ Var(warn_sign_compare) Warning LangEnabledBy(C++ ObjC++,Wall)
Warn about signed-unsigned comparisons
diff --git gcc/c/c-typeck.c gcc/c/c-typeck.c
index 91735b5..36cebc6 100644
--- gcc/c/c-typeck.c
+++ gcc/c/c-typeck.c
@@ -10707,6 +10707,15 @@ build_binary_op (location_t location, enum tree_code
code,
&& code1 == INTEGER_TYPE)
{
doing_shift = true;
+ if (TREE_CODE (op0) == INTEGER_CST
+ && tree_int_cst_sgn (op0) < 0
+ && flag_isoc99)
+ {
+ int_const = false;
+ if (c_inhibit_evaluation_warnings == 0)
+ warning_at (location, OPT_Wshift_negative_value,
+ "left shift of negative value");
+ }
if (TREE_CODE (op1) == INTEGER_CST)
{
if (tree_int_cst_sgn (op1) < 0)
diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index 91db32a..3cb5a8a 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -4326,11 +4326,21 @@ cp_build_binary_op (location_t location,
}
else if (code0 == INTEGER_TYPE && code1 == INTEGER_TYPE)
{
+ tree const_op0 = fold_non_dependent_expr (op0);
+ if (TREE_CODE (const_op0) != INTEGER_CST)
+ const_op0 = op0;
tree const_op1 = fold_non_dependent_expr (op1);
if (TREE_CODE (const_op1) != INTEGER_CST)
const_op1 = op1;
result_type = type0;
doing_shift = true;
+ if (TREE_CODE (const_op0) == INTEGER_CST
+ && tree_int_cst_sgn (const_op0) < 0
+ && (complain & tf_warning)
+ && c_inhibit_evaluation_warnings == 0
+ && cxx_dialect >= cxx11)
+ warning (OPT_Wshift_negative_value,
+ "left shift of negative value");
if (TREE_CODE (const_op1) == INTEGER_CST)
{
if (tree_int_cst_lt (const_op1, integer_zero_node))
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index 7d2f6e5..dcfe4cf 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -271,7 +271,7 @@ Objective-C and Objective-C++ Dialects}.
-Wpointer-arith -Wno-pointer-to-int-cast @gol
-Wredundant-decls -Wno-return-local-addr @gol
-Wreturn-type -Wsequence-point -Wshadow -Wno-shadow-ivar @gol
--Wshift-count-negative -Wshift-count-overflow @gol
+-Wshift-count-negative -Wshift-count-overflow -Wshift-negative-value @gol
-Wsign-compare -Wsign-conversion -Wfloat-conversion @gol
-Wsizeof-pointer-memaccess -Wsizeof-array-argument @gol
-Wstack-protector -Wstack-usage=@var{len} -Wstrict-aliasing @gol
@@ -3481,6 +3481,7 @@ name is still supported, but the newer name is more
descriptive.)
-Wsign-compare @gol
-Wtype-limits @gol
-Wuninitialized @gol
+-Wshift-negative-value @gol
-Wunused-parameter @r{(only with} @option{-Wunused} @r{or} @option{-Wall}@r{)}
@gol
-Wunused-but-set-parameter @r{(only with} @option{-Wunused} @r{or}
@option{-Wall}@r{)} @gol
}
@@ -3914,6 +3915,12 @@ Warn if shift count is negative. This warning is enabled
by default.
@opindex Wno-shift-count-overflow
Warn if shift count >= width of type. This warning is enabled by default.
+@item -Wshift-negative-value
+@opindex Wshift-negative-value
+@opindex Wno-shift-negative-value
+Warn if left shifting a negative value. This warning only occurs in C99 and
+C++11 modes (and newer). This warning is also enabled by @option{-Wextra}.
+
@item -Wswitch
@opindex Wswitch
@opindex Wno-switch
diff --git gcc/testsuite/c-c++-common/Wshift-negative-value-1.c
gcc/testsuite/c-c++-common/Wshift-negative-value-1.c
index e69de29..5d803ad 100644
--- gcc/testsuite/c-c++-common/Wshift-negative-value-1.c
+++ gcc/testsuite/c-c++-common/Wshift-negative-value-1.c
@@ -0,0 +1,49 @@
+/* PR c/65179 */
+/* { dg-do compile } */
+/* { dg-options "-O -Wextra" } */
+/* { dg-additional-options "-std=c++11" { target c++ } } */
+
+enum E {
+ A = 0 << 1,
+ B = 1 << 1,
+ C = -1 << 1, /* { dg-warning "left shift of negative value|not an integer
constant" } */
+ D = 0 >> 1,
+ E = 1 >> 1,
+ F = -1 >> 1
+};
+
+int
+left (int x)
+{
+ /* Warn for LSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z << x;
+ r += o << x;
+ r += m << x; /* { dg-warning "left shift of negative value" } */
+ r += 0 << x;
+ r += 1 << x;
+ r += -1 << x; /* { dg-warning "left shift of negative value" } */
+ r += -1U << x;
+ return r;
+}
+
+int
+right (int x)
+{
+ /* Shouldn't warn for RSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z >> x;
+ r += o >> x;
+ r += m >> x;
+ r += 0 >> x;
+ r += 1 >> x;
+ r += -1 >> x;
+ r += -1U >> x;
+ return r;
+}
diff --git gcc/testsuite/c-c++-common/Wshift-negative-value-2.c
gcc/testsuite/c-c++-common/Wshift-negative-value-2.c
index e69de29..fc89af1 100644
--- gcc/testsuite/c-c++-common/Wshift-negative-value-2.c
+++ gcc/testsuite/c-c++-common/Wshift-negative-value-2.c
@@ -0,0 +1,49 @@
+/* PR c/65179 */
+/* { dg-do compile } */
+/* { dg-options "-O -Wshift-negative-value" } */
+/* { dg-additional-options "-std=c++11" { target c++ } } */
+
+enum E {
+ A = 0 << 1,
+ B = 1 << 1,
+ C = -1 << 1, /* { dg-warning "left shift of negative value" } */
+ D = 0 >> 1,
+ E = 1 >> 1,
+ F = -1 >> 1
+};
+
+int
+left (int x)
+{
+ /* Warn for LSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z << x;
+ r += o << x;
+ r += m << x; /* { dg-warning "left shift of negative value" } */
+ r += 0 << x;
+ r += 1 << x;
+ r += -1 << x; /* { dg-warning "left shift of negative value" } */
+ r += -1U << x;
+ return r;
+}
+
+int
+right (int x)
+{
+ /* Shouldn't warn for RSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z >> x;
+ r += o >> x;
+ r += m >> x;
+ r += 0 >> x;
+ r += 1 >> x;
+ r += -1 >> x;
+ r += -1U >> x;
+ return r;
+}
diff --git gcc/testsuite/c-c++-common/Wshift-negative-value-3.c
gcc/testsuite/c-c++-common/Wshift-negative-value-3.c
index e69de29..bf9b1a0 100644
--- gcc/testsuite/c-c++-common/Wshift-negative-value-3.c
+++ gcc/testsuite/c-c++-common/Wshift-negative-value-3.c
@@ -0,0 +1,49 @@
+/* PR c/65179 */
+/* { dg-do compile } */
+/* { dg-options "-O -Wextra -Wno-shift-negative-value" } */
+/* { dg-additional-options "-std=c++11" { target c++ } } */
+
+enum E {
+ A = 0 << 1,
+ B = 1 << 1,
+ C = -1 << 1,
+ D = 0 >> 1,
+ E = 1 >> 1,
+ F = -1 >> 1
+};
+
+int
+left (int x)
+{
+ /* Warn for LSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z << x;
+ r += o << x;
+ r += m << x; /* { dg-bogus "left shift of negative value" } */
+ r += 0 << x;
+ r += 1 << x;
+ r += -1 << x; /* { dg-bogus "left shift of negative value" } */
+ r += -1U << x;
+ return r;
+}
+
+int
+right (int x)
+{
+ /* Shouldn't warn for RSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z >> x;
+ r += o >> x;
+ r += m >> x;
+ r += 0 >> x;
+ r += 1 >> x;
+ r += -1 >> x;
+ r += -1U >> x;
+ return r;
+}
diff --git gcc/testsuite/c-c++-common/Wshift-negative-value-4.c
gcc/testsuite/c-c++-common/Wshift-negative-value-4.c
index e69de29..85fbd0e 100644
--- gcc/testsuite/c-c++-common/Wshift-negative-value-4.c
+++ gcc/testsuite/c-c++-common/Wshift-negative-value-4.c
@@ -0,0 +1,49 @@
+/* PR c/65179 */
+/* { dg-do compile } */
+/* { dg-options "-O" } */
+/* { dg-additional-options "-std=c++11" { target c++ } } */
+
+enum E {
+ A = 0 << 1,
+ B = 1 << 1,
+ C = -1 << 1,
+ D = 0 >> 1,
+ E = 1 >> 1,
+ F = -1 >> 1
+};
+
+int
+left (int x)
+{
+ /* Warn for LSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z << x;
+ r += o << x;
+ r += m << x; /* { dg-bogus "left shift of negative value" } */
+ r += 0 << x;
+ r += 1 << x;
+ r += -1 << x; /* { dg-bogus "left shift of negative value" } */
+ r += -1U << x;
+ return r;
+}
+
+int
+right (int x)
+{
+ /* Shouldn't warn for RSHIFT_EXPR. */
+ const int z = 0;
+ const int o = 1;
+ const int m = -1;
+ int r = 0;
+ r += z >> x;
+ r += o >> x;
+ r += m >> x;
+ r += 0 >> x;
+ r += 1 >> x;
+ r += -1 >> x;
+ r += -1U >> x;
+ return r;
+}
diff --git gcc/testsuite/gcc.dg/c99-const-expr-7.c
gcc/testsuite/gcc.dg/c99-const-expr-7.c
index b872077..75b0567 100644
--- gcc/testsuite/gcc.dg/c99-const-expr-7.c
+++ gcc/testsuite/gcc.dg/c99-const-expr-7.c
@@ -30,8 +30,8 @@ int f1 = (0 ? 0 << -1 : 0);
int g1 = (0 ? 0 >> 1000 : 0);
int h1 = (0 ? 0 >> -1: 0);
-/* Allowed for now, but actually undefined behavior in C99. */
int i = -1 << 0;
+/* { dg-error "constant" "constant" { target *-*-* } 33 } */
int j[1] = { DBL_MAX }; /* { dg-warning "overflow in implicit constant
conversion" } */
/* { dg-error "overflow in constant expression" "constant" { target *-*-* } 36
} */
Marek
