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[C/C++ PATCH] Implement -Wbool-compare (PR c++/62153)


The testcase here
https://gcc.gnu.org/ml/gcc-patches/2014-08/msg01557.html
prodded me into implementing the -Wbool-compare warning, which
should pipe down such a code.  This warning detects dubious code
such as (i1 == i2) == 2 - which can never be true.  The implementation
was quite straightforward in the C front end, but in the C++ front
end I hit a problem with enums - that for e.g.
enum { A = 4 };
bool b;
b == A;
the b was coverted into (int) b before build_binary_op by convert_like,
so it wouldn't be possible to differentiate between the testcase above
and explicit cast of b to int.  So I had to handle this warning in
build_new_op_1, where I can see the original operands of the comparison.

Unfortunately, this warning cannot be enabled by -Wall yet, because
of a few blunders we have in the codebase.  But we really should iron out
that soon.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2014-08-18  Marek Polacek  <polacek@redhat.com>

	PR c++/62153
	* doc/invoke.texi: Document -Wbool-compare.
c-family/
	* c-common.c (maybe_warn_bool_compare): New function.
	* c-common.h (maybe_warn_bool_compare): Declare.
	* c.opt (Wbool-compare): New option.
c/
	* c-typeck.c (build_binary_op): If either operand of a comparison
	is a boolean expression, call maybe_warn_bool_compare.
cp/
	* call.c (build_new_op_1): Remember the type of arguments for
	a comparison.  If either operand of a comparison is a boolean
	expression, call maybe_warn_bool_compare.
testsuite/
	* c-c++-common/Wbool-compare-1.c: New test.

diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
index acc9a20..f221d80 100644
--- gcc/c-family/c-common.c
+++ gcc/c-family/c-common.c
@@ -11612,6 +11612,53 @@ maybe_warn_unused_local_typedefs (void)
   vec_free (l->local_typedefs);
 }
 
+/* Warn about boolean expression compared with an integer value different
+   from true/false.  Warns also e.g. about "(i1 == i2) == 2".
+   LOC is the location of the comparison, CODE is its code, OP0 and OP1
+   are the operands of the comparison.  The caller must ensure that
+   either operand is a boolean expression.  */
+
+void
+maybe_warn_bool_compare (location_t loc, enum tree_code code, tree op0,
+			 tree op1)
+{
+  if (TREE_CODE_CLASS (code) != tcc_comparison)
+    return;
+
+  /* <boolean> CMP <cst> */
+  if (TREE_CODE (op1) == INTEGER_CST
+      && !integer_zerop (op1)
+      && !integer_onep (op1))
+    {
+      if (code == EQ_EXPR
+	  || ((code == GT_EXPR || code == GE_EXPR)
+	      && tree_int_cst_sgn (op1) == 1)
+	  || ((code == LT_EXPR || code == LE_EXPR)
+	      && tree_int_cst_sgn (op1) == -1))
+	warning_at (loc, OPT_Wbool_compare, "comparison of constant %qE "
+		    "with boolean expression is always false", op1);
+      else
+	warning_at (loc, OPT_Wbool_compare, "comparison of constant %qE "
+		    "with boolean expression is always true", op1);
+    }
+  /* <cst> CMP <boolean> */
+  else if (TREE_CODE (op0) == INTEGER_CST
+	   && !integer_zerop (op0)
+	   && !integer_onep (op0))
+    {
+      if (code == EQ_EXPR
+	  || ((code == GT_EXPR || code == GE_EXPR)
+	      && tree_int_cst_sgn (op0) == -1)
+	  || ((code == LT_EXPR || code == LE_EXPR)
+	      && tree_int_cst_sgn (op0) == 1))
+	warning_at (loc, OPT_Wbool_compare, "comparison of constant %qE "
+		    "with boolean expression is always false", op0);
+      else
+	warning_at (loc, OPT_Wbool_compare, "comparison of constant %qE "
+		    "with boolean expression is always true", op0);
+    }
+}
+
 /* The C and C++ parsers both use vectors to hold function arguments.
    For efficiency, we keep a cache of unused vectors.  This is the
    cache.  */
diff --git gcc/c-family/c-common.h gcc/c-family/c-common.h
index 26aaee2..995bc8c 100644
--- gcc/c-family/c-common.h
+++ gcc/c-family/c-common.h
@@ -1015,6 +1015,7 @@ extern void record_types_used_by_current_var_decl (tree);
 extern void record_locally_defined_typedef (tree);
 extern void maybe_record_typedef_use (tree);
 extern void maybe_warn_unused_local_typedefs (void);
+extern void maybe_warn_bool_compare (location_t, enum tree_code, tree, tree);
 extern vec<tree, va_gc> *make_tree_vector (void);
 extern void release_tree_vector (vec<tree, va_gc> *);
 extern vec<tree, va_gc> *make_tree_vector_single (tree);
diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 356a79f..5a529b6 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -287,6 +287,10 @@ Wbad-function-cast
 C ObjC Var(warn_bad_function_cast) Warning
 Warn about casting functions to incompatible types
 
+Wbool-compare
+C ObjC C++ ObjC++ Var(warn_bool_compare) Warning
+Warn about boolean expression compared with an integer value different from true/false
+
 Wbuiltin-macro-redefined
 C ObjC C++ ObjC++ Warning
 Warn when a built-in preprocessor macro is undefined or redefined
diff --git gcc/c/c-typeck.c gcc/c/c-typeck.c
index 0ed92c6..8241b09 100644
--- gcc/c/c-typeck.c
+++ gcc/c/c-typeck.c
@@ -10673,6 +10673,11 @@ build_binary_op (location_t location, enum tree_code code,
 	  result_type = type1;
 	  pedwarn (location, 0, "comparison between pointer and integer");
 	}
+      if ((TREE_CODE (TREE_TYPE (orig_op0)) == BOOLEAN_TYPE
+	   || truth_value_p (TREE_CODE (orig_op0)))
+	  ^ (TREE_CODE (TREE_TYPE (orig_op1)) == BOOLEAN_TYPE
+	     || truth_value_p (TREE_CODE (orig_op1))))
+	maybe_warn_bool_compare (location, code, orig_op0, orig_op1);
       break;
 
     case LE_EXPR:
@@ -10777,6 +10782,11 @@ build_binary_op (location_t location, enum tree_code code,
 	  result_type = type1;
 	  pedwarn (location, 0, "comparison between pointer and integer");
 	}G
+      if ((TREE_CODE (TREE_TYPE (orig_op0)) == BOOLEAN_TYPE
+	   || truth_value_p (TREE_CODE (orig_op0)))
+	  ^ (TREE_CODE (TREE_TYPE (orig_op1)) == BOOLEAN_TYPE
+	     || truth_value_p (TREE_CODE (orig_op1))))
+	maybe_warn_bool_compare (location, code, orig_op0, orig_op1);
       break;
 
     default:
diff --git gcc/cp/call.c gcc/cp/call.c
index 7044e14..161235b 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -5318,7 +5318,17 @@ build_new_op_1 (location_t loc, enum tree_code code, int flags, tree arg1,
       /* These are saved for the sake of warn_logical_operator.  */
       code_orig_arg1 = TREE_CODE (arg1);
       code_orig_arg2 = TREE_CODE (arg2);
-
+      break;
+    case GT_EXPR:
+    case LT_EXPR:
+    case GE_EXPR:
+    case LE_EXPR:
+    case EQ_EXPR:
+    case NE_EXPR:
+      /* These are saved for the sake of maybe_warn_bool_compare.  */
+      code_orig_arg1 = TREE_CODE (TREE_TYPE (arg1));
+      code_orig_arg2 = TREE_CODE (TREE_TYPE (arg2));
+      break;
     default:
       break;
     }
@@ -5625,16 +5635,20 @@ build_new_op_1 (location_t loc, enum tree_code code, int flags, tree arg1,
       warn_logical_operator (loc, code, boolean_type_node,
 			     code_orig_arg1, arg1, code_orig_arg2, arg2);
       /* Fall through.  */
-    case PLUS_EXPR:
-    case MINUS_EXPR:
-    case MULT_EXPR:
-    case TRUNC_DIV_EXPR:
     case GT_EXPR:
     case LT_EXPR:
     case GE_EXPR:
     case LE_EXPR:
     case EQ_EXPR:
     case NE_EXPR:
+      if ((code_orig_arg1 == BOOLEAN_TYPE)
+	  ^ (code_orig_arg2 == BOOLEAN_TYPE))
+	maybe_warn_bool_compare (loc, code, arg1, arg2);
+      /* Fall through.  */
+    case PLUS_EXPR:
+    case MINUS_EXPR:
+    case MULT_EXPR:
+    case TRUNC_DIV_EXPR:
     case MAX_EXPR:
     case MIN_EXPR:
     case LSHIFT_EXPR:
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index 6374261..bafd606 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -240,6 +240,7 @@ Objective-C and Objective-C++ Dialects}.
 -pedantic-errors @gol
 -w  -Wextra  -Wall  -Waddress  -Waggregate-return  @gol
 -Waggressive-loop-optimizations -Warray-bounds @gol
+-Wbool-compare @gol
 -Wno-attributes -Wno-builtin-macro-redefined @gol
 -Wc90-c99-compat @gol
 -Wc++-compat -Wc++11-compat -Wcast-align  -Wcast-qual  @gol
@@ -4221,6 +4222,18 @@ This option is only active when @option{-ftree-vrp} is active
 (default for @option{-O2} and above). It warns about subscripts to arrays
 that are always out of bounds. This warning is enabled by @option{-Wall}.
 
+@item -Wbool-compare
+@opindex Wno-bool-compare
+@opindex Wbool-compare
+Warn about boolean expression compared with an integer value different from
+@code{true}/@code{false}.  For instance, the following comparison is
+always false:
+@smallexample
+int n = 5;
+@dots{}
+if ((n > 1) == 2) @{ @dots{} @}
+@end smallexample
+
 @item -Wno-discarded-qualifiers @r{(C and Objective-C only)}
 @opindex Wno-discarded-qualifiers
 @opindex Wdiscarded-qualifiers
diff --git gcc/testsuite/c-c++-common/Wbool-compare-1.c gcc/testsuite/c-c++-common/Wbool-compare-1.c
index e69de29..52adcea 100644
--- gcc/testsuite/c-c++-common/Wbool-compare-1.c
+++ gcc/testsuite/c-c++-common/Wbool-compare-1.c
@@ -0,0 +1,128 @@
+/* PR c++/62153 */
+/* { dg-do compile } */
+/* { dg-options "-Wbool-compare" } */
+
+#ifndef __cplusplus
+# define bool _Bool
+# define true 1
+# define false 0
+#endif
+
+extern bool foo (void);
+bool r;
+
+enum { E = 4 };
+
+void
+fn1 (bool b)
+{
+  r = b == 2; /* { dg-warning "with boolean expression is always false" } */
+  r = b != 2; /* { dg-warning "with boolean expression is always true" } */
+  r = b < 2; /* { dg-warning "with boolean expression is always true" } */
+  r = b > 2; /* { dg-warning "with boolean expression is always false" } */
+  r = b <= 2; /* { dg-warning "with boolean expression is always true" } */
+  r = b >= 2; /* { dg-warning "with boolean expression is always false" } */
+
+  r = b == -1; /* { dg-warning "with boolean expression is always false" } */
+  r = b != -1; /* { dg-warning "with boolean expression is always true" } */
+  r = b < -1; /* { dg-warning "with boolean expression is always false" } */
+  r = b > -1; /* { dg-warning "with boolean expression is always true" } */
+  r = b <= -1; /* { dg-warning "with boolean expression is always false" } */
+  r = b >= -1; /* { dg-warning "with boolean expression is always true" } */
+
+  r = foo () == 2; /* { dg-warning "with boolean expression is always false" } */
+  r = foo () != 2; /* { dg-warning "with boolean expression is always true" } */
+  r = foo () < 2; /* { dg-warning "with boolean expression is always true" } */
+  r = foo () > 2; /* { dg-warning "with boolean expression is always false" } */
+  r = foo () <= 2; /* { dg-warning "with boolean expression is always true" } */
+  r = foo () >= 2; /* { dg-warning "with boolean expression is always false" } */
+
+  r = b == E; /* { dg-warning "with boolean expression is always false" } */
+  r = b != E; /* { dg-warning "with boolean expression is always true" } */
+  r = b < E; /* { dg-warning "with boolean expression is always true" } */
+  r = b > E; /* { dg-warning "with boolean expression is always false" } */
+  r = b <= E; /* { dg-warning "with boolean expression is always true" } */
+  r = b >= E; /* { dg-warning "with boolean expression is always false" } */
+
+  /* Swap LHS and RHS.  */
+  r = 2 == b; /* { dg-warning "with boolean expression is always false" } */
+  r = 2 != b; /* { dg-warning "with boolean expression is always true" } */
+  r = 2 < b; /* { dg-warning "with boolean expression is always false" } */
+  r = 2 > b; /* { dg-warning "with boolean expression is always true" } */
+  r = 2 <= b; /* { dg-warning "with boolean expression is always false" } */
+  r = 2 >= b; /* { dg-warning "with boolean expression is always true" } */
+
+  r = -1 == b; /* { dg-warning "with boolean expression is always false" } */
+  r = -1 != b; /* { dg-warning "with boolean expression is always true" } */
+  r = -1 < b; /* { dg-warning "with boolean expression is always true" } */
+  r = -1 > b; /* { dg-warning "with boolean expression is always false" } */
+  r = -1 <= b; /* { dg-warning "with boolean expression is always true" } */
+  r = -1 >= b; /* { dg-warning "with boolean expression is always false" } */
+
+  r = E == b; /* { dg-warning "with boolean expression is always false" } */
+  r = E != b; /* { dg-warning "with boolean expression is always true" } */
+  r = E < b; /* { dg-warning "with boolean expression is always false" } */
+  r = E > b; /* { dg-warning "with boolean expression is always true" } */
+  r = E <= b; /* { dg-warning "with boolean expression is always false" } */
+  r = E >= b; /* { dg-warning "with boolean expression is always true" } */
+
+  /* These are of course fine.  */
+  r = b == false;
+  r = b != false;
+  r = b == true;
+  r = b != true;
+
+  /* Some of these don't make much sense, but we don't warn.  */
+  r = b < false;
+  r = b >= false;
+  r = b <= false;
+  r = b > false;
+  r = b < true;
+  r = b >= true;
+  r = b <= true;
+  r = b > true;
+}
+
+void
+fn2 (int i1, int i2)
+{
+  r = (i1 == i2) == 2; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) != 2; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) < 2; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) > 2; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) <= 2; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) >= 2; /* { dg-warning "with boolean expression is always false" } */
+
+  r = (i1 == i2) == -1; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) != -1; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) < -1; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) > -1; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) <= -1; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) >= -1; /* { dg-warning "with boolean expression is always true" } */
+
+  r = (i1 == i2) == E; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) != E; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) < E; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) > E; /* { dg-warning "with boolean expression is always false" } */
+  r = (i1 == i2) <= E; /* { dg-warning "with boolean expression is always true" } */
+  r = (i1 == i2) >= E; /* { dg-warning "with boolean expression is always false" } */
+}
+
+void
+fn3 (int n, bool b)
+{
+  /* Don't warn here.  */
+  r = b == n;
+  r = b != n;
+  r = b < n;
+  r = b > n;
+  r = b <= n;
+  r = b >= n;
+
+  r = n == E;
+  r = n != E;
+  r = n < E;
+  r = n > E;
+  r = n <= E;
+  r = n >= E;
+}

	Marek


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