[PATCH] Add a warning for suspicious use of conditional expressions in boolean context

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

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Hi!

As reported in PR77434 and PR77421 there should be a warning for
suspicious uses of conditional expressions with non-boolean arguments.

This warning triggers on conditional expressions in boolean context,
when both possible results are non-zero integer constants, so that
the resulting truth value does in fact not depend on the condition
itself.  Thus something like "if (a == b ? 1 : 2)" is always bogus,
and was most likely meant to be "if (a == (b ? 1 : 2))".


Boot-strap and reg-testing on x86_64-pc-linux-gnu without regressions.
Is it OK for trunk.


Thanks
Bernd.

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gcc:
2016-09-02  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	PR c++/77434
	* doc/invoke.texi: Document -Wcond-in-bool-context.

	PR middle-end/77421
	* dwarf2out.c (output_loc_operands): Fix assertion.

c-family:
2016-09-02  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	PR c++/77434
	* c.opt (Wcond-in-bool-context): New warning.
	* c-common.c (c_common_truthvalue_conversion): Warn on integer
	constants in boolean context.

testsuite:
2016-09-02  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	PR c++/77434
	* c-c++-common/Wcond-in-bool-context.c: New test.

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Index: gcc/c-family/c-common.c
===================================================================
--- gcc/c-family/c-common.c	(revision 239953)
+++ gcc/c-family/c-common.c	(working copy)
@@ -4618,6 +4618,14 @@ c_common_truthvalue_conversion (location_t locatio
 					       TREE_OPERAND (expr, 0));
 
     case COND_EXPR:
+      if (TREE_CODE (TREE_OPERAND (expr, 1)) == INTEGER_CST
+	  && TREE_CODE (TREE_OPERAND (expr, 2)) == INTEGER_CST
+	  && !integer_zerop (TREE_OPERAND (expr, 1))
+	  && !integer_zerop (TREE_OPERAND (expr, 2))
+	  && (!integer_onep (TREE_OPERAND (expr, 1))
+	      || !integer_onep (TREE_OPERAND (expr, 2))))
+	warning_at (EXPR_LOCATION (expr), OPT_Wcond_in_bool_context,
+		    "?: using integer constants in boolean context");
       /* Distribute the conversion into the arms of a COND_EXPR.  */
       if (c_dialect_cxx ())
 	{
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt	(revision 239953)
+++ gcc/c-family/c.opt	(working copy)
@@ -350,6 +350,10 @@ Wcomments
 C ObjC C++ ObjC++ Warning Alias(Wcomment)
 Synonym for -Wcomment.
 
+Wcond-in-bool-context
+C ObjC C++ ObjC++ Var(warn_cond_in_bool_context) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
+Warn for conditional expressions (?:) using integer constants in boolean context.
+
 Wconditionally-supported
 C++ ObjC++ Var(warn_conditionally_supported) Warning
 Warn for conditionally-supported constructs.
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 239953)
+++ gcc/doc/invoke.texi	(working copy)
@@ -259,7 +259,8 @@ Objective-C and Objective-C++ Dialects}.
 -Wno-attributes -Wbool-compare -Wno-builtin-macro-redefined @gol
 -Wc90-c99-compat -Wc99-c11-compat @gol
 -Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align  -Wcast-qual  @gol
--Wchar-subscripts -Wclobbered  -Wcomment -Wconditionally-supported  @gol
+-Wchar-subscripts -Wclobbered -Wcomment @gol
+-Wcond-in-bool-context -Wconditionally-supported  @gol
 -Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time @gol
 -Wdelete-incomplete @gol
 -Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init @gol
@@ -5179,6 +5180,13 @@ programs.
 Warn for variables that might be changed by @code{longjmp} or
 @code{vfork}.  This warning is also enabled by @option{-Wextra}.
 
+@item -Wcond-in-bool-context
+@opindex Wcond-in-bool-context
+@opindex Wno-cond-in-bool-context
+Warn for conditional expressions (?:) using non-boolean integer constants in
+boolean context, like @code{if (a <= b ? 2 : 3)}.  This warning is enabled
+by @option{-Wall}.
+
 @item -Wconditionally-supported @r{(C++ and Objective-C++ only)}
 @opindex Wconditionally-supported
 @opindex Wno-conditionally-supported
Index: gcc/dwarf2out.c
===================================================================
--- gcc/dwarf2out.c	(revision 239953)
+++ gcc/dwarf2out.c	(working copy)
@@ -2051,9 +2051,9 @@ output_loc_operands (dw_loc_descr_ref loc, int for
 	/* Make sure the offset has been computed and that we can encode it as
 	   an operand.  */
 	gcc_assert (die_offset > 0
-		    && die_offset <= (loc->dw_loc_opc == DW_OP_call2)
+		    && die_offset <= (loc->dw_loc_opc == DW_OP_call2
 				     ? 0xffff
-				     : 0xffffffff);
+				     : 0xffffffff));
 	dw2_asm_output_data ((loc->dw_loc_opc == DW_OP_call2) ? 2 : 4,
 			     die_offset, NULL);
       }
Index: gcc/testsuite/c-c++-common/Wcond-in-bool-context.c
===================================================================
--- gcc/testsuite/c-c++-common/Wcond-in-bool-context.c	(revision 0)
+++ gcc/testsuite/c-c++-common/Wcond-in-bool-context.c	(working copy)
@@ -0,0 +1,17 @@
+/* PR c++/77434 */
+/* { dg-options "-Wcond-in-bool-context" } */
+/* { dg-do compile } */
+
+int foo (int a, int b)
+{
+  if (a > 0 && a <= (b == 1) ? 1 : 2) /* { dg-warning "using integer constants in boolean context" } */
+    return 1;
+
+  if (a > 0 && a <= (b == 2) ? 1 : 1) /* { dg-bogus "using integer constants in boolean context" } */
+    return 2;
+
+  if (a > 0 && a <= (b == 3) ? 0 : 2) /* { dg-bogus "using integer constants in boolean context" } */
+    return 3;
+
+  return 0;
+}