Andi Kleen - [PATCH] Add -Womitted-conditional-op warning (original) (raw)

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Since it seems to be currently envogue to add new warnings: I had this one lying around for quite some time. It checks for a (relatively obscure) mistake in using a GCC extension that I once made. Gcc allows omitting the middle operand in ?:. Now when one writes e.g.

foo() != NULL ?: bar

then the result is typically unexpected because the omitted value will be always

  1. But at least I foolishly expected it to be the return value of foo(), because reusing the value from the guarding expression is normally the reason for using ?:.

I implemented a warning for this case. I guess it's relatively obscure, but will not hurt. The warning is enabled by default, but can be disabled with -Wno-omitted-conditional-op.

Passed C/C++ bootstrap and a test suite run on x86_64-linux.

-Andi

2007-02-02 Andi Kleen ak@suse.de

* c-common.c, c-common.h (warn_for_omitted_condop): Add.
* cp/parser.c (cp_parser_question_colon_clause): 
  Call warn_for_omitted_condop.
* c-parser.c (c_parser_conditional_expression): 
  Call warn_for_omitted_condop.	
* common.opt: Add -Womitted-conditional-op.
* doc/invoke.text: Document -Wno-omitted-conditional-op.
* testsuite/gcc.dg/warn-omitted-condop.c: Add.
* testsuite/g++.dg/warn-omitted-condop.C: Add.

Index: gcc/doc/invoke.texi

--- gcc/doc/invoke.texi (revision 121493) +++ gcc/doc/invoke.texi (working copy) @@ -253,7 +253,7 @@ -Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol -Wunused-value -Wunused-variable -Wvariadic-macros @gol --Wvolatile-register-var -Wwrite-strings} +-Wvolatile-register-var -Wwrite-strings -Wno-omitted-conditional-op} @item C-only Warning Options @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol @@ -3613,6 +3613,15 @@ This option is implied by @option{-pedantic}, and can be disabled with @option{-Wno-overlength-strings}. + +@item -Wno-omitted-conditional-op +@opindex Wno-omitted-conditional-op +Don't warn for dangerous uses of the +?: with omitted middle operand GNU extension. When the condition +in the ?: operator is a computed boolean the omitted value will +be always 1. Often the user expects it to be a value computed +inside the conditional expression instead. Gcc by default warns +for this, but this option disables it. @end table @node Debugging Options Index: gcc/cp/parser.c

--- gcc/cp/parser.c (revision 121493) +++ gcc/cp/parser.c (working copy) @@ -5856,11 +5856,16 @@ cp_lexer_consume_token (parser->lexer); if (cp_parser_allow_gnu_extensions_p (parser) && cp_lexer_next_token_is (parser->lexer, CPP_COLON)) + { + warn_for_omitted_condop (logical_or_expr); /* Implicit true clause. / expr = NULL_TREE; + } else + { / Parse the expression. / expr = cp_parser_expression (parser, /cast_p=/false); + } / The next token should be a :'. */ cp_parser_require (parser, CPP_COLON, ":'"); Index: gcc/common.opt

--- gcc/common.opt (revision 121493) +++ gcc/common.opt (working copy) @@ -201,6 +201,10 @@ Common RejectNegative Var(warn_coverage_mismatch) Warn instead of error in case profiles in -fprofile-use do not match +Womitted-conditional-op +Common Var(warn_omitted_condop) Init(1) +Warn for omitted middle operands in ?: with unexpected meaning + aux-info Common Separate -aux-info Emit declaration information into Index: gcc/c-common.c

--- gcc/c-common.c (revision 121493) +++ gcc/c-common.c (working copy) @@ -6783,4 +6828,26 @@ } } +/* Warn for A ?: C expressions (with B omitted) where A is a computed + boolean expression. */ + +void +warn_for_omitted_condop (tree cond) +{ + enum tree_code code = TREE_CODE (cond); + + if (!warn_omitted_condop) + return; + if (TREE_CODE_CLASS (code) == tcc_comparison || + code == TRUTH_ANDIF_EXPR || + code == TRUTH_ORIF_EXPR || + code == TRUTH_ANDIF_EXPR || + code == TRUTH_NOT_EXPR) + { + warning (OPT_Womitted_conditional_op, + "Omitting middle operand in ?: with computed " + "boolean conditional has unexpected meaning"); + }
+} + #include "gt-c-common.h" Index: gcc/c-common.h

--- gcc/c-common.h (revision 121493) +++ gcc/c-common.h (working copy) @@ -870,6 +870,7 @@ enum tree_code); extern void warn_for_unused_label (tree label); +extern void warn_for_omitted_condop (tree cond); /* In c-gimplify.c */ extern void c_genericize (tree); Index: gcc/c-parser.c

--- gcc/c-parser.c (revision 121493) +++ gcc/c-parser.c (working copy) @@ -4388,6 +4388,7 @@ { if (pedantic) pedwarn ("ISO C forbids omitting the middle term of a ?: expression");

--- /dev/null 2006-11-25 12:43:29.000000000 +0100 +++ gcc/testsuite/gcc.dg/warn-omitted-condop.c 2007-02-02 15:46:10.000000000 +0100 @@ -0,0 +1,29 @@ +/* { dg-options "-Womitted-conditional-op" } */ + +extern void f2 (int); + +void bar (int x, int y, int z) +{ +#define T(op) f2 (x op y ? : 1) +#define T2(op) f2 (x op y ? 2 : 1) +

+#define T2(op) f2 (x op y ? 2 : 1) +

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