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Make -Wint-in-bool-context warn on suspicious shift ops

Message ID AM4PR0701MB2162D459F858784668E47F78E4CE0@AM4PR0701MB2162.eurprd07.prod.outlook.com
State New
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Commit Message

Bernd Edlinger Sept. 29, 2016, 7:58 p.m. UTC 
On 09/29/16 20:52, Bernd Edlinger wrote:
> On 09/29/16 20:03, Jason Merrill wrote:

>>

>> What do you think about dropping the TYPE_UNSIGNED exception as well?

>> I don't see what difference that makes.

>>

>

>

> If I drop that exception, then I could also drop the check for

> INTEGER_TYPE and the whole if, because I think other types can not

> happen, but if they are allowed they are as well bogus here.

>

> I can try a bootstrap and see if there are false positives.

>

> But I can do that as well in a follow-up patch, this should probably

> be done step by step, especially when it may trigger some false

> positives.

>

> I think I could also add more stuff, like unary + or - ?

> or maybe also binary +, -, * and / ?

>

> We already discussed making this a multi-level option,

> and maybe enabling the higher level explicitly in the

> boot-strap.

>

> As long as the warning continues to find more bugs than false

> positives, it is probably worth extending it to more cases.

>

> However unsigned integer shift are not undefined if they overflow.

>

> It is possible that this warning will then trigger also on valid

> code that does loop termination with unsigned int left shifting.

> I dont have a real example, but maybe  like this hypothetical C-code:

>

>   unsigned int x=1, bits=0;

>   while (x << bits) bits++;

>   printf("bits=%d\n", bits);

>

>

> Is it OK for everybody to warn for this on -Wall, or maybe only

> when -Wextra or for instance -Wint-in-bool-context=2 is used ?

>

>


Unfortunately, without that exception there is a false positive:

In file included from ../../gcc-trunk/gcc/ada/gcc-interface/decl.c:30:0:
../../gcc-trunk/gcc/ada/gcc-interface/decl.c: In function 'int 
adjust_packed(tree, tree, int)':
../../gcc-trunk/gcc/tree.h:1874:22: error: << on signed integer in 
boolean context [-Werror=int-in-bool-context]
       ? ((unsigned)1) << ((NODE)->type_common.align - 1) : 0)
         ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../gcc-trunk/gcc/ada/gcc-interface/decl.c:6928:7: note: in expansion 
of macro 'TYPE_ALIGN'
    if (TYPE_ALIGN (record_type)
        ^~~~~~~~~~


But that did not happen with this version:



Is that version OK for you?


Bernd.

Comments

Jason Merrill Sept. 29, 2016, 8:38 p.m. UTC | #1 
On Thu, Sep 29, 2016 at 3:58 PM, Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
> Unfortunately, without that exception there is a false positive:
>
> In file included from ../../gcc-trunk/gcc/ada/gcc-interface/decl.c:30:0:
> ../../gcc-trunk/gcc/ada/gcc-interface/decl.c: In function 'int
> adjust_packed(tree, tree, int)':
> ../../gcc-trunk/gcc/tree.h:1874:22: error: << on signed integer in
> boolean context [-Werror=int-in-bool-context]
>        ? ((unsigned)1) << ((NODE)->type_common.align - 1) : 0)
>          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ah, this issue again: the shift isn't in boolean context, it's in
integer context.  I think we want to be a lot more conservative about
these warnings in the arms of a COND_EXPR.  In fact, I think the
entire

      /* Distribute the conversion into the arms of a COND_EXPR.  */

section is wrong now that we're doing delayed folding.

Jason
diff mbox

Patch

Index: c-common.c
===================================================================
--- c-common.c	(revision 240571)
+++ c-common.c	(working copy)
@@ -4655,6 +4655,14 @@  c_common_truthvalue_conversion (location_t locatio
  	return c_common_truthvalue_conversion (location,
  					       TREE_OPERAND (expr, 0));

+    case LSHIFT_EXPR:
+      /* Warn on signed integer left shift.  */
+      if (TREE_CODE (TREE_TYPE (expr)) == INTEGER_TYPE
+	  && !TYPE_UNSIGNED (TREE_TYPE (expr)))
+	warning_at (EXPR_LOCATION (expr), OPT_Wint_in_bool_context,
+		    "<< on signed integer in boolean context");
+      break;
+
      case COND_EXPR:
        if (warn_int_in_bool_context
  	  && !from_macro_definition_at (EXPR_LOCATION (expr)))