Re: c-family PATCH to improve -Wtautological-compare (PR c/81783)

[David Malcolm <dmalcolm at redhat dot com>]

On Wed, 2017-08-16 at 16:29 +0200, Marek Polacek wrote:
> This patch improves -Wtautological-compare so that it also detects
> bitwise comparisons involving & and | that are always true or false,
> e.g.
> 
>   if ((a & 16) == 10)
>     return 1;
> 
> can never be true.  Note that e.g. "(a & 9) == 8" is *not* always
> false
> or true.
> 
> I think it's pretty straightforward with one snag: we shouldn't warn
> if
> the constant part of the bitwise operation comes from a macro, but
> currently
> that's not possible, so I XFAILed this in the new test.

Maybe I'm missing something here, but why shouldn't it warn when the
constant comes from a macro?

At the end of your testcase you have this example:

#define N 0x10
  if ((a & N) == 10) /* { dg-bogus "bitwise comparison always evaluates to false" "" { xfail *-*-* } } */
     return 1;
  if ((a | N) == 10) /* { dg-bogus "bitwise comparison always evaluates to false" "" { xfail *-*-* } } */
   return 1;

That code looks bogus to me (and if the defn of "N" is further away,
it's harder to spot that it's wrong): shouldn't we warn about it?

> 
> This has found one issue in the GCC codebase and it's a genuine bug:
> <https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00757.html>.  

In this example GOVD_WRITTEN is from an enum, not a macro, but if
GOVD_WRITTEN had been a macro, shouldn't we still issue a warning?

Hope this is constructive
Dave

> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> 
> 2017-08-16  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c/81783
> 	* c-warn.c (warn_tautological_bitwise_comparison): New
> function.
> 	(warn_tautological_cmp): Call it.
> 
> 	* doc/invoke.texi: Update -Wtautological-compare documentation.
> 
> 	* c-c++-common/Wtautological-compare-5.c: New test.
> 
> diff --git gcc/c-family/c-warn.c gcc/c-family/c-warn.c
> index 9c3073444cf..0749d16a50f 100644
> --- gcc/c-family/c-warn.c
> +++ gcc/c-family/c-warn.c
> @@ -321,6 +321,59 @@ find_array_ref_with_const_idx_r (tree *expr_p,
> int *, void *)
>    return NULL_TREE;
>  }
>  
> +/* Subroutine of warn_tautological_cmp.  Warn about bitwise
> comparison
> +   that always evaluate to true or false.  LOC is the location of
> the
> +   ==/!= comparison specified by CODE; LHS and RHS are the usual
> operands
> +   of this comparison.  */
> +
> +static void
> +warn_tautological_bitwise_comparison (location_t loc, tree_code
> code,
> +				      tree lhs, tree rhs)
> +{
> +  if (code != EQ_EXPR && code != NE_EXPR)
> +    return;
> +
> +  /* Extract the operands from e.g. (x & 8) == 4.  */
> +  tree bitop;
> +  tree cst;
> +  if ((TREE_CODE (lhs) == BIT_AND_EXPR
> +       || TREE_CODE (lhs) == BIT_IOR_EXPR)
> +      && TREE_CODE (rhs) == INTEGER_CST)
> +    bitop = lhs, cst = rhs;
> +  else if ((TREE_CODE (rhs) == BIT_AND_EXPR
> +	    || TREE_CODE (rhs) == BIT_IOR_EXPR)
> +	   && TREE_CODE (lhs) == INTEGER_CST)
> +    bitop = rhs, cst = lhs;
> +  else
> +    return;
> +
> +  tree bitopcst;
> +  if (TREE_CODE (TREE_OPERAND (bitop, 0)) == INTEGER_CST)
> +    bitopcst = TREE_OPERAND (bitop, 0);
> +  else if (TREE_CODE (TREE_OPERAND (bitop, 1)) == INTEGER_CST)
> +    bitopcst = TREE_OPERAND (bitop, 1);
> +  else
> +    return;
> +
> +  wide_int res;
> +  if (TREE_CODE (bitop) == BIT_AND_EXPR)
> +    res = wi::bit_and (bitopcst, cst);
> +  else
> +    res = wi::bit_or (bitopcst, cst);
> +
> +  /* For BIT_AND only warn if (CST2 & CST1) != CST1, and
> +     for BIT_OR only if (CST2 | CST1) != CST1.  */
> +  if (res == cst)
> +    return;
> +
> +  if (code == EQ_EXPR)
> +    warning_at (loc, OPT_Wtautological_compare,
> +		"bitwise comparison always evaluates to false");
> +  else
> +    warning_at (loc, OPT_Wtautological_compare,
> +		"bitwise comparison always evaluates to true");
> +}
> +
>  /* Warn if a self-comparison always evaluates to true or false.  LOC
>     is the location of the comparison with code CODE, LHS and RHS are
>     operands of the comparison.  */
> @@ -337,6 +390,8 @@ warn_tautological_cmp (location_t loc, enum
> tree_code code, tree lhs, tree rhs)
>        || from_macro_expansion_at (EXPR_LOCATION (rhs)))
>      return;
>  
> +  warn_tautological_bitwise_comparison (loc, code, lhs, rhs);
> +
>    /* We do not warn for constants because they are typical of macro
>       expansions that test for features, sizeof, and similar.  */
>    if (CONSTANT_CLASS_P (fold_for_warn (lhs))
> diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
> index ec29f1d629e..72a16a19711 100644
> --- gcc/doc/invoke.texi
> +++ gcc/doc/invoke.texi
> @@ -5484,6 +5484,14 @@ int i = 1;
>  @dots{}
>  if (i > i) @{ @dots{} @}
>  @end smallexample
> +
> +This warning also warns about bitwise comparisons that always
> evaluate
> +to true or false, for instance:
> +@smallexample
> +if ((a & 16) == 10) @{ @dots{} @}
> +@end smallexample
> +will always be false.
> +
>  This warning is enabled by @option{-Wall}.
>  
>  @item -Wtrampolines
> diff --git gcc/testsuite/c-c++-common/Wtautological-compare-5.c
> gcc/testsuite/c-c++-common/Wtautological-compare-5.c
> index e69de29bb2d..4664bfdeae6 100644
> --- gcc/testsuite/c-c++-common/Wtautological-compare-5.c
> +++ gcc/testsuite/c-c++-common/Wtautological-compare-5.c
> @@ -0,0 +1,106 @@
> +/* PR c/81783 */
> +/* { dg-do compile } */
> +/* { dg-options "-Wtautological-compare" } */
> +
> +enum E { FOO = 128 };
> +
> +int
> +f (int a)
> +{
> +  if ((a & 16) == 10) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((16 & a) == 10) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (10 == (a & 16)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (10 == (16 & a)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +
> +  if ((a & 16) != 10) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((16 & a) != 10) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (10 != (a & 16)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (10 != (16 & a)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +
> +  if ((a & 9) == 8)
> +    return 1;
> +  if ((9 & a) == 8)
> +    return 1;
> +  if (8 == (a & 9))
> +    return 1;
> +  if (8 == (9 & a))
> +    return 1;
> +
> +  if ((a & 9) != 8)
> +    return 1;
> +  if ((9 & a) != 8)
> +    return 1;
> +  if (8 != (a & 9))
> +    return 1;
> +  if (8 != (9 & a))
> +    return 1;
> +
> +  if ((a | 16) == 10) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((16 | a) == 10) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (10 == (a | 16)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (10 == (16 | a)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +
> +  if ((a | 16) != 10) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((16 | a) != 10) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (10 != (a | 16)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (10 != (16 | a)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +
> +  if ((a | 9) == 8) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((9 | a) == 8) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (8 == (a | 9)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if (8 == (9 | a)) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +
> +  if ((a | 9) != 8) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((9 | a) != 8) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (8 != (a | 9)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if (8 != (9 | a)) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +
> +  if ((a & 128) != 1) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((128 & a) != 1) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((a & FOO) != 1) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((FOO & a) != 1) /* { dg-warning "bitwise comparison always
> evaluates to true" } */
> +    return 1;
> +  if ((a & 128) == 1) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((128 & a) == 1) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((a & FOO) == 1) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +  if ((FOO & a) == 1) /* { dg-warning "bitwise comparison always
> evaluates to false" } */
> +    return 1;
> +
> +#define N 0x10
> +  if ((a & N) == 10) /* { dg-bogus "bitwise comparison always
> evaluates to false" "" { xfail *-*-* } } */
> +    return 1;
> +  if ((a | N) == 10) /* { dg-bogus "bitwise comparison always
> evaluates to false" "" { xfail *-*-* } } */
> +    return 1;
> +
> +  return 0;
> +}
> 
> 	Marek