-Wstrict-aliasing=2 does not warn about all problems

-Wstrict-aliasing=2 does not warn about all problems

[Ian Lance Taylor]

Hi Jim, the documentation for -Wstrict-aliasing=2 says:

    It warns about all code which might break the strict aliasing
    rules that the compiler is using for optimization.  This warning
    catches all cases, but it will also give a warning for some
    ambiguous cases that are safe.

However, there are a number of cases of aliasing which it does not
catch.  For example, see PR 23106.  In general, the warning is
disabled by casts through void* or char*.  Also, the warning only
applies to casts of the address of a variable.

In general, I think the risky code is when we see some ordering of
these three statements:
    p2 = (TYPE *) p1;
    *p2;
    *p1;
However, computing that is probably non-trivial.

Anyhow, I think we need to change the documentation for
-Wstrict-aliasing=2.  It warns about more possible problems than
-Wstrict-aliasing, but it does not warn about all possible problems.

Any thoughts?

Ian

Re: -Wstrict-aliasing=2 does not warn about all problems

[James E Wilson]

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Ian Lance Taylor wrote:
> -Wstrict-aliasing=2.  It warns about more possible problems than
> -Wstrict-aliasing, but it does not warn about all possible problems.

This is the important point that I was trying to get across, though I do 
see that using "all" can be misconstrued here.  How about something like 
this?
-- 
Jim Wilson, GNU Tools Support, http://www.specifix.com

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2005-07-28  James E Wilson  <wilson@specifixinc.com>

	* doc/invoke.texi (Wstrict-aliasing=2): Reword.

Index: invoke.texi
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doc/invoke.texi,v
retrieving revision 1.655
diff -p -p -r1.655 invoke.texi
*** invoke.texi	25 Jul 2005 19:42:02 -0000	1.655
--- invoke.texi	28 Jul 2005 19:17:44 -0000
*************** included in @option{-Wall}.
*** 2726,2734 ****
  @item -Wstrict-aliasing=2
  @opindex Wstrict-aliasing=2
  This option is only active when @option{-fstrict-aliasing} is active.
! It warns about all code which might break the strict aliasing rules that the
! compiler is using for optimization.  This warning catches all cases, but
! it will also give a warning for some ambiguous cases that are safe.
  
  @item -Wall
  @opindex Wall
--- 2726,2735 ----
  @item -Wstrict-aliasing=2
  @opindex Wstrict-aliasing=2
  This option is only active when @option{-fstrict-aliasing} is active.
! It warns about code which might break the strict aliasing rules that the
! compiler is using for optimization.  This warning catches more cases than
! @option{-Wstrict-aliasing}, but it will also give a warning for some ambiguous
! cases that are safe.
  
  @item -Wall
  @opindex Wall

Re: -Wstrict-aliasing=2 does not warn about all problems

[Ian Lance Taylor]

James E Wilson <wilson@specifix.com> writes:

> Ian Lance Taylor wrote:
> > -Wstrict-aliasing=2.  It warns about more possible problems than
> > -Wstrict-aliasing, but it does not warn about all possible problems.
> 
> This is the important point that I was trying to get across, though I
> do see that using "all" can be misconstrued here.  How about something
> like this?

Works for me.  Thanks.

Ian