Re: [PATCH] call mark_dfs_back_edges() before testing EDGE_DFS_BACK [PR104761]

[Richard Biener via Gcc-patches]

> Am 03.03.2022 um 09:02 schrieb Jakub Jelinek via Gcc-patches 
> <gcc-patches@gcc.gnu.org>:
> 
> On Wed, Mar 02, 2022 at 04:15:09PM -0700, Martin Sebor via Gcc-patches wrote:
>> The -Wdangling-pointer code tests the EDGE_DFS_BACK but the pass never
>> calls the mark_dfs_back_edges() function that initializes the bit (I
>> didn't know about it).  As a result the bit is not set when expected,
>> which can cause false positives under the right conditions.
> 
> Not a review because I also had to look up what computes EDGE_DFS_BACK,
> so I don't feel the right person to ack the patch.

The patch looks OK.  The questions below might be all valid but they can be 
addressed with followup changes.

Richard.


> So, just a few questions.
> 
> The code in question is:
>      auto gsi = gsi_for_stmt (use_stmt);
> 
>      auto_bitmap visited;
> 
>      /* A use statement in the last basic block in a function or one that
>         falls through to it is after any other prior clobber of the used
>         variable unless it's followed by a clobber of the same variable. */
>      basic_block bb = use_bb;
>      while (bb != inval_bb
>             && single_succ_p (bb)
>             && !(single_succ_edge (bb)->flags & (EDGE_EH|EDGE_DFS_BACK)))
>        {
>          if (!bitmap_set_bit (visited, bb->index))
>            /* Avoid cycles. */
>            return true;
> 
>          for (; !gsi_end_p (gsi); gsi_next_nondebug (&gsi))
>            {
>              gimple *stmt = gsi_stmt (gsi);
>              if (gimple_clobber_p (stmt))
>                {
>                  if (clobvar == gimple_assign_lhs (stmt))
>                    /* The use is followed by a clobber.  */
>                    return false;
>                }
>            }
> 
>          bb = single_succ (bb);
>          gsi = gsi_start_bb (bb);
>        }
> 
> 1) shouldn't it give up for EDGE_ABNORMAL too?  I mean, e.g.
>   following a non-local goto forced edge from a noreturn call
>   to a non-local label (if there is just one) doesn't seem
>   right to me
> 2) if EDGE_DFS_BACK is computed and 1) is done, is there any
>   reason why you need 2 levels of protection, i.e. the EDGE_DFS_BACK
>   check as well as the visited bitmap (and having them use
>   very different answers, if EDGE_DFS_BACK is seen, the function
>   will return false, if visited bitmap has a bb, it will return true)?
>   Can't the visited bitmap go away?
> 3) I'm concerned about compile time with the above, consider you have
>   1000000 use_stmts and 1000000 corresponding inv_stmts and in each
>   case you enter this loop and go through a series of very large basic
>   blocks that don't clobber those stmts; shouldn't it bail out
>   (return false) after walking some param
>   controlled number of non-debug stmts (say 1000 by default)?
>   There is an early exit if
>   if (dominated_by_p (CDI_DOMINATORS, use_bb, inval_bb))
>     return true;
>   (I admit I haven't read the code what happens if there is more than
>   one clobber for the same variable)
> 
>> The attached patch adds a call to the warning pass to initialize
>> the bit.  Tested on x86_64-linux.
>> 
>> Martin
> 
>> Call mark_dfs_back_edges before testing EDGE_DFS_BACK [PR104761].
>> 
>> Resolves:
>> PR middle-end/104761 - bogus -Wdangling-pointer with cleanup and infinite 
>> loop
>> 
>> gcc/ChangeLog:
>> 
>>    PR middle-end/104761
>>    * gimple-ssa-warn-access.cc (pass_waccess::execute): Call
>>    mark_dfs_back_edges.
>> 
>> gcc/testsuite/ChangeLog:
>> 
>>    PR middle-end/104761
>>    * g++.dg/warn/Wdangling-pointer-4.C: New test.
>>    * gcc.dg/Wdangling-pointer-4.c: New test.
>> 
>> diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc
>> index b7cdad517b3..b519712d76e 100644
>> --- a/gcc/gimple-ssa-warn-access.cc
>> +++ b/gcc/gimple-ssa-warn-access.cc
>> @@ -47,7 +47,7 @@
>> #include "tree-object-size.h"
>> #include "tree-ssa-strlen.h"
>> #include "calls.h"
>> -#include "cfgloop.h"
>> +#include "cfganal.h"
>> #include "intl.h"
>> #include "gimple-range.h"
>> #include "stringpool.h"
>> @@ -4710,6 +4710,9 @@ pass_waccess::execute (function *fun)
>>   calculate_dominance_info (CDI_DOMINATORS);
>>   calculate_dominance_info (CDI_POST_DOMINATORS);
>> 
>> +  /* Set or clear EDGE_DFS_BACK bits on back edges.  */
>> +  mark_dfs_back_edges (fun);
>> +
>>   /* Create a new ranger instance and associate it with FUN.  */
>>   m_ptr_qry.rvals = enable_ranger (fun);
>>   m_func = fun;
> 
>    Jakub
>