[PATCH] PR91195: fix -Wmaybe-uninitialized warning for conditional store optimization

[Jeff Law]

On 7/23/19 10:20 AM, Martin Sebor wrote:
> On 7/22/19 10:26 PM, JiangNing OS wrote:
>> This patch is to fix PR91195. Is it OK for trunk?
>>
>> diff --git a/gcc/ChangeLog b/gcc/ChangeLog
>> index 711a31ea597..4db36644160 100644
>> --- a/gcc/ChangeLog
>> +++ b/gcc/ChangeLog
>> @@ -1,3 +1,9 @@
>> +2019-07-22  Jiangning Liu  <jiangning.liu@amperecomputing.com>
>> +
>> +Â Â Â  PR middle-end/91195
>> +Â Â Â  * tree-ssa-phiopt.c (cond_store_replacement): Work around
>> +Â Â Â  -Wmaybe-uninitialized warning.
>> +
>>   2019-07-22  Stafford Horne  <shorne@gmail.com>
>> Â  Â Â Â Â Â  * config/or1k/or1k.c (or1k_expand_compare): Check for int before
>> diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
>> index b64bde695f4..7a86007d087 100644
>> --- a/gcc/tree-ssa-phiopt.c
>> +++ b/gcc/tree-ssa-phiopt.c
>> @@ -2240,6 +2240,14 @@ cond_store_replacement (basic_block middle_bb,
>> basic_block join_bb,
>> Â Â Â Â Â Â Â  tree base = get_base_address (lhs);
>> Â Â Â Â Â Â Â  if (!auto_var_p (base) || TREE_ADDRESSABLE (base))
>> Â Â Â Â Â  return false;
>> +
>> +Â Â Â Â Â  /* The transformation below will inherently introduce a memory
>> load,
>> +Â Â Â Â  for which LHS may not be initialized yet if it is not in NOTRAP,
>> +Â Â Â Â  so a -Wmaybe-uninitialized warning message could be triggered.
>> +Â Â Â Â  Since it's a bit hard to have a very accurate uninitialization
>> +Â Â Â Â  analysis to memory reference, we disable the warning here to avoid
>> +     confusion.  */
>> +Â Â Â Â Â  TREE_NO_WARNING (lhs) = 1;
> 
> The no-warning bit is sometimes (typically?) set by the middle-end
> after a warning has been issued, to avoid triggering other warnings
> down the line for an already diagnosed expression.  Unless it's
> done just for the purposes of a single pass and the bit is cleared
> afterwards, using it to avoid potential false positives doesn't
> seem like a robust solution.  It will mask warnings for constructs
> that have been determined to be invalid.
> 
> FWIW, the middle-end is susceptible to quite a few false positives
> that would nice to avoid.  We have discussed various approaches to
> the problem but setting the no-warning bit seems like too blunt of
> an instrument.
All true.

But in the case JiangNing is working with the transformation inherently
can introduce an uninitialized read.  It seems reasonable to mark those
loads he generates that don't have a dominating read.

His code takes something like

  if (x)
    *p = <someval>

And turns it into

  t1 = *p;
  t2 = x ? <someval> : t1;
  *p = t2;

In the past we required there be a dominating read from *p (among other
restrictions).  That requirement was meant to ensure *p isn't going to
fault.  Jiang's work relaxes that requirement somewhat for objects that
we can prove aren't going to fault via other means.

Can setting TREE_NO_WARNING on the inserted loads inhibit warnings?
Certainly.  However, I believe we use it in other places where we know
the code we're emitting is safe, but can cause a warning.  I think
Jiang's work falls into that category.

I do think the bit should only be set if we don't have a dominating load
to minimize cases where we might inhibit a valid warning.

jeff