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RE: [PATCH] Fix PR tree-optimization/58137


On Fri, 30 Aug 2013 12:34:51 Richard Biener wrote:
> On Tue, Aug 20, 2013 at 1:01 PM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
>> This patch fixes a bug in the vectorized pointer arithmetic in the forwprop
>> optimization pass.
>>
>> Although it seems to be impossible to create a vector of pointers with the
>> __attribute__((vector_size(x))) syntax, the vector loop optimization together
>> with the loop unrolling can create code which adds a vector of ptroff_t
>> repeatedly to a vector of pointers.
>>
>> The code in tree-ssa-forwprop.c handles program transformations of the
>> form (PTR +- CST1) +- CST2 => PTR +- (CST1+-CST2) where PTR is
>> a vector of pointers and CST1 and CST2 are vectors of ptroff_t, without the
>> fix the result type of CST1+-CST2 was vector of pointer, which causes the
>> ICE in tree-cfg.c, which sees an undefined pointer + pointer operation.
>>
>> Additionally the check in tree-cfg.c allows expressions of the form CST - PTR,
>> which is clearly wrong. That should be fixed too.
>>
>> The patch was bootstrapped and regression tested on i686-pc-linux-gnu.
>
> It seems to me that the forwprop code does not handle the fact that we
> are permissive as to using PLUS_EXPR instead of POINTER_PLUS_EXPR
> for vector pointer - offset additions. So instead of doing this dance the
> better (and more easily backportable) fix is to simply disable the transform
> on pointer valued PLUS_EXPR. Like with
>
> if (POINTER_TYPE_P (TREE_TYPE (gimple_assign_lhs (stmt))))
> return false;
>
> at the beginning of the function.
>

the condition would have to be:

  if (TREE_CODE (TREE_TYPE (gimple_assign_lhs (stmt))) == VECTOR_TYPE
      && POINTER_TYPE_P (TREE_TYPE (TREE_TYPE (gimple_assign_lhs (stmt)))))
    return false;

I tried this, and it fixes the ICE. However the generated code was still vectorized but
much less efficient and used more registers.

Fortunately there will be no need to backport this, since this bug does not happen in
gcc 4.8.2 I checked that. I believe it was introduced with the checkin r200059 by
Marc Glisse where associate_plusminus was enhanced to handle vector values.
Before that only TREE_CODE (rhs2) == INTEGER_CST was handled.

Frankly I would prefer the initial version of the patch, because the code is more
efficient this way. The vector data is folded correctly, only the data type was wrong
and triggered the ICE in tree-cfg.c.

Please advise.

Thanks
Bernd.

> The real fix is of course to make vector pointer operations properly
> use POINTER_PLUS_EXPR ...
>
> Richard.
>
>
>
>> Regards
>> Bernd Edlinger 		 	   		  

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