ssa-pre-21.c / PRE for truncated indices

ssa-pre-21.c / PRE for truncated indices

[Joern Rennecke]

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I see that gcc.dg/tree-ssa/ssa-pre-21.c fails for avr.
The computation of k + 1L is done as unsigned int (16 bit),
but later ++k is performed as unsigned long (32 bit), and thus
array[k+1] is not re-used.

Is this to be considered a big in the optimizers for not
optimizing this, or in the testsuite for insisting that it
should be?

FWIW, equivalent behaviour can be seen natively on
i686-pc-linux-gnu by replacing "long" with "long long",
as shown in the attachment.

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/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-pre" } */

long long
NumSift (long long *array, unsigned long long k)
{
  if (array[k] < array[k + 1LL])
    ++k;
  return array[k];
}

/* There should be only two loads left.  */

/* { dg-final { scan-tree-dump-times "= \\\*\[^\n;\]*;" 2 "pre" } } */
/* { dg-final { cleanup-tree-dump "pre" } } */

Re: ssa-pre-21.c / PRE for truncated indices

[Richard Biener]

On 8/15/13 1:35 PM, Joern Rennecke wrote:
> I see that gcc.dg/tree-ssa/ssa-pre-21.c fails for avr.
> The computation of k + 1L is done as unsigned int (16 bit),
> but later ++k is performed as unsigned long (32 bit), and thus
> array[k+1] is not re-used.
> 
> Is this to be considered a big in the optimizers for not
> optimizing this, or in the testsuite for insisting that it
> should be?

It's a but in the optimizer, the modified testcase should
optimize well on i686.  The issue is very likely that
the frontend demotes array indices to sizetype.

You may want to file a bugreport.

Richard.

> FWIW, equivalent behaviour can be seen natively on
> i686-pc-linux-gnu by replacing "long" with "long long",
> as shown in the attachment.