Re: Fix ICE in predictive commoning

[Richard Guenther <richard dot guenther at gmail dot com>]

On Sat, Feb 14, 2009 at 1:43 PM, Joseph S. Myers
<joseph@codesourcery.com> wrote:
> This patch fixes an ICE in predictive commoning seen on
> arm-none-linux-gnueabi.  (I do not know why it is target-dependent.)
>
> The ICE arises from simple_iv being passed a NULL statement from
> first_stmt (loop->header) called in ref_at_iteration, because the
> block loop->header is empty.  In the 087t.loopinit dump we have
>
> ;; Loop 1
> ;;  header 4, latch 12
>
> with nonempty block 4, but the next dump (088t.copyprop4) has empty
> block 4, and this persists through the 089t.dceloop1 and 090t.lim
> dumps before leading to the ICE.
>
> As far as I can tell it's valid for basic blocks to be empty, so this
> patch (which should only affect cases that previously ICEd) makes this
> code pass from an empty block to its successor as many times as
> needed.  Tested with no regressions with cross to
> arm-none-linux-gnueabi.  OK to commit?

I'll defer to Zdenek for this particular patch, but a patch just
returning NULL_TREE
for the empty block case is pre-approved for 4.4.

Thanks,
Richard.

> 2009-02-14  Joseph Myers  <joseph@codesourcery.com>
>
>        * tree-predcom.c (ref_at_iteration): Look for successor of empty
>        loop header rather than passing NULL statement to simple_iv.
>
> testsuite:
> 2009-02-14  Nathan Sidwell  <nathan@codesourcery.com>
>
>        * g++.dg/torture/predcom-1.C: New test.
>
> Index: tree-predcom.c
> ===================================================================
> --- tree-predcom.c      (revision 144159)
> +++ tree-predcom.c      (working copy)
> @@ -1327,6 +1327,8 @@ ref_at_iteration (struct loop *loop, tre
>  {
>   tree idx, *idx_p, type, val, op0 = NULL_TREE, ret;
>   affine_iv iv;
> +  basic_block header;
> +  gimple fs;
>   bool ok;
>
>   if (handled_component_p (ref))
> @@ -1374,7 +1376,14 @@ ref_at_iteration (struct loop *loop, tre
>   else
>     return NULL_TREE;
>
> -  ok = simple_iv (loop, first_stmt (loop->header), idx, &iv, true);
> +  header = loop->header;
> +  while ((fs = first_stmt (header)) == NULL)
> +    {
> +      if (!single_succ_p (header))
> +       return NULL_TREE;
> +      header = single_succ (header);
> +    }
> +  ok = simple_iv (loop, fs, idx, &iv, true);
>   if (!ok)
>     return NULL_TREE;
>   iv.base = expand_simple_operations (iv.base);
> Index: testsuite/g++.dg/torture/predcom-1.C
> ===================================================================
> --- testsuite/g++.dg/torture/predcom-1.C        (revision 0)
> +++ testsuite/g++.dg/torture/predcom-1.C        (revision 0)
> @@ -0,0 +1,29 @@
> +/* Test for ICE in predictive commoning with empty loop header block
> +   on arm-none-linux-gnueabi.  */
> +
> +struct Foo
> +{
> +  double *ptr;
> +
> +  Foo (double *ptr_)
> +    : ptr (ptr_)
> +  {
> +  }
> +
> +  Foo PostInc ()
> +  {
> +    return Foo (ptr++);
> +  }
> +};
> +
> +bool Baz (Foo first, double *last)
> +{
> +  Foo prev (first.ptr);
> +
> +  first.ptr++;
> +
> +  while (first.ptr != last)
> +    if (*first.PostInc ().ptr < *prev.PostInc ().ptr)
> +      return false;
> +}
> +
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
>