[PR50878, PATCH] Fix for verify_dominators in -ftree-tail-merge

[Tom de Vries]

Richard,

I have a fix for PR50878.

A simplified form of the problem from the test-case of the PR is shown in this
cfg. Block 12 has as direct dominator block 5.

        5
       / \
      /   \
     *     *
     6     7
     |     |
     |     |
     *     *
     8     9
      \   /
       \ /
        *
       12

tail_merge_optimize finds that blocks 6 and 7 are duplicates. After replacing
block 7 by block 6, the cfg looks like this:

        5
        |
        |
        *
        6
       / \
      /   \
     *     *
     8     9
      \   /
       \ /
        *
       12

The new direct dominator of block 12 is block 6, but the current algorithm only
recalculates dominator info for blocks 6, 8 and 9.

The patch fixes this by additionally recalculating the dominator info for blocks
immediately dominated by bb2 (block 6 in the example), if bb2 has a single
predecessor after replacement.

Bootstapped and reg-tested on x86_64 and i686. Build and reg-tested on MIPS and 
ARM.

Ok for trunk?

Thanks,
- Tom

2011-10-30  Tom de Vries  <t...@codesourcery.com>

        PR tree-optimization/50878
        * tree-ssa-tail-merge.c (replace_block_by): Recalculate dominator info
        for blocks immediately dominated by bb2, if bb2 has a single predecessor
        after replacement.