[PATCH] conditional load hoisting

[Tehila Meyzels <TEHILA at il dot ibm dot com>]

Hi,

This patch is aimed to perform conditional load hoisting (in tree level),
to increase opportunities for if-conversion (in RTL, ifcvt.c),
It was written for a GCC 4.1.1 based compiler, and I'm posting it, as a
reference, for a discussion I want to raise
with respect tree-ssa-if-conversion improvements (coming up) .
 I'm not submitting this for inclusion in GCC as is, since I think it
should be rewritten to be part of tree-if-conv (details will followed).

This patch performs hoisting of a load insn from the THEN or the ELSE block
(one of them, where it appears) to the IF block above.
It has several restrictions when to hoist, including a checking that the
BBs are if-convertable.

For example:

Replace:

  bb0:
       if (c) goto bb1; else goto bb2;
     bb1:
       a = arr[index];
     bb2:

 with

     bb0:
       a' = arr[index];
       if (c) goto bb1; else goto bb2;
     bb1:
       tmp = a';
     bb2:

In order to activate this transformation, 2 flags should be enabled:
1. -ftree-clhoist (the pass flag)
2. -fsched-spec-load-dangerous - since we create speculative loads, which
is dangerous.

Tehila.

(See attached file: cond_load_hoist.diff)


-----------------------------------------------------------------------------

Tehila Meyzels
Tel: 972-4-829-6190
Email: tehila@il.ibm.com

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