Change the handling of stack check on backward branches

Change the handling of stack check on backward branches

The hydrogen stack check instruction is now added to each loop and the stack check handling on the back edge has been removed.

This change causes regression on small tight loops as the stack check is now at the top of the loop instead of at the bottom, and that requires one additional unconditional jump per loop iteration. However the reason for this change is to avoid worse regressions for upcoming changes to correctly support debugger break in optimized code.

R=fschneider@chromium.org

BUG=none
TEST=none

Committed: http://code.google.com/p/v8/source/detail?r=8428

[fschneider, 2011-06-22 07:41:18]

LGTM.

http://codereview.chromium.org/7216009/diff/1001/src/hydrogen.cc
File src/hydrogen.cc (right):

http://codereview.chromium.org/7216009/diff/1001/src/hydrogen.cc#newcode1254
src/hydrogen.cc:1254:
block->loop_information()->stack_check()->set_eliminated(true);
Idea for improvement: Can you delete the HStackCheck instruction from the graph
instead of marking it here and eliminating it in the code generator? This would
eliminate the environment uses before doing register allocation and could
potentially help shortening live ranges.

[Søren Thygesen Gjesse, 2011-06-27 11:07:52]

New patch uploaded.

http://codereview.chromium.org/7216009/diff/1001/src/hydrogen.cc
File src/hydrogen.cc (right):

http://codereview.chromium.org/7216009/diff/1001/src/hydrogen.cc#newcode1254
src/hydrogen.cc:1254:
block->loop_information()->stack_check()->set_eliminated(true);
On 2011/06/22 07:41:18, fschneider wrote:
> Idea for improvement: Can you delete the HStackCheck instruction from the
graph
> instead of marking it here and eliminating it in the code generator? This
would
> eliminate the environment uses before doing register allocation and could
> potentially help shortening live ranges.

Good point. Changed the elimination to unlink the stack check instruction from
the graph instead of having a boolean flag on it.