1900316 - BacktrackingAllocator.cpp: reduce RANGE_SIZE_CUTOFF to 250000

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement, P3)

Description

[Julian Seward [:jseward]]

Bug 1728781 added a test in BacktrackingAllocator::tryMergeReusedRegister that
avoided some worst-case quadratic behaviour, at the (very minor) expense of
forcing a reg-reg copy in the middle of very large live ranges.

In bug 1897427 comment 9 we see a case where even that mechanism doesn't avoid
register allocation lasting for several seconds (possibly above 10). The
simplest fix seem to be to lower the threshold from 1 million to 500 thousand.
That avoids the big delay whilst not penalising large live ranges; a live
range of 500000 is still huge.

For the sake of safety it seems prudent to put the threshold a bit lower, so
as to ensure it kicks in on all architectures and won't need changing yet
again in future. I'm suggesting 250000.

Comment 1

[Julian Seward [:jseward]]

Attachment: Bug 1900316 - BacktrackingAllocator.cpp: reduce RANGE_SIZE_CUTOFF to 250000. r=jandem.

Bug 1728781 added a test in BacktrackingAllocator::tryMergeReusedRegister that
avoided some worst-case quadratic behaviour, at the (very minor) expense of
forcing a reg-reg copy in the middle of very large live ranges.

In bug 1897427 comment 9 we see a case where even that mechanism doesn't avoid
register allocation lasting for several seconds (possibly above 10). The
simplest fix seem to be to lower the threshold from 1 million to 500 thousand.
That avoids the big delay whilst not penalising large live ranges; a live
range of 500000 is still huge.

For the sake of safety it seems prudent to put the threshold a bit lower, so
as to ensure it kicks in on all architectures and won't need changing yet
again in future. I'm suggesting 250000.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/2c87317e5a67
BacktrackingAllocator.cpp: reduce RANGE_SIZE_CUTOFF to 250000.  r=jandem.

Comment 3

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/2c87317e5a67