code review 94810044: cmd/gc: don't give credit for NOPs during register allo...

cmd/gc: don't give credit for NOPs during register allocation

The register allocator decides which variables should be placed into registers by charging for each load/store and crediting for each use, and then selecting an allocation with minimal cost. NOPs will be eliminated, however, so using a variable in a NOP should not generate credit.

Issue 7867 arises from attempted registerization of multi-word variables because they are used in NOPs. By not crediting for that use, they will no longer be considered for registerization.

This fix could theoretically lead to better register allocation, but NOPs are rare relative to other instructions.

Fixes issue 7867.

[josharian, 2014-04-28 17:36:01]

Hello rsc@golang.org (cc: golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go

[josharian, 2014-04-28 17:37:28]

Third time's a charm, I hope. I feel much more confident about the safety of
this fix. Thanks for your patience.

I was tempted to add a ProgInfo flag to indicate that a Prog should not generate
usage credit, but opted for a minimal diff. Happy to change that.

[rsc, 2014-05-02 15:55:33]

Is this still relevant?

[josharian, 2014-05-02 15:56:55]

> Is this still relevant?

I think so. The issue is unfixed, and I've not come up with a better fix in the
interim.

[josharian, 2014-05-03 20:41:04]

PTAL

No actual rush, just getting the dashboard to update.

[rsc, 2014-05-09 16:06:04]

LGTM

[josharian, 2014-05-09 16:55:20]

*** Submitted as https://code.google.com/p/go/source/detail?r=4ffd1c7fbadb ***

cmd/gc: don't give credit for NOPs during register allocation

The register allocator decides which variables should be placed into registers
by charging for each load/store and crediting for each use, and then selecting
an allocation with minimal cost. NOPs will be eliminated, however, so using a
variable in a NOP should not generate credit.

Issue 7867 arises from attempted registerization of multi-word variables because
they are used in NOPs. By not crediting for that use, they will no longer be
considered for registerization.

This fix could theoretically lead to better register allocation, but NOPs are
rare relative to other instructions.

Fixes issue 7867.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/94810044

[gobot, 2014-05-09 17:29:23]

This CL appears to have broken the netbsd-amd64-bsiegert builder.
See http://build.golang.org/log/739f52ad3967dfd0b55cde67a8d4da7d2b2b9499

[josharian, 2014-05-09 17:35:55]

> This CL appears to have broken the netbsd-amd64-bsiegert builder.
> See http://build.golang.org/log/739f52ad3967dfd0b55cde67a8d4da7d2b2b9499

Nope. I'll reopen issue 7328.

--- FAIL: TestFinalizerType-4 (6.00 seconds)
mfinal_test.go:61: finalizer for type func(*int) didn't run
FAIL