code review 6002043: 6c, 6g, 6l: add MOVQL to make truncation explicit

6c, 6g, 6l: add MOVQL to make truncation explicit

Without an explicit signal for a truncation, copy propagation
will sometimes propagate a 32-bit truncation and end up
overwriting uses of the original 64-bit value.

The case that arose in practice is in C but I believe
that the same could plausibly happen in Go.
The main reason we didn't run into the same in Go
is that I (perhaps incorrectly?) drop MOVL AX, AX
during gins, so the truncation was never generated, so
it didn't confuse the optimizer.

Fixes issue 1315.
Fixes issue 3488.

[rsc, 2012-04-10 16:51:56]

Hello ken2 (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[rsc, 2012-04-10 16:52:03]

*** Submitted as http://code.google.com/p/go/source/detail?r=a9e09f1fe373 ***

6c, 6g, 6l: add MOVQL to make truncation explicit

Without an explicit signal for a truncation, copy propagation
will sometimes propagate a 32-bit truncation and end up
overwriting uses of the original 64-bit value.

The case that arose in practice is in C but I believe
that the same could plausibly happen in Go.
The main reason we didn't run into the same in Go
is that I (perhaps incorrectly?) drop MOVL AX, AX
during gins, so the truncation was never generated, so
it didn't confuse the optimizer.

Fixes issue 1315.
Fixes issue 3488.

R=ken2
CC=golang-dev
http://codereview.appspot.com/6002043

[ken3, 2012-04-11 00:59:57]

lgtm