code review 5646044: 6g,8g: make constant propagation inlining-friendly.

6g,8g: make constant propagation inlining-friendly.

This changes makes constant propagation compare 'from' values using node
pointers rather than symbol names when checking to see whether a set
operation is redundant. When a function is inlined multiple times in a
calling function its arguments will share symbol names even though the values
are different. Prior to this fix the bug409 test would hit a case with 6g
where an LEAQ instruction was incorrectly eliminated from the second inlined
function call. 8g appears to have had the same bug, but the test did not fail
there.

[jgennis, 2012-02-08 07:28:47]

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

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

[rsc, 2012-02-08 13:40:22]

LGTM

Thanks very much for finding this!

I grepped through the rest of the optimizer and
confirmed that this is the only use of .sym in
an inappropriate context.

[rsc, 2012-02-08 15:25:18]

*** Submitted as c46561b27b39 ***

6g,8g: make constant propagation inlining-friendly.

This changes makes constant propagation compare 'from' values using node
pointers rather than symbol names when checking to see whether a set
operation is redundant. When a function is inlined multiple times in a
calling function its arguments will share symbol names even though the values
are different. Prior to this fix the bug409 test would hit a case with 6g
where an LEAQ instruction was incorrectly eliminated from the second inlined
function call. 8g appears to have had the same bug, but the test did not fail
there.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5646044

Committer: Russ Cox <rsc@golang.org>