code review 14920053: cmd/gc: reject incorrect use of fallthrough.

cmd/gc: reject incorrect use of fallthrough.

Fixes issue 6500.

[remyoudompheng, 2013-10-22 05:56:25]

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

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

[rsc, 2013-10-23 14:11:58]

Thanks, but let's wait until after Go 1.2.
This is not going to cause problems for anyone if we wait.

[remyoudompheng, 2013-12-20 07:35:21]

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

Please take another look.

[gobot, 2013-12-20 16:26:08]

Replacing golang-dev with golang-codereviews.

[gobot, 2013-12-20 16:26:09]

Replacing golang-dev with golang-codereviews.

[rsc, 2014-01-07 02:00:13]

Can we fix the problem without introducing a new syntax error?
I've tried to be as permissive as possible in the grammar so that errors can be
generated at higher levels in the compiler, where the errors can be better. 
'syntax error near fallthrough' is not nearly as good as something like
'fallthrough statement out of place', which is what you get for most uses.

For example, in the production that does $$->op = OSWITCH you could add
$$->xoffset = block;, and then the same for the OXFALL creation. And then the
fallthrough check in swt.c would be if(last->op == OXFALL && last->xoffset ==
sw->xoffset+1) to make sure it is not inside a nested block.

[remyoudompheng, 2014-02-11 00:10:21]

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

Please take another look.

[rsc, 2014-02-11 01:14:40]

https://codereview.appspot.com/14920053/diff/190001/src/cmd/gc/swt.c
File src/cmd/gc/swt.c (right):

https://codereview.appspot.com/14920053/diff/190001/src/cmd/gc/swt.c#newcode331
src/cmd/gc/swt.c:331: yyerror("cannot fallthrough a closing brace");
it may be a closing brace but it may not be. there's already code in walk.c to
diagnose non-last fallthroughs. let's let it do its job, by not changing
last->op here. that is, revert the changes in this file and instead change line
320 to

if(last->xoffset == n->xoffset && last->op == OXFALL) {

[remyoudompheng, 2014-02-11 07:03:49]

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

Please take another look.

[remyoudompheng, 2014-02-18 23:05:39]

ping

[rsc, 2014-02-19 03:15:50]

LGTM

https://codereview.appspot.com/14920053/diff/200001/test/fixedbugs/issue6500.go
File test/fixedbugs/issue6500.go (right):

https://codereview.appspot.com/14920053/diff/200001/test/fixedbugs/issue6500....
test/fixedbugs/issue6500.go:3: // Copyright 2013 The Go Authors. All rights
reserved.
2014

[remyoudompheng, 2014-02-19 06:55:08]

*** Submitted as https://code.google.com/p/go/source/detail?r=1b2554280cc6 ***

cmd/gc: reject incorrect use of fallthrough.

Fixes issue 6500.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://codereview.appspot.com/14920053

https://codereview.appspot.com/14920053/diff/200001/test/fixedbugs/issue6500.go
File test/fixedbugs/issue6500.go (right):

https://codereview.appspot.com/14920053/diff/200001/test/fixedbugs/issue6500....
test/fixedbugs/issue6500.go:3: // Copyright 2013 The Go Authors. All rights
reserved.
On 2014/02/19 03:15:51, rsc wrote:
> 2014

Done.