commit f062f48c1f8862ff0ef7baccf563497339129743
author Keith Randall <khr@google.com> Tue Mar 05 17:42:48 2019 -0800
committer Brad Fitzpatrick <bradfitz@golang.org> Wed Mar 13 19:54:41 2019 +0000
tree d30650fe6aa62dbc97dad88cfd705f9e822748f0
parent a40b76a40c024fb018045d1d2fd94f9e7b186a15

[release-branch.go1.12] cmd/compile: fix ordering for short-circuiting ops

Make sure the side effects inside short-circuited operations (&& and ||)
happen correctly.

Before this CL, we attached the side effects to the node itself using
exprInPlace. That caused other side effects in sibling expressions
to get reordered with respect to the short circuit side effect.

Instead, rewrite a && b like:

r := a
if r {
  r = b
}

That code we can keep correctly ordered with respect to other
side-effects extracted from part of a big expression.

exprInPlace seems generally unsafe. But this was the only case where
exprInPlace is called not at the top level of an expression, so I
don't think the other uses can actually trigger an issue (there can't
be a sibling expression). TODO: maybe those cases don't need "in
place", and we can retire that function generally.

This CL needed a small tweak to the SSA generation of OIF so that the
short circuit optimization still triggers. The short circuit optimization
looks for triangle but not diamonds, so don't bother allocating a block
if it will be empty.

Go 1 benchmarks are in the noise.

Fixes #30567

Change-Id: I19c04296bea63cbd6ad05f87a63b005029123610
Reviewed-on: https://go-review.googlesource.com/c/go/+/165617
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit 4a9064ef41ccc65454564536f40cf7d5a00db8ad)
Reviewed-on: https://go-review.googlesource.com/c/go/+/165858
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>