cmd/compile: sporadic memory corruption on 386 (32-bit) builders

[bcmills]

Various sweep increased allocation count errors have started cropping up in or near go/build invocations in various go commands.

2020-03-15T08:13:55-32dbccd/linux-386-clang
2020-03-14T07:03:15-d774d97/linux-386-sid
2020-03-16T20:59:27-ff1eb42/linux-386-387
2020-03-15T08:13:55-32dbccd/linux-386-clang
2020-03-14T07:03:15-d774d97/linux-386-sid
2020-03-13T20:43:12-e2a9ea0/openbsd-386-62

Since go/build is involved, this may be related to the go/types crash cluster (#37602, #37507, #37690).

CC @griesemer @matloob @aclements @danscales @mknyszek @cherrymui

[bcmills]

These found bad pointer in Go heap errors seem to fit the same root cause:

2020-03-16T22:31:39-2fbca94/linux-386-sid
2020-03-13T19:43:47-cbcb031/openbsd-386-62

[bcmills]

This appears to be a dramatic uptick in memory corruption across the board on the 32-bit builders.

Given that it is only appearing on the 32-bit builders, and that we have not (to my knowledge) made any particularly racy or dangerous changes in cmd/go this cycle, marking as release-blocker for Go 1.15.

[bcmills]

I also can't rule out a compiler bug as the cause, given the number of changes to the rewrite rules so far. (CC @josharian @randall77)

[bcmills]

The first failure in this cluster was the same day that CL 222782 (#36468) was merged, so I suspect that may be related.

[bcmills]

Note that some of the affected builders are also TryBots:
https://go-review.googlesource.com/c/go/+/223745/5#message-7c0b4f426d49ae03a1ec88007cf714c5134d4800

[ianlancetaylor]

https://storage.googleapis.com/go-build-log/a19069d2/linux-386_272d029f.log

[jayconrod]

https://storage.googleapis.com/go-build-log/29e1c579/linux-386_8cf5204b.log

[randall77]

I'm kinda stumped on this one. I've tried to reproduce to no avail, and pouring over CL 222782 doesn't reveal anything that might cause intermittent issues like this.

On a possibly related note, I just mailed a change to add more addressing mode modifications for amd64. When that goes in, whether or not we start seeing this on amd64 will be illuminating.