cmd/compile: merge temporaries more aggressively

[josharian]

CL 12829043 added a compiler pass that merges temporary variables with equal types and
non-overlapping lifetimes.

I believe that "equal types" is sufficient but not necessary and that
"equivalent w.r.t. GC" is both necessary and sufficient. (Please correct me if
I am wrong!) This should allow more temporaries to be merged, further reducing stack
usage.

[josharian]

I took a quick hack at this. It does appear to offer some nice stack use wins, but it breaks our dwarf generation. Also, twobitwalktype1 is a compiler performance bottleneck. This will need to wait at the least until the compiler transition to Go is completed.

My ugly hack code: https://github.com/josharian/go/compare/issue-8740. From the commit message there:

In godoc binary

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[dvyukov]

Nice! This can significantly reduce memory consumption and frequency of GCs for servers with lots of goroutines.

[rsc]

The compare link doesn't seem to work anymore. Can you refresh it somehow?

[josharian]

CL 10251

[gopherbot]

CL https://golang.org/cl/10251 mentions this issue.

[bradfitz]

/cc @randall77

[josharian]

I have a prototype of this for SSA, although it needs some cleanup. It still helps some, although not as dramatically as before SSA. I plan to finish it up and mail it early in the Go 1.8 cycle.

[quentinmit]

@josharian Have you had a chance to look at this yet for 1.8?

[josharian]

I did. My recollection (now fuzzy) is that it helped but not enough to inspire me to finish re-implementing it.

I'll also admit to being a bit confused at the moment about the relationship between the stackalloc pass and the temporary merging code found in pgen.go. I've implemented this independently in both places, but never tried applying it to both at the same time.