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cmd/compile: non-deterministic build, breaking toolstash -cmp #17288

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mdempsky opened this issue Sep 29, 2016 · 8 comments
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cmd/compile: non-deterministic build, breaking toolstash -cmp #17288

mdempsky opened this issue Sep 29, 2016 · 8 comments
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Go1.8

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@mdempsky
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Currently at master you can't do:

./make.bash
toolstash save
go build -a -toolexec='toolstash -cmp' std cmd

because cmd/compile isn't producing deterministic output. This used to work.

It looks like instructions are now being ordered non-deterministically within a basic block. I'll begin a bisect to track down when this regressed.

/cc @randall77 @cherrymui
@bradfitz
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@aclements, is this why your performance numbers were bouncing all over the other day?

@bradfitz bradfitz added this to the Go1.8 milestone Sep 29, 2016
@mdempsky
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bisect identifies CL 29732 (f876fb9).

/cc @cherrymui @dr2chase

@bradfitz
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for c, used := range s.copies {

@aclements
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@bradfitz, my problem looked like it had to do with spills, but it's possible.

@mdempsky
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Do we make copies of copies? Is it possible that copy A is the only use of copy B, and depending on whether we visit A before B will influence whether we're able to eliminate B?

@cherrymui
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I thought the order of iteration doesn't matter as it will delete all of them. Well, maybe it is, as deleting one copy may decrement Uses of another.
Will try to fix it tonight.

@mdempsky
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FYI, running that loop until we reach a fixed point seems to solve the non-determinism. I can upload a CL for that if you're interested.

@cherrymui
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Yes, I am doing this, too. Go ahead and send the CL.

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@bradfitz @mdempsky @aclements @gopherbot @cherrymui