cmd/internal/obj: delete Cputime and all its uses

[josharian]

obj.Cputime is used to print elapsed time in various places in the assemblers. It is protected by ctxt.Debugvlog. However, toolstash -cmp sets Debugvlog to true, causing rare spurious timing-based failures. They are rare because the elapsed time is basically always ~0. We could introduce a special flag to turn on/off this timing printing, but I'd rather just wipe it all out. We have good profiling and benchmarking tools; we should use them instead, if/when the assemblers ever become an actual performance bottleneck.

Objections? Not sure who quite who to cc here for input, but maybe: @minux @cherrymui @randall77

[cherrymui]

IIRC obj.Cputime is also used in the linker when -v is set.
Just curious, why toolstash -cmp needs to set Debugvlog?

[josharian]

Just curious, why toolstash -cmp needs to set Debugvlog?

Debugvlog is pretty heavily overloaded. toolstash -cmp uses it to ask the compiler to emit much more detailed versions of the generated assembly, to better confirm that it is identical.

Why does the linker need obj.Cputime? Any reason we couldn't use pprof instead?

[cherrymui]

Why does the linker need obj.Cputime? Any reason we couldn't use pprof instead?

I believe it is just historical reason.

[cherrymui]

At long time, we probably should get rid of obj.Cputime and use pprof.
As a workaround, could toolstash -cmp filter out Cputime outputs?

[josharian]

I guess the question is: Why not now?

[josharian]

Slight misdiagnosis: It's not that variance in the timing causes toolstash -cmp failures. Rather, when toolstash -cmp fails, it causes the output to be about timing rather than about something useful.

I'll send a note to golang-dev to see whether anyone is actively using this.

[mdempsky]

Yeah, those timing lines are obnoxious when diffing toolstash log files. I for one wouldn't miss them.

[cherrymui]

I guess the question is: Why not now?

No problem with me, if you want to take it :)

[josharian]

CL 39914 deletes all the assembler usages, which are the ones currently driving me crazy. If no one speaks up on behalf of the linker usages in the next few days, I'll do them too.

[gopherbot]

CL https://golang.org/cl/39914 mentions this issue.

[gopherbot]

Change https://golang.org/cl/171730 mentions this issue: cmd/link/internal: Eliminate all ld.Cputime() usages