runtime: performance regression due to bad instruction used in morestack_noctxt for ppc64 in CL 425396

[laboger]

What version of Go are you using (go version)?

$ go version
latest

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
ppc64le power10

What did you do?

What did you expect to see?

Consistent performance of runtime benchmarks.

What did you see instead?

Some benchmarks in the runtime package show a 3X or more degradation on power10 after CL 425396 was merged. Removing this line in the latest also removes the degradation. For example, Hash65536 is one example.

name          old time/op    new time/op   delta
Hash65536-64    4.08µs ± 2%  18.89µs ±25%  +363.16%  (p=0.029 n=4+4)

name          old speed      new speed     delta
Hash65536-64  16.1GB/s ± 2%  3.5GB/s ±22%   -77.96%  (p=0.029 n=4+4)

The MOVD R1, R1 results in
or r1,r1,r1
which is a special instruction on ppc64 that changes thread priority. This is described in section 3.2 of ISA 3.1.

Changing it to OR $0, R1 has the same effect wrt SPWRITE but does not affect thread priority.

@cherrymui @pmur @archanaravindar

[cherrymui]

Interesting. Good finding!
Mind sending a CL? Or I could send one, but I probably won't be able to validate the performance impact myself. Thanks.

[laboger]

We can do it. We have been testing it out.

[gopherbot]

Change https://go.dev/cl/461597 mentions this issue: runtime: fix performance regression in morestack_noctxt on ppc64

[laboger]

@cherrymui we want this in Go 1.20, so is there anything special needed on the CL now that the tree is open for 1.21?

[cherrymui]

The tree hasn't opened yet. So the CL can be submitted as usual.

[archanaravindar]

@gopherbot please backport this to previous releases Go1.19 and Go1.18
Since the fix 54332 that causes this regression on ppc64 has been likewise back ported to these two releases
@laboger @cherrymui @pmur

[gopherbot]

Backport issue(s) opened: #57811 (for 1.18), #57812 (for 1.19).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.