cmd/compile: several arm64 code-gen issues

[shawndx]

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

$ go version
tip

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

What did you do?

Several code-gen (optimization) issues relevant to utilizing CMN/CMP instructions on arm64 were spotted recently, it will take time to identify all of them, create a placeholder issue tracker for updating.

example:

A condition like "if (x1+3) < 0" gets translated to the following instructions, that is transforming to "if x1 < -3"
cmn x0, #0x3
b.ge

But such transformation doesn't hold true, in case executing 'cmn' set the overflow flag, internally 'cmn' is an alias of the 'adds' instruction which performs add with carry set operation.

What did you expect to see?

What did you see instead?

[shawndx]

working on collecting all scenarios that hit the problem and a fix.

[shawndx]

rewriteARM64_test.txt
Attaching an initial version of the negative scenarios identified so far.

[gopherbot]

Change https://golang.org/cl/233097 mentions this issue: cmd/compile: remove unexpected rewriting to conditional expressions

[randall77]

This issue is causing incorrect code generation on (at least) arm64 and arm.
It's a pretty esoteric case (comparisons after integer overflow), so I don't think it should block beta1, but should be fixed before release.

[randall77]

... and will probably need backporting as well, depending on what the fix looks like.

[randall77]

I think this is now fixed. Please reopen if you think there's more to do here.