cmd/compile: Duplicated sequence of instructions in runtime.adjustframe on AMD64

[jake-ciolek]

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

go tip

Does this issue reproduce with the latest release?

Yes

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

go env Output

Darwin AMD64

What did you do?

Looked at the assembly code of runtime.adjustframe.

What did you expect to see?

movl	$1, %eax
movq	128(%rsp), %rbp
addq	$136, %rsp
retq

I expected to see this once since there's a ret.

What did you see instead?

movl	$1, %eax
movq	128(%rsp), %rbp
addq	$136, %rsp
retq
movl	$1, %eax
movq	128(%rsp), %rbp
addq	$136, %rsp
retq
movl	$1, %eax
movq	128(%rsp), %rbp
addq	$136, %rsp
retq

Perhaps I am missing something and there's a valid reason for this, but it seems to me that we could use this sequence once and save 40+ bytes of instructions. Not sure if it's occuring anywhere else.

[ianlancetaylor]

If you look at the line table information, you'll see that there are different line numbers associated with each of those instruction blocks. There are three of them because there are three different return true statements in the function. It's not possible for a Go compiler to merge instruction sequences in the general case, because that will cause a panic to show the wrong line number. This particular instruction sequence probably can't panic, but a signal is still a possibility. It's not clear to me whether these instructions can be merged or not.

[randall77]

See #24936 (#24936 (comment))
I had a CL to do something like this, never really got around to completing it https://go-review.googlesource.com/c/go/+/265100

[ianlancetaylor]

Thanks, closing as dup of #24936.