cmd/compile: unnecessary register moves

[rip-create-your-account]

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

$ go version
go version devel go1.21-3ed8a1e629 Tue Mar 28 05:41:44 2023 +0000 linux/amd64

Does this issue reproduce with the latest release?

Not on go1.20.2. The generated assembly is in other ways very different to gotip.

What did you do?

https://go.dev/play/p/eKdUtP-irZa

The generated assembly for the Checksum function contains instructions that to my understanding are unnecessary. By removing the []byte return value (like in the Checksum2 function) the generated code for the loop body is what I expected.

What did you expect to see?

The generated assembly for the Checksum function:

...
MOVQ 0x30(AX), DI
BSWAP DI
...

What did you see instead?

There is one occurrence of:

...
MOVQ 0x28(AX), R13
BSWAP R13
vvv
MOVQ 0x30(AX), R15
BSWAP R15
MOVQ R15, DI
^^^
MOVQ 0x38(AX), R15
BSWAP R15
...

My understanding is that the MOVQ first to R15 just to then move it to DI is unnecessary. MOVQ directly to DI is what I expected.

[randall77]

The register allocator is trying and failing to keep DI unoccupied because it thinks it needs it for the return value (the uint16 one).
I think the priorities are just wrong. Saving the register may help later. Moving the value definitely costs now.

[gopherbot]

Change https://go.dev/cl/479895 mentions this issue: cmd/compile: lower priority of avoiding registers