cmd/compile: rotate instructions are not always emitted when they could be

[dgryski]

go version devel +41908a5453 Thu Dec 1 02:54:21 2016 +0000 linux/amd64
(1.8beta1)

What did you do?

https://play.golang.org/p/eyIlsgnmc4

What did you expect to see?

Both of the rotate functions can be inlined. For rot7, a ROLL instruction is generated:

ROLL    $7, AX

However, the constant propagation is incomplete for rot:

MOVQ    AX, CX
SHLL    $7, AX
SHRL    $25, CX
ORL     CX, AX

Rotate instructions are common in crypto code and hashing algorithms. Here's an example of a significant speedup by forcing the compiler to emit the ROLQ:
cespare/xxhash@4a66a12

[bradfitz]

/cc @randall77

[randall77]

This issue is a side effect of doing rotate detection in walk instead of the SSA rewriter.
We just need to put rotate-generation rules in the SSA rewriter and remove them from walk.
Then we can retire the OLROT opcode. And for that matter, all of walkrotate.

[dgryski]

I might try to tackle this.

[randall77]

See https://go-review.googlesource.com/#/c/34232/

[gopherbot]

CL https://golang.org/cl/34232 mentions this issue.

[rasky]

This function does not generate a rotation, even after CL 34232 is applied:

func testRotate(op uint32) uint32 {
	rot := uint((op >> 7) & 0x1E)
	return ((op & 0xFF) >> rot) | ((op & 0xFF) << (32 - rot))
}

Should I open a different issue?

[randall77]

@rasky Yes, if you could please open a different issue.
This issue is about rotates by a constant amount.
Your issue is rotates by a non-constant amount.