cmd/compile: trim more blocks?

[josharian]

In ssa/trim.go, func emptyBlock, the only op that counts as not generating instructions is OpPhi. But OpVarDef, OpVarKill, OpVarLive, and OpKeepAlive could probably qualify as "not generating instructions" as well. This would allow trimming more blocks.

cc @randall77

[randall77]

Do you have an example where this happens?

[josharian]

Sure. func pcvalue in package runtime:

b7: ← b8
v63 = VarKill <mem> {.autotmp_22} v30
Plain → b15

It looks like this is killing the temp copy of cache.entries on the way out of the cache checking loop.

Instrumenting the compiler shows 295 new trimmable blocks in make.bash.

func emptyBlock(b *Block) bool {
	if emptyBlock2(b) && !emptyBlock1(b) {
		fmt.Printf("%v in %v\n", b, b.Func.Name)
	}
	return emptyBlock1(b)
}

func emptyBlock1(b *Block) bool {
	for _, v := range b.Values {
		switch v.Op {
		case OpPhi:
			// not an actual instruction
		default:
			return false
		}
	}
	return true
}

func emptyBlock2(b *Block) bool {
	for _, v := range b.Values {
		switch v.Op {
		case OpPhi, OpVarDef, OpVarKill, OpVarLive, OpKeepAlive:
			// not an actual instruction
		default:
			return false
		}
	}
	return true
}

[josharian]

The new “zero width” op flag might be helpful for making this more powerful: https://go-review.googlesource.com/#/c/97957/