cmd/compile: DCE before inlining does not remove dead `if`s

[CAFxX]

DCE performed before inlining seems to eliminate the body of ifs where the condition can be proven to always evaluate to false, but fails to eliminate the if statement and the condition themselves. So, for example, the following code:

// code before if 
if condition_known_to_be_false {
  // code inside if
}
// code after if

that should be DCEd (before inlining) to:

// code before if
// (if statement has been completely removed)
// code after if

but currently DCE transforms it to:

// code before if
if condition_known_to_be_false { /* empty body */ }
// code after if

Found by going over the output of GO_GCFLAGS=-m ./make.bash (the following is just a sample, see #8421 (comment) for details):

src/math/bits/bits.go:283:6: can inline Len as: func(uint) int { if UintSize == 32 {  }; return Len64(uint64(x)) }
src/runtime/cgocall.go:176:14: inlining call to dolockOSThread func() { if GOARCH == "wasm" {  }; _g_ := getg(); _g_.m.lockedg.set(_g_); _g_.lockedm.set(_g_.m) }
src/runtime/stack.go:1277:24: inlining call to stackmapdata func(*stackmap, int32) bitvector { if stackDebug > 0 {  }; return bitvector literal }
src/runtime/malloc.go:1105:6: can inline nextSample as: func() int32 { if GOOS == "plan9" {  }; return fastexprand(MemProfileRate) }
src/go/token/position.go:452:15: inlining call to sync.(*RWMutex).RLock method(*sync.RWMutex) func() { if bool(false) {  }; if atomic.AddInt32(&sync.rw.readerCount, int32(1)) < int32(0) { sync.runtime_SemacquireMutex(&sync.rw.readerSem, bool(false)) }; if bool(false) {  } }
src/runtime/type.go:269:20: inlining call to reflectOffsUnlock func() { if raceenabled {  }; unlock(&reflectOffs.lock) }

This is a problem because part of the inlining budget will be consumed by the vestigial ifs. If DCE before inlining completely removed those ifs more functions would likely become candidate for inlining.

Likely related to #23521 and possibly to #29095.

[mvdan]

Are you positive that the inlining call to messages show you the nodes as they are being inlined? For example, perhaps it's showing you the original source code minus comments.

I haven't actually checked; I'm simply pointing out that this might be an issue with the -m messages and not the DCE or inline passes.

[josharian]

On my phone but the condition is probably being printed from n.Orig. But there are still extraneous OIF and OLITERAL (“if” and “true”) nodes there.

[CAFxX]

@mvdan I don't have complete certainty, but for sure those if bodies are not empty, see e.g.

go/src/runtime/malloc.go

Lines 1105 to 1114 in 944a9c7

	func nextSample() int32 {
	if GOOS == "plan9" {
	// Plan 9 doesn't support floating point in note handler.
	if g := getg(); g == g.m.gsignal {
	return nextSampleNoFP()
	}
	}
	return fastexprand(MemProfileRate)
	}

src/runtime/malloc.go:1105:6: can inline nextSample as: func() int32 { if GOOS == "plan9" {  }; return fastexprand(MemProfileRate) }

[mdempsky]

The inliner has logic for making sure constant if expressions don't cost anything extra:

go/src/cmd/compile/internal/gc/inl.go

Lines 396 to 401 in e6ae4e8

	case OIF:
	if Isconst(n.Left, CTBOOL) {
	// This if and the condition cost nothing.
	return v.visitList(n.Ninit) || v.visitList(n.Nbody) ||
	v.visitList(n.Rlist)
	}

The diagnostic message might be a little confusing, but I don't think it should be affecting the output binaries at all.

The if bodies are empty because of the deadcode elimination pass done after typechecking function bodies:

go/src/cmd/compile/internal/gc/typecheck.go

Lines 3882 to 3892 in e6ae4e8

	if n.Op == OIF {
	n.Left = deadcodeexpr(n.Left)
	if Isconst(n.Left, CTBOOL) {
	var body Nodes
	if n.Left.Bool() {
	n.Rlist = Nodes{}
	body = n.Nbody
	} else {
	n.Nbody = Nodes{}
	body = n.Rlist
	}

[mdempsky]

Closing because I think this is working as intended. Please let us know to reopen though if you have evidence that a constant if statement is actually affecting inlining (e.g., either it's affecting budgeting, or somehow affecting the resulting assembly negatively).