cmd/compile: get rid of unnecessary inline marks

If no other instruction mentions an inline mark, we can get rid of it.
This normally happens when the inlined function is empty, or when all
of its code is folded into other instructions.

Also use consistent statement-ness for inline mark positions, so that
more of them can be removed in favor of existing instructions.

Update #29571
Fixes #31172

Change-Id: I71f84d355101f37a27960d9e8528f42f92767496
Reviewed-on: https://go-review.googlesource.com/c/go/+/170445
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -1054,7 +1054,7 @@ func mkinlcall(n, fn *Node, maxCost int32) *Node {
 	// to put a breakpoint. Not sure if that's really necessary or not
 	// (in which case it could go at the end of the function instead).
 	inlMark := nod(OINLMARK, nil, nil)
-	inlMark.Pos = n.Pos
+	inlMark.Pos = n.Pos.WithDefaultStmt()
 	inlMark.Xoffset = int64(newIndex)
 	ninit.Append(inlMark)
 

src/cmd/compile/internal/ssa/deadcode.go

@@ -76,6 +76,30 @@ func liveValues(f *Func, reachable []bool) (live []bool, liveOrderStmts []*Value
 		return
 	}
 
+	// Record all the inline indexes we need
+	var liveInlIdx map[int]bool
+	pt := f.Config.ctxt.PosTable
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			i := pt.Pos(v.Pos).Base().InliningIndex()
+			if i < 0 {
+				continue
+			}
+			if liveInlIdx == nil {
+				liveInlIdx = map[int]bool{}
+			}
+			liveInlIdx[i] = true
+		}
+		i := pt.Pos(b.Pos).Base().InliningIndex()
+		if i < 0 {
+			continue
+		}
+		if liveInlIdx == nil {
+			liveInlIdx = map[int]bool{}
+		}
+		liveInlIdx[i] = true
+	}
+
 	// Find all live values
 	q := f.Cache.deadcode.q[:0]
 	defer func() { f.Cache.deadcode.q = q }()
@@ -103,6 +127,13 @@ func liveValues(f *Func, reachable []bool) (live []bool, liveOrderStmts []*Value
 			}
 			if v.Type.IsVoid() && !live[v.ID] {
 				// The only Void ops are nil checks and inline marks.  We must keep these.
+				if v.Op == OpInlMark && !liveInlIdx[int(v.AuxInt)] {
+					// We don't need marks for bodies that
+					// have been completely optimized away.
+					// TODO: save marks only for bodies which
+					// have a faulting instruction or a call?
+					continue
+				}
 				live[v.ID] = true
 				q = append(q, v)
 				if v.Pos.IsStmt() != src.PosNotStmt {