cmd/compile/internal: add a PGO devirt post-lookup cleanup hook

The PGO-based devirtualization helper pgoir.addIndirectEdges makes a
series of calls into the unified IR reader to import functions that
would not normally be imported but may be the target of a hot indirect
call from the current package. This importing primarily targets at
non-generic functions and methods, but as part of the process we can
encounter types that have methods (including generic methods) whose
bodies need to be read in. When the reader encounters an inlinable
func of this sort, it may (depending on the context) decide not to
read the body right away, but instead adds the func to a list
("todoBodies") to be read in later on in a more convenient context.

In the bug in question, a hot method lookup takes place in
pgoir.addIndirectEdges, and as part of the import process we wind up
with a type T with method M that is in this partially created state,
and in addition T gets added to the unified IR's list of types that
may need method wrappers. During wrapper generation we create a new
wrapper "(*T).M" whose body has a call to "T.M", then farther on down
the pike during escape analysis we try to analyze the two functions;
this causes a crash due to "T.M" being in partially constructed state.

As a fix, add a new "PostLookupCleanup" hook (in the unified IR
reader) that pgoir.addIndirectEdges can invoke that takes care of
reading in the bodies of any functions that have been added to the
"todoBodies" list.

[Note: creating a test case for this problem is proving to be very
tricky; a new test will be added in a subsequent patch].

Fixes #67746.

Change-Id: Ibc47ee79e08a55421728d35341df80a865231cff
Reviewed-on: https://go-review.googlesource.com/c/go/+/591075
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/devirtualize/pgo.go

@@ -364,11 +364,15 @@ func constructCallStat(p *pgoir.Profile, fn *ir.Func, name string, call *ir.Call
 		return e.Dst.Name() < stat.Hottest
 	}
 
+	callerNode := p.WeightedCG.IRNodes[name]
+	if callerNode == nil {
+		return nil
+	}
+
 	// Sum of all edges from this callsite, regardless of callee.
 	// For direct calls, this should be the same as the single edge
 	// weight (except for multiple calls on one line, which we
 	// can't distinguish).
-	callerNode := p.WeightedCG.IRNodes[name]
 	for _, edge := range callerNode.OutEdges {
 		if edge.CallSiteOffset != offset {
 			continue
@@ -656,6 +660,10 @@ func findHotConcreteCallee(p *pgoir.Profile, caller *ir.Func, call *ir.CallExpr,
 	callerNode := p.WeightedCG.IRNodes[callerName]
 	callOffset := pgoir.NodeLineOffset(call, caller)
 
+	if callerNode == nil {
+		return nil, 0
+	}
+
 	var hottest *pgoir.IREdge
 
 	// Returns true if e is hotter than hottest.

src/cmd/compile/internal/noder/unified.go

@@ -69,6 +69,14 @@ func LookupFunc(fullName string) (*ir.Func, error) {
 	return nil, fmt.Errorf("%s is not a function (%v) or method (%v)", fullName, err, mErr)
 }
 
+// PostLookupCleanup performs cleanup operations needed
+// after a series of calls to LookupFunc, specifically invoking
+// readBodies to post-process any funcs on the "todoBodies" list
+// that were added as a result of the lookup operations.
+func PostLookupCleanup() {
+	readBodies(typecheck.Target, false)
+}
+
 func lookupFunction(pkg *types.Pkg, symName string) (*ir.Func, error) {
 	sym := pkg.Lookup(symName)
 
@@ -179,6 +187,7 @@ func unified(m posMap, noders []*noder) {
 	inline.InlineCall = unifiedInlineCall
 	typecheck.HaveInlineBody = unifiedHaveInlineBody
 	pgoir.LookupFunc = LookupFunc
+	pgoir.PostLookupCleanup = PostLookupCleanup
 
 	data := writePkgStub(m, noders)
 

src/cmd/compile/internal/pgoir/irgraph.go

@@ -267,7 +267,16 @@ func addIREdge(callerNode *IRNode, callerName string, call ir.Node, callee *ir.F
 // LookupFunc looks up a function or method in export data. It is expected to
 // be overridden by package noder, to break a dependency cycle.
 var LookupFunc = func(fullName string) (*ir.Func, error) {
-	base.Fatalf("pgo.LookupMethodFunc not overridden")
+	base.Fatalf("pgoir.LookupMethodFunc not overridden")
+	panic("unreachable")
+}
+
+// PostLookupCleanup performs any remaining cleanup operations needed
+// after a series of calls to LookupFunc, specifically reading in the
+// bodies of functions that may have been delayed due being encountered
+// in a stage where the reader's curfn state was not set up.
+var PostLookupCleanup = func() {
+	base.Fatalf("pgoir.PostLookupCleanup not overridden")
 	panic("unreachable")
 }
 
@@ -386,6 +395,8 @@ func addIndirectEdges(g *IRGraph, namedEdgeMap pgo.NamedEdgeMap) {
 		}
 		callerNode.OutEdges[key] = edge
 	}
+
+	PostLookupCleanup()
 }
 
 // PrintWeightedCallGraphDOT prints IRGraph in DOT format.