cmd/compile/internal/inline: no-return flag analysis for inline heuristics

Add code to compute whether a given function appears to
unconditionally call panic or exit, as a means of driving inlining
decisions. Note that this determination is based on
heuristics/guesses, as opposed to strict safety analysis; in some
cases we may miss a function that does indeed always panic, or mark a
function as always invoking panic when it doesn't; the intent is get
the right answer in "most" cases.

Updates #61502.

Change-Id: Ibba3e60c06c2e54cf29b3ffa0f816518aaacb9a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/511558
Reviewed-by: Matthew Dempsky <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: thanm

src/cmd/compile/internal/inline/inlheur/analyze.go

@@ -18,8 +18,24 @@ import (
 
 const (
 	debugTraceFuncs = 1 << iota
+	debugTraceFuncFlags
 )
 
+// propAnalyzer interface is used for defining one or more analyzer
+// helper objects, each tasked with computing some specific subset of
+// the properties we're interested in. The assumption is that
+// properties are independent, so each new analyzer that implements
+// this interface can operate entirely on its own. For a given analyzer
+// there will be a sequence of calls to nodeVisitPre and nodeVisitPost
+// as the nodes within a function are visited, then a followup call to
+// setResults so that the analyzer can transfer its results into the
+// final properties object.
+type propAnalyzer interface {
+	nodeVisitPre(n ir.Node)
+	nodeVisitPost(n ir.Node)
+	setResults(fp *FuncProps)
+}
+
 // fnInlHeur contains inline heuristics state information about
 // a specific Go function being analyzed/considered by the inliner.
 type fnInlHeur struct {
@@ -37,8 +53,29 @@ func computeFuncProps(fn *ir.Func) *FuncProps {
 		fmt.Fprintf(os.Stderr, "=-= starting analysis of func %v:\n%+v\n",
 			fn.Sym().Name, fn)
 	}
-	// implementation stubbed out for now
-	return &FuncProps{}
+	ffa := makeFuncFlagsAnalyzer(fn)
+	analyzers := []propAnalyzer{ffa}
+	fp := new(FuncProps)
+	runAnalyzersOnFunction(fn, analyzers)
+	for _, a := range analyzers {
+		a.setResults(fp)
+	}
+	return fp
+}
+
+func runAnalyzersOnFunction(fn *ir.Func, analyzers []propAnalyzer) {
+	var doNode func(ir.Node) bool
+	doNode = func(n ir.Node) bool {
+		for _, a := range analyzers {
+			a.nodeVisitPre(n)
+		}
+		ir.DoChildren(n, doNode)
+		for _, a := range analyzers {
+			a.nodeVisitPost(n)
+		}
+		return false
+	}
+	doNode(fn)
 }
 
 func fnFileLine(fn *ir.Func) (string, uint) {

src/cmd/compile/internal/inline/inlheur/analyze_func_flags.go

@@ -0,0 +1,338 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package inlheur
+
+import (
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/types"
+	"fmt"
+	"os"
+)
+
+// funcFlagsAnalyzer computes the "Flags" value for the FuncProps
+// object we're computing. The main item of interest here is "nstate",
+// which stores the disposition of a given ir Node with respect to the
+// flags/properties we're trying to compute.
+type funcFlagsAnalyzer struct {
+	fn     *ir.Func
+	nstate map[ir.Node]pstate
+	noInfo bool // set if we see something inscrutable/un-analyzable
+}
+
+// pstate keeps track of the disposition of a given node and its
+// children with respect to panic/exit calls.
+type pstate int
+
+const (
+	psNoInfo     pstate = iota // nothing interesting about this node
+	psCallsPanic               // node causes call to panic or os.Exit
+	psMayReturn                // executing node may trigger a "return" stmt
+	psTop                      // dataflow lattice "top" element
+)
+
+func makeFuncFlagsAnalyzer(fn *ir.Func) *funcFlagsAnalyzer {
+	return &funcFlagsAnalyzer{
+		fn:     fn,
+		nstate: make(map[ir.Node]pstate),
+	}
+}
+
+// setResults transfers func flag results to 'fp'.
+func (ffa *funcFlagsAnalyzer) setResults(fp *FuncProps) {
+	var rv FuncPropBits
+	if !ffa.noInfo && ffa.stateForList(ffa.fn.Body) == psCallsPanic {
+		rv = FuncPropNeverReturns
+	}
+	// This is slightly hacky and not at all required, but include a
+	// special case for main.main, which often ends in a call to
+	// os.Exit. People who write code like this (very common I
+	// imagine)
+	//
+	//   func main() {
+	//     rc = perform()
+	//     ...
+	//     foo()
+	//     os.Exit(rc)
+	//   }
+	//
+	// will be constantly surprised when foo() is inlined in many
+	// other spots in the program but not in main().
+	if isMainMain(ffa.fn) {
+		rv &^= FuncPropNeverReturns
+	}
+	fp.Flags = rv
+}
+
+func (ffa *funcFlagsAnalyzer) getstate(n ir.Node) pstate {
+	val, ok := ffa.nstate[n]
+	if !ok {
+		base.Fatalf("funcFlagsAnalyzer: fn %q node %s line %s: internal error, no setting for node:\n%+v\n", ffa.fn.Sym().Name, n.Op().String(), ir.Line(n), n)
+	}
+	return val
+}
+
+func (ffa *funcFlagsAnalyzer) setstate(n ir.Node, st pstate) {
+	if _, ok := ffa.nstate[n]; ok {
+		base.Fatalf("funcFlagsAnalyzer: fn %q internal error, existing setting for node:\n%+v\n", ffa.fn.Sym().Name, n)
+	} else {
+		ffa.nstate[n] = st
+	}
+}
+
+func (ffa *funcFlagsAnalyzer) setstateSoft(n ir.Node, st pstate) {
+	ffa.nstate[n] = st
+}
+
+// blockCombine merges together states as part of a linear sequence of
+// statements, where 'pred' and 'succ' are analysis results for a pair
+// of consecutive statements. Examples:
+//
+//	case 1:             case 2:
+//	    panic("foo")      if q { return x }        <-pred
+//	    return x          panic("boo")             <-succ
+//
+// In case 1, since the pred state is "always panic" it doesn't matter
+// what the succ state is, hence the state for the combination of the
+// two blocks is "always panics". In case 2, because there is a path
+// to return that avoids the panic in succ, the state for the
+// combination of the two statements is "may return".
+func blockCombine(pred, succ pstate) pstate {
+	switch succ {
+	case psTop:
+		return pred
+	case psMayReturn:
+		if pred == psCallsPanic {
+			return psCallsPanic
+		}
+		return psMayReturn
+	case psNoInfo:
+		return pred
+	case psCallsPanic:
+		if pred == psMayReturn {
+			return psMayReturn
+		}
+		return psCallsPanic
+	}
+	panic("should never execute")
+}
+
+// branchCombine combines two states at a control flow branch point where
+// either p1 or p2 executes (as in an "if" statement).
+func branchCombine(p1, p2 pstate) pstate {
+	if p1 == psCallsPanic && p2 == psCallsPanic {
+		return psCallsPanic
+	}
+	if p1 == psMayReturn || p2 == psMayReturn {
+		return psMayReturn
+	}
+	return psNoInfo
+}
+
+// stateForList walks through a list of statements and computes the
+// state/diposition for the entire list as a whole.
+func (ffa *funcFlagsAnalyzer) stateForList(list ir.Nodes) pstate {
+	st := psTop
+	for i := range list {
+		n := list[i]
+		psi := ffa.getstate(n)
+		if debugTrace&debugTraceFuncFlags != 0 {
+			fmt.Fprintf(os.Stderr, "=-= %v: stateForList n=%s ps=%s\n",
+				ir.Line(n), n.Op().String(), psi.String())
+		}
+		st = blockCombine(st, psi)
+	}
+	if st == psTop {
+		st = psNoInfo
+	}
+	return st
+}
+
+func isMainMain(fn *ir.Func) bool {
+	s := fn.Sym()
+	return (s.Pkg.Name == "main" && s.Name == "main")
+}
+
+func isWellKnownFunc(s *types.Sym, pkg, name string) bool {
+	return s.Pkg.Path == pkg && s.Name == name
+}
+
+// isExitCall reports TRUE if the node itself is an unconditional
+// call to os.Exit(), a panic, or a function that does likewise.
+func isExitCall(n ir.Node) bool {
+	if n.Op() != ir.OCALLFUNC {
+		return false
+	}
+	cx := n.(*ir.CallExpr)
+	name := ir.StaticCalleeName(cx.X)
+	if name == nil {
+		return false
+	}
+	s := name.Sym()
+	if isWellKnownFunc(s, "os", "Exit") ||
+		isWellKnownFunc(s, "runtime", "throw") {
+		return true
+	}
+	// FIXME: consult results of flags computation for
+	// previously analyzed Go functions, including props
+	// read from export data for functions in other packages.
+	return false
+}
+
+// pessimize is called to record the fact that we saw something in the
+// function that renders it entirely impossible to analyze.
+func (ffa *funcFlagsAnalyzer) pessimize() {
+	ffa.noInfo = true
+}
+
+// shouldVisit reports TRUE if this is an interesting node from the
+// perspective of computing function flags. NB: due to the fact that
+// ir.CallExpr implements the Stmt interface, we wind up visiting
+// a lot of nodes that we don't really need to, but these can
+// simply be screened out as part of the visit.
+func shouldVisit(n ir.Node) bool {
+	_, isStmt := n.(ir.Stmt)
+	return n.Op() != ir.ODCL &&
+		(isStmt || n.Op() == ir.OCALLFUNC || n.Op() == ir.OPANIC)
+}
+
+// nodeVisitPost helps implement the propAnalyzer interface; when
+// called on a given node, it decides the disposition of that node
+// based on the state(s) of the node's children.
+func (ffa *funcFlagsAnalyzer) nodeVisitPost(n ir.Node) {
+	if debugTrace&debugTraceFuncFlags != 0 {
+		fmt.Fprintf(os.Stderr, "=+= nodevis %v %s should=%v\n",
+			ir.Line(n), n.Op().String(), shouldVisit(n))
+	}
+	if !shouldVisit(n) {
+		// invoke soft set, since node may be shared (e.g. ONAME)
+		ffa.setstateSoft(n, psNoInfo)
+		return
+	}
+	var st pstate
+	switch n.Op() {
+	case ir.OCALLFUNC:
+		if isExitCall(n) {
+			st = psCallsPanic
+		}
+	case ir.OPANIC:
+		st = psCallsPanic
+	case ir.ORETURN:
+		st = psMayReturn
+	case ir.OBREAK, ir.OCONTINUE:
+		// FIXME: this handling of break/continue is sub-optimal; we
+		// have them as "mayReturn" in order to help with this case:
+		//
+		//   for {
+		//     if q() { break }
+		//     panic(...)
+		//   }
+		//
+		// where the effect of the 'break' is to cause the subsequent
+		// panic to be skipped. One possible improvement would be to
+		// track whether the currently enclosing loop is a "for {" or
+		// a for/range with condition, then use mayReturn only for the
+		// former. Note also that "break X" or "continue X" is treated
+		// the same as "goto", since we don't have a good way to track
+		// the target of the branch.
+		st = psMayReturn
+		n := n.(*ir.BranchStmt)
+		if n.Label != nil {
+			ffa.pessimize()
+		}
+	case ir.OBLOCK:
+		n := n.(*ir.BlockStmt)
+		st = ffa.stateForList(n.List)
+	case ir.OCASE:
+		if ccst, ok := n.(*ir.CaseClause); ok {
+			st = ffa.stateForList(ccst.Body)
+		} else if ccst, ok := n.(*ir.CommClause); ok {
+			st = ffa.stateForList(ccst.Body)
+		} else {
+			panic("unexpected")
+		}
+	case ir.OIF:
+		n := n.(*ir.IfStmt)
+		st = branchCombine(ffa.stateForList(n.Body), ffa.stateForList(n.Else))
+	case ir.OFOR:
+		// Treat for { XXX } like a block.
+		// Treat for <cond> { XXX } like an if statement with no else.
+		n := n.(*ir.ForStmt)
+		bst := ffa.stateForList(n.Body)
+		if n.Cond == nil {
+			st = bst
+		} else {
+			if bst == psMayReturn {
+				st = psMayReturn
+			}
+		}
+	case ir.ORANGE:
+		// Treat for range { XXX } like an if statement with no else.
+		n := n.(*ir.RangeStmt)
+		if ffa.stateForList(n.Body) == psMayReturn {
+			st = psMayReturn
+		}
+	case ir.OGOTO:
+		// punt if we see even one goto. if we built a control
+		// flow graph we could do more, but this is just a tree walk.
+		ffa.pessimize()
+	case ir.OSELECT:
+		// process selects for "may return" but not "always panics",
+		// the latter case seems very improbable.
+		n := n.(*ir.SelectStmt)
+		if len(n.Cases) != 0 {
+			st = psTop
+			for _, c := range n.Cases {
+				st = branchCombine(ffa.stateForList(c.Body), st)
+			}
+		}
+	case ir.OSWITCH:
+		n := n.(*ir.SwitchStmt)
+		if len(n.Cases) != 0 {
+			st = psTop
+			for _, c := range n.Cases {
+				st = branchCombine(ffa.stateForList(c.Body), st)
+			}
+		}
+
+		st, fall := psTop, psNoInfo
+		for i := len(n.Cases) - 1; i >= 0; i-- {
+			cas := n.Cases[i]
+			cst := ffa.stateForList(cas.Body)
+			endsInFallthrough := false
+			if len(cas.Body) != 0 {
+				endsInFallthrough = cas.Body[0].Op() == ir.OFALL
+			}
+			if endsInFallthrough {
+				cst = blockCombine(cst, fall)
+			}
+			st = branchCombine(st, cst)
+			fall = cst
+		}
+	case ir.OFALL:
+		// Not important.
+	case ir.ODCLFUNC, ir.ORECOVER, ir.OAS, ir.OAS2, ir.OAS2FUNC, ir.OASOP,
+		ir.OPRINTN, ir.OPRINT, ir.OLABEL, ir.OCALLINTER, ir.ODEFER,
+		ir.OSEND, ir.ORECV, ir.OSELRECV2, ir.OGO, ir.OAPPEND, ir.OAS2DOTTYPE,
+		ir.OAS2MAPR, ir.OGETG, ir.ODELETE, ir.OINLMARK, ir.OAS2RECV,
+		ir.OMIN, ir.OMAX, ir.OMAKE, ir.ORECOVERFP, ir.OGETCALLERSP:
+		// these should all be benign/uninteresting
+	case ir.OTAILCALL, ir.OJUMPTABLE, ir.OTYPESW:
+		// don't expect to see these at all.
+		base.Fatalf("unexpected op %s in func %s",
+			n.Op().String(), ir.FuncName(ffa.fn))
+	default:
+		base.Fatalf("%v: unhandled op %s in func %v",
+			ir.Line(n), n.Op().String(), ir.FuncName(ffa.fn))
+	}
+	if debugTrace&debugTraceFuncFlags != 0 {
+		fmt.Fprintf(os.Stderr, "=-= %v: visit n=%s returns %s\n",
+			ir.Line(n), n.Op().String(), st.String())
+	}
+	ffa.setstate(n, st)
+}
+
+func (ffa *funcFlagsAnalyzer) nodeVisitPre(n ir.Node) {
+}