cmd/compile: preserve statements in late nilcheckelim optimization

When a subsequent load/store of a ptr makes the nil check of that pointer
unnecessary, if their lines differ, change the line of the load/store
to that of the nilcheck, and attempt to rehome the load/store position
instead.

This fix makes profiling less accurate in order to make panics more
informative.

Fixes #33724

Change-Id: Ib9afaac12fe0d0320aea1bf493617facc34034b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/200197
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -199,8 +199,8 @@ var faultOnLoad = objabi.GOOS != "aix"
 // nilcheckelim2 eliminates unnecessary nil checks.
 // Runs after lowering and scheduling.
 func nilcheckelim2(f *Func) {
-	unnecessary := f.newSparseSet(f.NumValues())
-	defer f.retSparseSet(unnecessary)
+	unnecessary := f.newSparseMap(f.NumValues()) // map from pointer that will be dereferenced to index of dereferencing value in b.Values[]
+	defer f.retSparseMap(unnecessary)
 
 	pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block
 
@@ -218,9 +218,21 @@ func nilcheckelim2(f *Func) {
 				if f.fe.Debug_checknil() && v.Pos.Line() > 1 {
 					f.Warnl(v.Pos, "removed nil check")
 				}
-				if v.Pos.IsStmt() == src.PosIsStmt {
+				// For bug 33724, policy is that we might choose to bump an existing position
+				// off the faulting load/store in favor of the one from the nil check.
+
+				// Iteration order means that first nilcheck in the chain wins, others
+				// are bumped into the ordinary statement preservation algorithm.
+				u := b.Values[unnecessary.get(v.Args[0].ID)]
+				if !u.Pos.SameFileAndLine(v.Pos) {
+					if u.Pos.IsStmt() == src.PosIsStmt {
+						pendingLines.add(u.Pos)
+					}
+					u.Pos = v.Pos
+				} else if v.Pos.IsStmt() == src.PosIsStmt {
 					pendingLines.add(v.Pos)
 				}
+
 				v.reset(OpUnknown)
 				firstToRemove = i
 				continue
@@ -294,7 +306,7 @@ func nilcheckelim2(f *Func) {
 				}
 				// This instruction is guaranteed to fault if ptr is nil.
 				// Any previous nil check op is unnecessary.
-				unnecessary.add(ptr.ID)
+				unnecessary.set(ptr.ID, int32(i), src.NoXPos)
 			}
 		}
 		// Remove values we've clobbered with OpUnknown.
@@ -302,7 +314,7 @@ func nilcheckelim2(f *Func) {
 		for j := i; j < len(b.Values); j++ {
 			v := b.Values[j]
 			if v.Op != OpUnknown {
-				if v.Pos.IsStmt() != src.PosNotStmt && pendingLines.contains(v.Pos) {
+				if !notStmtBoundary(v.Op) && pendingLines.contains(v.Pos) { // Late in compilation, so any remaining NotStmt values are probably okay now.
 					v.Pos = v.Pos.WithIsStmt()
 					pendingLines.remove(v.Pos)
 				}

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

@@ -74,7 +74,7 @@ func nextGoodStatementIndex(v *Value, i int, b *Block) int {
 // rewrite.
 func notStmtBoundary(op Op) bool {
 	switch op {
-	case OpCopy, OpPhi, OpVarKill, OpVarDef, OpUnknown, OpFwdRef, OpArg:
+	case OpCopy, OpPhi, OpVarKill, OpVarDef, OpVarLive, OpUnknown, OpFwdRef, OpArg:
 		return true
 	}
 	return false

test/codegen/memcombine.go

@@ -321,8 +321,8 @@ func fcall_uint32(a, b uint32) (uint32, uint32) {
 // We want to merge load+op in the first function, but not in the
 // second. See Issue 19595.
 func load_op_merge(p, q *int) {
-	x := *p
-	*q += x // amd64:`ADDQ\t\(`
+	x := *p // amd64:`ADDQ\t\(`
+	*q += x // The combined nilcheck and load would normally have this line number, but we want that combined operation to have the line number of the nil check instead (see #33724).
 }
 func load_op_no_merge(p, q *int) {
 	x := *p

test/fixedbugs/issue33724.go

@@ -0,0 +1,45 @@
+// run
+package main
+
+import (
+	"fmt"
+	"runtime/debug"
+	"strings"
+)
+
+type Inner struct {
+	Err int
+}
+
+func (i *Inner) NotExpectedInStackTrace() int {
+	if i == nil {
+		return 86
+	}
+	return 17 + i.Err
+}
+
+type Outer struct {
+	Inner
+}
+
+func ExpectedInStackTrace() {
+	var o *Outer
+	println(o.NotExpectedInStackTrace())
+}
+
+func main() {
+    defer func() {
+        if r := recover(); r != nil {
+        	stacktrace := string(debug.Stack())
+        	if strings.Contains(stacktrace, "NotExpectedInStackTrace") {
+        		fmt.Println("FAIL, stacktrace contains NotExpectedInStackTrace")
+        	}
+        	if !strings.Contains(stacktrace, "ExpectedInStackTrace") {
+        		fmt.Println("FAIL, stacktrace does not contain ExpectedInStackTrace")
+        	}
+        } else {
+        	fmt.Println("FAIL, should have panicked but did not")
+        }
+    }()
+    ExpectedInStackTrace()
+}