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>

2 files changed, 47 insertions, 2 deletions

diff --git a/test/codegen/memcombine.go b/test/codegen/memcombine.go
index d5f3af7692..e2d703cb0c 100644
--- a/test/codegen/memcombine.go
+++ b/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
diff --git a/test/fixedbugs/issue33724.go b/test/fixedbugs/issue33724.go
new file mode 100644
index 0000000000..a4ecddc0b3
--- /dev/null
+++ b/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()
+}