cmd/compile: try to preserve IsStmt marks from OpConvert

Note when a statement mark was not consumed during Prog
generation, and try to use it on a subsequent opcode so
that the statement marker will not be lost.

And a test.

Fixes #49628.

Change-Id: I03f7782a9809cc4a0a5870df92b3e182cf124554
Reviewed-on: https://go-review.googlesource.com/c/go/+/366694
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -82,6 +82,25 @@ func TestDebugLinesPushback(t *testing.T) {
 	}
 }
 
+func TestDebugLinesConvert(t *testing.T) {
+	if runtime.GOOS != "linux" && runtime.GOOS != "darwin" { // in particular, it could be windows.
+		t.Skip("this test depends on creating a file with a wonky name, only works for sure on Linux and Darwin")
+	}
+
+	switch testGoArch() {
+	default:
+		t.Skip("skipped for many architectures")
+
+	case "arm64", "amd64": // register ABI
+		fn := "G[go.shape.int_0]"
+		if buildcfg.Experiment.Unified {
+			// Unified mangles differently
+			fn = "G[int]"
+		}
+		testDebugLines(t, "-N -l -G=3", "convertline.go", fn, []int{9, 10, 11}, true)
+	}
+}
+
 func TestInlineLines(t *testing.T) {
 	if runtime.GOARCH != "amd64" && *testGoArchFlag == "" {
 		// As of september 2021, works for everything except mips64, but still potentially fragile

src/cmd/compile/internal/ssa/testdata/convertline.go

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+func F[T any](n T) {
+	fmt.Printf("called\n")
+}
+
+func G[T any](n T) {
+	F(n)
+	fmt.Printf("after\n")
+}
+
+func main() {
+	G(3)
+}

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

@@ -6577,6 +6577,22 @@ func (s *State) DebugFriendlySetPosFrom(v *ssa.Value) {
 			// explicit statement boundaries should appear
 			// in the generated code.
 			if p.IsStmt() != src.PosIsStmt {
+				if s.pp.Pos.IsStmt() == src.PosIsStmt && s.pp.Pos.SameFileAndLine(p) {
+					// If s.pp.Pos already has a statement mark, then it was set here (below) for
+					// the previous value.  If an actual instruction had been emitted for that
+					// value, then the statement mark would have been reset.  Since the statement
+					// mark of s.pp.Pos was not reset, this position (file/line) still needs a
+					// statement mark on an instruction.  If file and line for this value are
+					// the same as the previous value, then the first instruction for this
+					// value will work to take the statement mark.  Return early to avoid
+					// resetting the statement mark.
+					//
+					// The reset of s.pp.Pos occurs in (*Progs).Prog() -- if it emits
+					// an instruction, and the instruction's statement mark was set,
+					// and it is not one of the LosesStmtMark instructions,
+					// then Prog() resets the statement mark on the (*Progs).Pos.
+					return
+				}
 				p = p.WithNotStmt()
 				// Calls use the pos attached to v, but copy the statement mark from State
 			}
@@ -6818,6 +6834,7 @@ func genssa(f *ssa.Func, pp *objw.Progs) {
 	for i, b := range f.Blocks {
 		s.bstart[b.ID] = s.pp.Next
 		s.lineRunStart = nil
+		s.SetPos(s.pp.Pos.WithNotStmt()) // It needs a non-empty Pos, but cannot be a statement boundary (yet).
 
 		// Attach a "default" liveness info. Normally this will be
 		// overwritten in the Values loop below for each Value. But