cmd/compile: drop column info when line number saturates

When line number saturates, we can end up getting non-monotonic
position info, because the start of the next line after line=lineMax,col=2
is line=lineMax,col=1.

Instead, if line==lineMax, make the column always 0 (no column info).
If the line number is wrong, having column info probably isn't that helpful.

Fixes #51193

Change-Id: If3d90472691b1f6163654f3505e2cb98467f2383
Reviewed-on: https://go-review.googlesource.com/c/go/+/385795
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

2 files changed, 11 insertions, 8 deletions

diff --git a/src/cmd/internal/src/pos.go b/src/cmd/internal/src/pos.go
index b6816a56e0..373a22a7f2 100644
--- a/src/cmd/internal/src/pos.go
+++ b/src/cmd/internal/src/pos.go
@@ -389,9 +389,12 @@ func makeBogusLico() lico {
 }
 
 func makeLico(line, col uint) lico {
-	if line > lineMax {
+	if line >= lineMax {
 		// cannot represent line, use max. line so we have some information
 		line = lineMax
+		// Drop column information if line number saturates.
+		// Ensures line+col is monotonic. See issue 51193.
+		col = 0
 	}
 	if col > colMax {
 		// cannot represent column, use max. column so we have some information
diff --git a/src/cmd/internal/src/pos_test.go b/src/cmd/internal/src/pos_test.go
index d4cd0e7ff1..cdf4ab4081 100644
--- a/src/cmd/internal/src/pos_test.go
+++ b/src/cmd/internal/src/pos_test.go
@@ -140,8 +140,8 @@ func TestLico(t *testing.T) {
 		{makeLico(1, 0), ":1", 1, 0},
 		{makeLico(1, 1), ":1:1", 1, 1},
 		{makeLico(2, 3), ":2:3", 2, 3},
-		{makeLico(lineMax, 1), fmt.Sprintf(":%d:1", lineMax), lineMax, 1},
-		{makeLico(lineMax+1, 1), fmt.Sprintf(":%d:1", lineMax), lineMax, 1}, // line too large, stick with max. line
+		{makeLico(lineMax, 1), fmt.Sprintf(":%d", lineMax), lineMax, 1},
+		{makeLico(lineMax+1, 1), fmt.Sprintf(":%d", lineMax), lineMax, 1}, // line too large, stick with max. line
 		{makeLico(1, colMax), ":1", 1, colMax},
 		{makeLico(1, colMax+1), ":1", 1, 0}, // column too large
 		{makeLico(lineMax+1, colMax+1), fmt.Sprintf(":%d", lineMax), lineMax, 0},
@@ -170,8 +170,8 @@ func TestIsStmt(t *testing.T) {
 		{makeLico(1, 1), ":1:1" + def, 1, 1},
 		{makeLico(1, 1).withIsStmt(), ":1:1" + is, 1, 1},
 		{makeLico(1, 1).withNotStmt(), ":1:1" + not, 1, 1},
-		{makeLico(lineMax, 1), fmt.Sprintf(":%d:1", lineMax) + def, lineMax, 1},
-		{makeLico(lineMax+1, 1), fmt.Sprintf(":%d:1", lineMax) + def, lineMax, 1}, // line too large, stick with max. line
+		{makeLico(lineMax, 1), fmt.Sprintf(":%d", lineMax) + def, lineMax, 1},
+		{makeLico(lineMax+1, 1), fmt.Sprintf(":%d", lineMax) + def, lineMax, 1}, // line too large, stick with max. line
 		{makeLico(1, colMax), ":1" + def, 1, colMax},
 		{makeLico(1, colMax+1), ":1" + def, 1, 0}, // column too large
 		{makeLico(lineMax+1, colMax+1), fmt.Sprintf(":%d", lineMax) + def, lineMax, 0},
@@ -214,9 +214,9 @@ func TestLogue(t *testing.T) {
 		{makeLico(1, 1).withXlogue(PosPrologueEnd), ":1:1" + defs + pro, 1, 1},
 		{makeLico(1, 1).withXlogue(PosEpilogueBegin), ":1:1" + defs + epi, 1, 1},
 
-		{makeLico(lineMax, 1).withXlogue(PosDefaultLogue), fmt.Sprintf(":%d:1", lineMax) + defs + defp, lineMax, 1},
-		{makeLico(lineMax, 1).withXlogue(PosPrologueEnd), fmt.Sprintf(":%d:1", lineMax) + defs + pro, lineMax, 1},
-		{makeLico(lineMax, 1).withXlogue(PosEpilogueBegin), fmt.Sprintf(":%d:1", lineMax) + defs + epi, lineMax, 1},
+		{makeLico(lineMax, 1).withXlogue(PosDefaultLogue), fmt.Sprintf(":%d", lineMax) + defs + defp, lineMax, 1},
+		{makeLico(lineMax, 1).withXlogue(PosPrologueEnd), fmt.Sprintf(":%d", lineMax) + defs + pro, lineMax, 1},
+		{makeLico(lineMax, 1).withXlogue(PosEpilogueBegin), fmt.Sprintf(":%d", lineMax) + defs + epi, lineMax, 1},
 	} {
 		x := test.x
 		if got := formatstr("", x.Line(), x.Col(), true) + fmt.Sprintf(":%d:%d", x.IsStmt(), x.Xlogue()); got != test.string {