cmd/compile: use block starting position for phi line number

Fixes #75615

Change-Id: I2c7f0ea1203e8a97749c9f780c29a66050f0159d
Reviewed-on: https://go-review.googlesource.com/c/go/+/710355
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: randall77

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

@@ -137,17 +137,17 @@ func TestStmtLines(t *testing.T) {
 		}
 	}
 
-	var m int
+	var m float64
 	if runtime.GOARCH == "amd64" {
-		m = 1 // > 99% obtained on amd64, no backsliding
+		m = 0.011 // > 98.9% obtained on amd64, no backsliding
 	} else if runtime.GOARCH == "riscv64" {
-		m = 3 // XXX temporary update threshold to 97% for regabi
+		m = 0.03 // XXX temporary update threshold to 97% for regabi
 	} else {
-		m = 2 // expect 98% elsewhere.
+		m = 0.02 // expect 98% elsewhere.
 	}
 
-	if len(nonStmtLines)*100 > m*len(lines) {
-		t.Errorf("Saw too many (%s, > %d%%) lines without statement marks, total=%d, nostmt=%d ('-run TestStmtLines -v' lists failing lines)\n", runtime.GOARCH, m, len(lines), len(nonStmtLines))
+	if float64(len(nonStmtLines)) > m*float64(len(lines)) {
+		t.Errorf("Saw too many (%s, > %.1f%%) lines without statement marks, total=%d, nostmt=%d ('-run TestStmtLines -v' lists failing lines)\n", runtime.GOARCH, m*100, len(lines), len(nonStmtLines))
 	}
 	t.Logf("Saw %d out of %d lines without statement marks", len(nonStmtLines), len(lines))
 	if testing.Verbose() {

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

@@ -253,7 +253,7 @@ func (s *phiState) insertVarPhis(n int, var_ ir.Node, defs []*ssa.Block, typ *ty
 				}
 				// Add a phi to block c for variable n.
 				hasPhi.add(c.ID)
-				v := c.NewValue0I(currentRoot.Pos, ssa.OpPhi, typ, int64(n)) // TODO: line number right?
+				v := c.NewValue0I(s.s.blockStarts[b.ID], ssa.OpPhi, typ, int64(n))
 				// Note: we store the variable number in the phi's AuxInt field. Used temporarily by phi building.
 				if var_.Op() == ir.ONAME {
 					s.s.addNamedValue(var_.(*ir.Name), v)
@@ -513,6 +513,7 @@ loop:
 				v.Op = ssa.OpPhi
 				v.AddArgs(args...)
 				v.Aux = nil
+				v.Pos = s.s.blockStarts[b.ID]
 				continue loop
 			}
 			w = a // save witness

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

@@ -1088,6 +1088,9 @@ type state struct {
 
 	// First argument of append calls that could be stack allocated.
 	appendTargets map[ir.Node]bool
+
+	// Block starting position, indexed by block id.
+	blockStarts []src.XPos
 }
 
 type funcLine struct {
@@ -1146,6 +1149,9 @@ func (s *state) startBlock(b *ssa.Block) {
 	s.curBlock = b
 	s.vars = map[ir.Node]*ssa.Value{}
 	clear(s.fwdVars)
+	for len(s.blockStarts) <= int(b.ID) {
+		s.blockStarts = append(s.blockStarts, src.NoXPos)
+	}
 }
 
 // endBlock marks the end of generating code for the current block.
@@ -1172,6 +1178,9 @@ func (s *state) endBlock() *ssa.Block {
 		b.Pos = src.NoXPos
 	} else {
 		b.Pos = s.lastPos
+		if s.blockStarts[b.ID] == src.NoXPos {
+			s.blockStarts[b.ID] = s.lastPos
+		}
 	}
 	return b
 }
@@ -1188,6 +1197,11 @@ func (s *state) pushLine(line src.XPos) {
 	} else {
 		s.lastPos = line
 	}
+	// The first position we see for a new block is its starting position
+	// (the line number for its phis, if any).
+	if b := s.curBlock; b != nil && s.blockStarts[b.ID] == src.NoXPos {
+		s.blockStarts[b.ID] = line
+	}
 
 	s.line = append(s.line, line)
 }