cmd/compile: fix bug in logic for computing var abstract origins

The DWARF inline info generation code was using file/line/column (from
src.Pos) as a means of matching up pre- and post-optimization variable
nodes. This turns out to be problematic since it looks as though
distinct formals on the same line can be assigned the same column
number. Work around this issue by adding variable names to the
disambiguation code. Added a testpoint to the linker DWARF test that
checks to make sure each abstract origin offset of distinct within a
given DWARF DW_AT_inlined_routine body.

Fixes #23020.

Change-Id: Ie09bbe01dc60822d84d4085547b138e644036fb3
Reviewed-on: https://go-review.googlesource.com/82396Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/gc/dwinl.go

@@ -14,6 +14,7 @@ import (
 
 // To identify variables by original source position.
 type varPos struct {
+	DeclName string
 	DeclFile string
 	DeclLine uint
 	DeclCol  uint
@@ -96,14 +97,19 @@ func assembleInlines(fnsym *obj.LSym, fn *Node, dwVars []*dwarf.Var) dwarf.InlCa
 				n := dcl[i]
 				pos := Ctxt.InnermostPos(n.Pos)
 				vp := varPos{
+					DeclName: n.Sym.Name,
 					DeclFile: pos.Base().SymFilename(),
 					DeclLine: pos.Line(),
 					DeclCol:  pos.Col(),
 				}
+				if _, found := m[vp]; found {
+					Fatalf("child dcl collision on symbol %s within %v\n", n.Sym.Name, fnsym.Name)
+				}
 				m[vp] = i
 			}
 			for j := 0; j < len(sl); j++ {
 				vp := varPos{
+					DeclName: sl[j].Name,
 					DeclFile: sl[j].DeclFile,
 					DeclLine: sl[j].DeclLine,
 					DeclCol:  sl[j].DeclCol,

src/cmd/link/internal/ld/dwarf_test.go

@@ -522,8 +522,13 @@ package main
 
 var G int
 
+func noinline(x int) int {
+	defer func() { G += x }()
+	return x
+}
+
 func cand(x, y int) int {
-    return (x + y) ^ (y - x)
+	return noinline(x+y) ^ (y - x)
 }
 
 func main() {
@@ -599,6 +604,24 @@ func main() {
 				}
 			}
 			exCount++
+
+			omap := make(map[dwarf.Offset]bool)
+
+			// Walk the child variables of the inlined routine. Each
+			// of them should have a distinct abstract origin-- if two
+			// vars point to the same origin things are definitely broken.
+			inlIdx := ex.idxFromOffset(child.Offset)
+			inlChildDies := ex.Children(inlIdx)
+			for _, k := range inlChildDies {
+				ooff, originOK := k.Val(dwarf.AttrAbstractOrigin).(dwarf.Offset)
+				if !originOK {
+					t.Fatalf("no abstract origin attr for child of inlined subroutine at offset %v", k.Offset)
+				}
+				if _, found := omap[ooff]; found {
+					t.Fatalf("duplicate abstract origin at child of inlined subroutine at offset %v", k.Offset)
+				}
+				omap[ooff] = true
+			}
 		}
 	}
 	if exCount != len(expectedInl) {