cmd/compile: fail noder.LookupFunc gracefully if function generic

PGO uses noder.LookupFunc to look for devirtualization targets in
export data.  LookupFunc does not support type-parameterized
functions, and will currently fail the build when attempting to lookup
a type-parameterized function because objIdx is passed the wrong
number of type arguments.

This doesn't usually come up, as a PGO profile will report a generic
function with a symbol name like Func[.go.shape.foo]. In export data,
this is just Func, so when we do LookupFunc("Func[.go.shape.foo]")
lookup simply fails because the name doesn't exist.

However, if Func is not generic when the profile is collected, but the
source has since changed to make Func generic, then LookupFunc("Func")
will find the object successfully, only to fail the build because we
failed to provide type arguments.

Handle this with a objIdxMayFail, which allows graceful failure if the
object requires type arguments.

Bumping the language version to 1.21 in pgo_devirtualize_test.go is
required for type inference of the uses of mult.MultFn in
cmd/compile/internal/test/testdata/pgo/devirtualize/devirt_test.go.

Fixes #65615.

Change-Id: I84d9344840b851182f5321b8f7a29a591221b29f
Reviewed-on: https://go-review.googlesource.com/c/go/+/562737
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/noder/unified.go b/src/cmd/compile/internal/noder/unified.go
index d2ca1f37a9..492b00d256 100644
--- a/src/cmd/compile/internal/noder/unified.go
+++ b/src/cmd/compile/internal/noder/unified.go
@@ -80,7 +80,11 @@ func lookupFunction(pkg *types.Pkg, symName string) (*ir.Func, error) {
 		return nil, fmt.Errorf("func sym %v missing objReader", sym)
 	}
 
-	name := pri.pr.objIdx(pri.idx, nil, nil, false).(*ir.Name)
+	node, err := pri.pr.objIdxMayFail(pri.idx, nil, nil, false)
+	if err != nil {
+		return nil, fmt.Errorf("func sym %v lookup error: %w", sym, err)
+	}
+	name := node.(*ir.Name)
 	if name.Op() != ir.ONAME || name.Class != ir.PFUNC {
 		return nil, fmt.Errorf("func sym %v refers to non-function name: %v", sym, name)
 	}
@@ -105,7 +109,11 @@ func lookupMethod(pkg *types.Pkg, symName string) (*ir.Func, error) {
 		return nil, fmt.Errorf("type sym %v missing objReader", typ)
 	}
 
-	name := pri.pr.objIdx(pri.idx, nil, nil, false).(*ir.Name)
+	node, err := pri.pr.objIdxMayFail(pri.idx, nil, nil, false)
+	if err != nil {
+		return nil, fmt.Errorf("func sym %v lookup error: %w", typ, err)
+	}
+	name := node.(*ir.Name)
 	if name.Op() != ir.OTYPE {
 		return nil, fmt.Errorf("type sym %v refers to non-type name: %v", typ, name)
 	}