reflect, internal/abi: speed up TypeFor[T]

goos: linux
goarch: amd64
pkg: reflect
cpu: AMD Ryzen 5 4600G with Radeon Graphics
                 │ /tmp/before  │              /tmp/after              │
                 │    sec/op    │    sec/op     vs base                │
TypeForString-12    2.091n ± 1%    1.174n ± 1%  -43.84% (p=0.000 n=20)
TypeForError-12    7.5810n ± 3%   0.9372n ± 1%  -87.64% (p=0.000 n=20)

Change-Id: I22022f99b2dd2029f02d9ed8477b209adf7e9496
GitHub-Last-Rev: 64d2ac5bb2
GitHub-Pull-Request: golang/go#71654
Reviewed-on: https://go-review.googlesource.com/c/go/+/648395
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/internal/abi/type.go

@@ -187,11 +187,7 @@ func TypeOf(a any) *Type {
 
 // TypeFor returns the abi.Type for a type parameter.
 func TypeFor[T any]() *Type {
-	var v T
-	if t := TypeOf(v); t != nil {
-		return t // optimize for T being a non-interface kind
-	}
-	return TypeOf((*T)(nil)).Elem() // only for an interface kind
+	return (*PtrType)(unsafe.Pointer(TypeOf((*T)(nil)))).Elem
 }
 
 func (t *Type) Kind() Kind { return t.Kind_ & KindMask }

src/reflect/type.go

@@ -1303,6 +1303,11 @@ func TypeOf(i any) Type {
 	return toType(abi.TypeOf(i))
 }
 
+// TypeFor returns the [Type] that represents the type argument T.
+func TypeFor[T any]() Type {
+	return toType(abi.TypeFor[T]())
+}
+
 // rtypeOf directly extracts the *rtype of the provided value.
 func rtypeOf(i any) *abi.Type {
 	return abi.TypeOf(i)
@@ -2850,12 +2855,3 @@ func addTypeBits(bv *bitVector, offset uintptr, t *abi.Type) {
 		}
 	}
 }
-
-// TypeFor returns the [Type] that represents the type argument T.
-func TypeFor[T any]() Type {
-	var v T
-	if t := TypeOf(v); t != nil {
-		return t // optimize for T being a non-interface kind
-	}
-	return TypeOf((*T)(nil)).Elem() // only for an interface kind
-}