cmd/compile, reflect: further allow inlining of TypeFor

Previous CLs optimized direct use of abi.Type, but reflect.Type is
indirected, so was not benefiting.

For TypeFor, we can use toRType directly without a nil check because the
types are statically known.

Normally, I'd think SSA would remove the nil check, but due to some
oddity (specifically, late fuse being required to remove the nil check,
but opt doesn't run that late) means that the nil check persists and
gets in the way.

Manually writing the code in this instance seems to fix the problem.

It also exposed another problem; depending on the ordering, writeType
could get to a type symbol before SSA, thereby preventing Extra from
being created on the symbol for later lookups that don't go through
TypeLinksym directly. In writeType, for non-shape types, call
TypeLinksym to ensure that the type is set up for later callers. That
change itself passed toolstash -cmp.

All up, this stack put through compilecmp shows a lot of improvement in
various reflect-using packages, and reflect itself. It is too big to fit
in the commit message but here's some info:

compilecmp master -> HEAD
master (d767064170): cmd/compile: mark abi.PtrType.Elem sym as used
HEAD (846a94c568): cmd/compile, reflect: further allow inlining of TypeFor

file      before    after     Δ       %
addr2line 3735911   3735391   -520    -0.014%
asm       6382235   6382091   -144    -0.002%
buildid   3608568   3608360   -208    -0.006%
cgo       5951816   5951480   -336    -0.006%
compile   28362080  28339772  -22308  -0.079%
cover     6668686   6661414   -7272   -0.109%
dist      4311961   4311425   -536    -0.012%
fix       3771706   3771474   -232    -0.006%
link      8686073   8684993   -1080   -0.012%
nm        3715923   3715459   -464    -0.012%
objdump   6074366   6073774   -592    -0.010%
pack      3025653   3025277   -376    -0.012%
pprof     18269485  18261653  -7832   -0.043%
test2json 3442726   3438390   -4336   -0.126%
trace     16984831  16981767  -3064   -0.018%
vet       10701931  10696355  -5576   -0.052%
total     133693951 133639075 -54876  -0.041%

runtime
runtime.stkobjinit 240 -> 165  (-31.25%)

runtime [cmd/compile]
runtime.stkobjinit 240 -> 165  (-31.25%)

reflect
reflect.Value.Seq2.func3 309 -> 245  (-20.71%)
reflect.Value.Seq2.func1.1 281 -> 198  (-29.54%)
reflect.Value.Seq.func1.1 242 -> 165  (-31.82%)
reflect.Value.Seq2.func2 360 -> 285  (-20.83%)
reflect.Value.Seq.func4 281 -> 239  (-14.95%)
reflect.Value.Seq2.func4 399 -> 284  (-28.82%)
reflect.Value.Seq.func2 271 -> 230  (-15.13%)
reflect.TypeFor[go.shape.uint64] 33 -> 18  (-45.45%)
reflect.Value.Seq.func3 219 -> 178  (-18.72%)

reflect [cmd/compile]
reflect.Value.Seq2.func2 360 -> 285  (-20.83%)
reflect.Value.Seq.func4 281 -> 239  (-14.95%)
reflect.Value.Seq.func2 271 -> 230  (-15.13%)
reflect.Value.Seq.func1.1 242 -> 165  (-31.82%)
reflect.Value.Seq2.func1.1 281 -> 198  (-29.54%)
reflect.Value.Seq2.func3 309 -> 245  (-20.71%)
reflect.Value.Seq.func3 219 -> 178  (-18.72%)
reflect.TypeFor[go.shape.uint64] 33 -> 18  (-45.45%)
reflect.Value.Seq2.func4 399 -> 284  (-28.82%)

fmt
fmt.(*pp).fmtBytes 1723 -> 1691  (-1.86%)

database/sql/driver
reflect.TypeFor[go.shape.interface 33 -> 18  (-45.45%)
database/sql/driver.init 72 -> 57  (-20.83%)

Change-Id: I9eb750cf0b7ebf532589f939431feb0a899e42ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/701301
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -717,6 +717,10 @@ func writeType(t *types.Type) *obj.LSym {
 	}
 	s.SetSiggen(true)
 
+	if !tbase.HasShape() {
+		TypeLinksym(t) // ensure lsym.Extra is set
+	}
+
 	if !NeedEmit(tbase) {
 		if i := typecheck.BaseTypeIndex(t); i >= 0 {
 			lsym.Pkg = tbase.Sym().Pkg.Prefix

src/reflect/type.go

@@ -1314,7 +1314,8 @@ func TypeOf(i any) Type {
 
 // TypeFor returns the [Type] that represents the type argument T.
 func TypeFor[T any]() Type {
-	return toType(abi.TypeFor[T]())
+	// toRType is safe to use here; type is never nil as T is statically known.
+	return toRType(abi.TypeFor[T]())
 }
 
 // rtypeOf directly extracts the *rtype of the provided value.

test/codegen/reflect_type.go

@@ -0,0 +1,21 @@
+// asmcheck
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codegen
+
+import "reflect"
+
+func intPtrTypeSize() uintptr {
+	// amd64:"MOVL\t[$]8,",-"CALL"
+	// arm64:"MOVD\t[$]8,",-"CALL"
+	return reflect.TypeFor[*int]().Size()
+}
+
+func intPtrTypeKind() reflect.Kind {
+	// amd64:"MOVL\t[$]22,",-"CALL"
+	// arm64:"MOVD\t[$]22,",-"CALL"
+	return reflect.TypeFor[*int]().Kind()
+}