diff --git a/src/internal/abi/abi_amd64.go b/src/internal/abi/abi_amd64.go
index 77589d4c34..07c3ec1aab 100644
--- a/src/internal/abi/abi_amd64.go
+++ b/src/internal/abi/abi_amd64.go
@@ -10,16 +10,12 @@ package abi
 const (
 	// See abi_generic.go.
 
-	// Currently these values are zero because whatever uses
-	// them will expect the register ABI, which isn't ready
-	// yet.
-
 	// RAX, RBX, RCX, RDI, RSI, R8, R9, R10, R11.
-	IntArgRegs = 0 // 9
+	IntArgRegs = 9
 
 	// X0 -> X14.
-	FloatArgRegs = 0 // 15
+	FloatArgRegs = 15
 
 	// We use SSE2 registers which support 64-bit float operations.
-	EffectiveFloatRegSize = 0 // 8
+	EffectiveFloatRegSize = 8
 )
diff --git a/src/reflect/abi.go b/src/reflect/abi.go
index 36d6b3095b..618efd0980 100644
--- a/src/reflect/abi.go
+++ b/src/reflect/abi.go
@@ -9,6 +9,30 @@ import (
 	"unsafe"
 )
 
+// These variables are used by the register assignment
+// algorithm in this file.
+//
+// They should be modified with care (no other reflect code
+// may be executing) and are generally only modified
+// when testing this package.
+//
+// They should never be set higher than their internal/abi
+// constant counterparts, because the system relies on a
+// structure that is at least large enough to hold the
+// registers the system supports.
+//
+// Currently they're set to zero because using the actual
+// constants will break every part of the toolchain that
+// uses reflect to call functions (e.g. go test, or anything
+// that uses text/template). The values that are currently
+// commented out there should be the actual values once
+// we're ready to use the register ABI everywhere.
+var (
+	intArgRegs   = 0          // abi.IntArgRegs
+	floatArgRegs = 0          // abi.FloatArgRegs
+	floatRegSize = uintptr(0) // uintptr(abi.EffectiveFloatRegSize)
+)
+
 // abiStep represents an ABI "instruction." Each instruction
 // describes one part of how to translate between a Go value
 // in memory and a call frame.
@@ -226,7 +250,7 @@ func (a *abiSeq) assignIntN(offset, size uintptr, n int, ptrMap uint8) bool {
 	if ptrMap != 0 && size != ptrSize {
 		panic("non-empty pointer map passed for non-pointer-size values")
 	}
-	if a.iregs+n > abi.IntArgRegs {
+	if a.iregs+n > intArgRegs {
 		return false
 	}
 	for i := 0; i < n; i++ {
@@ -255,7 +279,7 @@ func (a *abiSeq) assignFloatN(offset, size uintptr, n int) bool {
 	if n < 0 {
 		panic("invalid n")
 	}
-	if a.fregs+n > abi.FloatArgRegs || abi.EffectiveFloatRegSize < size {
+	if a.fregs+n > floatArgRegs || floatRegSize < size {
 		return false
 	}
 	for i := 0; i < n; i++ {
diff --git a/src/runtime/asm_amd64.s b/src/runtime/asm_amd64.s
index 517c5a9d3e..ddd6a5bd5b 100644
--- a/src/runtime/asm_amd64.s
+++ b/src/runtime/asm_amd64.s
@@ -442,10 +442,7 @@ TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0
 	MOVL	$0, DX
 	JMP	runtime·morestack(SB)
 
-// REFLECTCALL_USE_REGABI is not defined. It must be defined in conjunction with the
-// register constants in the internal/abi package.
-
-#ifdef REFLECTCALL_USE_REGABI
+#ifdef GOEXPERIMENT_REGABI
 // spillArgs stores return values from registers to a *internal/abi.RegArgs in R12.
 TEXT spillArgs<>(SB),NOSPLIT,$0-0
 	MOVQ AX, 0(R12)