diff --git a/tests/alloc/gp8/asm.go b/tests/alloc/gp8/asm.go
new file mode 100644
index 0000000..88b0b9a
--- /dev/null
+++ b/tests/alloc/gp8/asm.go
@@ -0,0 +1,39 @@
+// +build ignore
+
+package main
+
+import (
+	"strconv"
+
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+	. "github.com/mmcloughlin/avo/reg"
+)
+
+func main() {
+	// n is the number of 8-bit registers to use.
+	// 15 low-byte registers (excluding SP)
+	// 4 high-byte registers AH,BH,CH,DH
+	const n = 19
+
+	TEXT("GP8", NOSPLIT, "func() uint8")
+	Doc("GP8 returns the sum 1+2+...+" + strconv.Itoa(n) + " using " + strconv.Itoa(n) + " distinct 8-bit registers.")
+
+	// Allocate registers and initialize.
+	x := make([]Register, n)
+	for i := 0; i < n; i++ {
+		x[i] = GP8()
+		MOVB(U8(i+1), x[i])
+	}
+
+	// Sum them up.
+	for i := 1; i < n; i++ {
+		ADDB(x[i], x[0])
+	}
+
+	// Return.
+	Store(x[0], ReturnIndex(0))
+	RET()
+
+	Generate()
+}
diff --git a/tests/alloc/gp8/doc.go b/tests/alloc/gp8/doc.go
new file mode 100644
index 0000000..703b569
--- /dev/null
+++ b/tests/alloc/gp8/doc.go
@@ -0,0 +1,2 @@
+// Package gp8 tests the register allocator by using as many 8-bit registers as possible.
+package gp8
diff --git a/tests/alloc/gp8/gp8.s b/tests/alloc/gp8/gp8.s
new file mode 100644
index 0000000..882496f
--- /dev/null
+++ b/tests/alloc/gp8/gp8.s
@@ -0,0 +1,45 @@
+// Code generated by command: go run asm.go -out gp8.s -stubs stub.go. DO NOT EDIT.
+
+#include "textflag.h"
+
+// func GP8() uint8
+TEXT ·GP8(SB), NOSPLIT, $0-1
+	MOVB $0x01, AL
+	MOVB $0x02, CL
+	MOVB $0x03, DL
+	MOVB $0x04, BL
+	MOVB $0x05, AH
+	MOVB $0x06, CH
+	MOVB $0x07, DH
+	MOVB $0x08, BH
+	MOVB $0x09, BP
+	MOVB $0x0a, SI
+	MOVB $0x0b, DI
+	MOVB $0x0c, R8
+	MOVB $0x0d, R9
+	MOVB $0x0e, R10
+	MOVB $0x0f, R11
+	MOVB $0x10, R12
+	MOVB $0x11, R13
+	MOVB $0x12, R14
+	MOVB $0x13, R15
+	ADDB CL, AL
+	ADDB DL, AL
+	ADDB BL, AL
+	ADDB AH, AL
+	ADDB CH, AL
+	ADDB DH, AL
+	ADDB BH, AL
+	ADDB BP, AL
+	ADDB SI, AL
+	ADDB DI, AL
+	ADDB R8, AL
+	ADDB R9, AL
+	ADDB R10, AL
+	ADDB R11, AL
+	ADDB R12, AL
+	ADDB R13, AL
+	ADDB R14, AL
+	ADDB R15, AL
+	MOVB AL, ret+0(FP)
+	RET
diff --git a/tests/alloc/gp8/gp8_test.go b/tests/alloc/gp8/gp8_test.go
new file mode 100644
index 0000000..b79e3c8
--- /dev/null
+++ b/tests/alloc/gp8/gp8_test.go
@@ -0,0 +1,15 @@
+package gp8
+
+import (
+	"testing"
+)
+
+//go:generate go run asm.go -out gp8.s -stubs stub.go
+
+func TestGP8(t *testing.T) {
+	const n = 19
+	expect := uint8(n * (n + 1) / 2)
+	if got := GP8(); got != expect {
+		t.Fatalf("GP8() = %d; expect %d", got, expect)
+	}
+}
diff --git a/tests/alloc/gp8/stub.go b/tests/alloc/gp8/stub.go
new file mode 100644
index 0000000..138217c
--- /dev/null
+++ b/tests/alloc/gp8/stub.go
@@ -0,0 +1,6 @@
+// Code generated by command: go run asm.go -out gp8.s -stubs stub.go. DO NOT EDIT.
+
+package gp8
+
+// GP8 returns the sum 1+2+...+19 using 19 distinct 8-bit registers.
+func GP8() uint8