diff --git a/asm/amd64_ext_asm_test.go b/asm/amd64_ext_asm_test.go
index 4c38737..37bc3ba 100644
--- a/asm/amd64_ext_asm_test.go
+++ b/asm/amd64_ext_asm_test.go
@@ -6,6 +6,7 @@ package asm
 import (
 	"bytes"
 	"encoding/hex"
+	"fmt"
 	"strings"
 	"testing"
 
@@ -282,3 +283,227 @@ func TestAmd64AssembleExtensionLeavesTheMainEncoderAlone(t *testing.T) {
 		t.Errorf("VPOPCNTD Z1, Z2: got %x, want the main encoder's %x", body, e.out)
 	}
 }
+
+// amd64SweepClass recomputes the vector class an entry encodes, the read of
+// the template's length field the arch package keeps private: EVEX.L'L in
+// byte three, VEX.L in byte two.
+func amd64SweepClass(in arch.ExtInstr) arch.ExtOperandKind {
+	if in.Vex {
+		if in.Bytes[2]&0x04 != 0 {
+			return arch.ExtYMM
+		}
+		return arch.ExtXMM
+	}
+	switch (in.Bytes[3] >> 5) & 3 {
+	case 0:
+		return arch.ExtXMM
+	case 1:
+		return arch.ExtYMM
+	default:
+		return arch.ExtZMM
+	}
+}
+
+// amd64SweepVec spells and models one vector register of the class, the
+// house names X, Y and Z the layer's text forms carry.
+func amd64SweepVec(kind arch.ExtOperandKind, n int) (string, arch.ExtOperand) {
+	switch kind {
+	case arch.ExtXMM:
+		return fmt.Sprintf("X%d", n), arch.ExtXmm(n)
+	case arch.ExtYMM:
+		return fmt.Sprintf("Y%d", n), arch.ExtYmm(n)
+	default:
+		return fmt.Sprintf("Z%d", n), arch.ExtZmm(n)
+	}
+}
+
+// amd64SweepGpr spells and models the general register of an entry: the W bit
+// picks the width, and an entry that ignores W takes the 64-bit spelling.
+func amd64SweepGpr(in arch.ExtInstr) (string, arch.ExtOperand) {
+	if in.Wig || in.Bytes[2]&0x80 != 0 {
+		return "R12", arch.ExtGpr64(12)
+	}
+	return "R12D", arch.ExtGpr32(12)
+}
+
+// amd64SweepImm spells and models the control immediate of an entry: the
+// mantissa control keeps its reserved upper nibble at zero, every other
+// layout takes a whole byte.
+func amd64SweepImm(in arch.ExtInstr) (string, arch.ExtOperand) {
+	if in.Imm8 == arch.ExtImm8GetMant {
+		return "$0x0b", arch.ExtImmediate(0x0b)
+	}
+	return "$0x7b", arch.ExtImmediate(0x7b)
+}
+
+// amd64SweepDest spells and models the register destination with the
+// decorations the entry carries on its register form: the write mask beside
+// every masked entry, the rounding or the exception suppression beside every
+// entry that takes one.
+func amd64SweepDest(in arch.ExtInstr, kind arch.ExtOperandKind, n int) (string, arch.ExtOperand) {
+	text, model := amd64SweepVec(kind, n)
+	dec := ""
+	if in.Mask {
+		dec += "{K5}"
+	}
+	switch {
+	case in.Er:
+		dec += "{RZ-SAE}"
+	case in.Sae:
+		dec += "{SAE}"
+	}
+	if dec == "" {
+		return text, model
+	}
+	if in.Mask {
+		model = arch.ExtWriteMasked(model, 5, false)
+	}
+	switch {
+	case in.Er:
+		model = arch.ExtRounded(model, arch.ExtRoundTruncate)
+	case in.Sae:
+		model = arch.ExtRounded(model, arch.ExtRoundSAE)
+	}
+	return text + dec, model
+}
+
+// amd64SweepMaskedDest spells and models the destination with the write mask
+// alone, the one decoration the memory shape keeps.
+func amd64SweepMaskedDest(in arch.ExtInstr, kind arch.ExtOperandKind, n int) (string, arch.ExtOperand) {
+	text, model := amd64SweepVec(kind, n)
+	if in.Mask {
+		return text + "{K5}", arch.ExtWriteMasked(model, 5, false)
+	}
+	return text, model
+}
+
+// amd64ExtSweepStatements builds, for the first registered entry of every
+// distinct amd64 mnemonic, the register-form statement the sweep drives and,
+// where the entry carries a memory position, the memory-form statement beside
+// it.  Each statement comes back with its mnemonic and the model operands the
+// text spells, so the sweep can pin the assembled bytes against
+// EncodeExtension.  The statements follow the registry: a mnemonic registered
+// on a form this builder knows lands in the sweep in the same change.
+func amd64ExtSweepStatements() (stmts, mnems []string, models [][]arch.ExtOperand, distinct int) {
+	memText, memModel := "(R9)", arch.ExtMemory(9, 0)
+	seen := make(map[string]bool)
+	for _, in := range arch.Extensions(arch.AMD64) {
+		if seen[in.Name] {
+			continue
+		}
+		seen[in.Name] = true
+		distinct++
+		class := amd64SweepClass(in)
+		// The two-vector forms narrow one side: the half form the
+		// destination, the wide form the source, and the quarter forms pin
+		// one side to the XMM class.
+		srcClass, destClass := class, class
+		switch in.Form {
+		case arch.ExtFormAmdVec2Half:
+			destClass = arch.ExtXMM
+			if class == arch.ExtZMM {
+				destClass = arch.ExtYMM
+			}
+		case arch.ExtFormAmdVec2Wide:
+			srcClass = arch.ExtXMM
+			if class == arch.ExtZMM {
+				srcClass = arch.ExtYMM
+			}
+		case arch.ExtFormAmdVec2Quarter:
+			srcClass = arch.ExtXMM
+		case arch.ExtFormAmdVec2ToQuarter:
+			destClass = arch.ExtXMM
+		}
+		srcText, srcModel := amd64SweepVec(srcClass, 1)
+		src2Text, src2Model := amd64SweepVec(class, 2)
+		gprText, gprModel := amd64SweepGpr(in)
+		immText, immModel := amd64SweepImm(in)
+		add := func(stmt, mnem string, ops ...arch.ExtOperand) {
+			stmts = append(stmts, stmt)
+			mnems = append(mnems, mnem)
+			models = append(models, ops)
+		}
+		switch in.Form {
+		case arch.ExtFormAmdVec3:
+			dText, dModel := amd64SweepDest(in, class, 3)
+			add(fmt.Sprintf("%s %s, %s, %s", in.Name, srcText, src2Text, dText), in.Name, srcModel, src2Model, dModel)
+			if in.Mem == 2 {
+				dText, dModel = amd64SweepMaskedDest(in, class, 3)
+				add(fmt.Sprintf("%s %s, %s, %s", in.Name, srcText, memText, dText), in.Name, srcModel, memModel, dModel)
+			}
+		case arch.ExtFormAmdVec2, arch.ExtFormAmdVec2Half, arch.ExtFormAmdVec2Wide,
+			arch.ExtFormAmdVec2Quarter, arch.ExtFormAmdVec2ToQuarter:
+			dText, dModel := amd64SweepDest(in, destClass, 2)
+			add(fmt.Sprintf("%s %s, %s", in.Name, srcText, dText), in.Name, srcModel, dModel)
+			if in.Mem == 1 {
+				dText, dModel = amd64SweepMaskedDest(in, destClass, 2)
+				add(fmt.Sprintf("%s %s, %s", in.Name, memText, dText), in.Name, memModel, dModel)
+			}
+		case arch.ExtFormAmdMask2:
+			add(fmt.Sprintf("%s %s, %s, K3", in.Name, srcText, src2Text), in.Name, srcModel, src2Model, arch.ExtMask(3))
+		case arch.ExtFormAmdVecGprVec:
+			dText, dModel := amd64SweepVec(class, 2)
+			add(fmt.Sprintf("%s %s, %s, %s", in.Name, srcText, gprText, dText), in.Name, srcModel, gprModel, dModel)
+		case arch.ExtFormAmdGprVec:
+			dText, dModel := amd64SweepVec(class, 1)
+			add(fmt.Sprintf("%s %s, %s", in.Name, gprText, dText), in.Name, gprModel, dModel)
+		case arch.ExtFormAmdVecGpr:
+			add(fmt.Sprintf("%s %s, %s", in.Name, srcText, gprText), in.Name, srcModel, gprModel)
+		case arch.ExtFormAmdVec3Imm:
+			dText, dModel := amd64SweepMaskedDest(in, class, 4)
+			add(fmt.Sprintf("%s %s, %s, %s, %s", in.Name, immText, srcText, src2Text, dText), in.Name, immModel, srcModel, src2Model, dModel)
+			if in.Mem == 3 {
+				add(fmt.Sprintf("%s %s, %s, %s, %s", in.Name, immText, srcText, memText, dText), in.Name, immModel, srcModel, memModel, dModel)
+			}
+		case arch.ExtFormAmdMask2Imm:
+			add(fmt.Sprintf("%s %s, %s, %s, K3", in.Name, immText, srcText, src2Text), in.Name, immModel, srcModel, src2Model, arch.ExtMask(3))
+			if in.Mem == 3 {
+				add(fmt.Sprintf("%s %s, %s, %s, K3", in.Name, immText, srcText, memText), in.Name, immModel, srcModel, memModel, arch.ExtMask(3))
+			}
+		case arch.ExtFormAmdVec2Imm:
+			dText, dModel := amd64SweepMaskedDest(in, class, 3)
+			add(fmt.Sprintf("%s %s, %s, %s", in.Name, immText, srcText, dText), in.Name, immModel, srcModel, dModel)
+			if in.Mem == 2 {
+				add(fmt.Sprintf("%s %s, %s, %s", in.Name, immText, memText, dText), in.Name, immModel, memModel, dModel)
+			}
+		case arch.ExtFormAmdMemVec:
+			dText, dModel := amd64SweepVec(class, 1)
+			add(fmt.Sprintf("%s %s, %s", in.Name, memText, dText), in.Name, memModel, dModel)
+		case arch.ExtFormAmdVecMem:
+			add(fmt.Sprintf("%s %s, %s", in.Name, srcText, memText), in.Name, srcModel, memModel)
+		default:
+			stmts = append(stmts, "")
+			mnems = append(mnems, in.Name)
+			models = append(models, nil)
+		}
+	}
+	return stmts, mnems, models, distinct
+}
+
+// TestAmd64AssembleExtensionSweep drives every distinct registered mnemonic
+// through the full assembler from .s text: the register-form statement of the
+// mnemonic's first entry, and the memory-form statement beside it where the
+// entry carries a memory position.  Every statement must assemble, and every
+// body must equal EncodeExtension's encoding of the model operands the text
+// spells, so the front end and the registry cannot drift apart on any family.
+func TestAmd64AssembleExtensionSweep(t *testing.T) {
+	stmts, mnems, models, distinct := amd64ExtSweepStatements()
+	if distinct != 86 {
+		t.Errorf("the amd64 layer registers %d distinct mnemonics, want 86", distinct)
+	}
+	for i, stmt := range stmts {
+		if stmt == "" {
+			t.Errorf("%s registers a form the sweep builder does not spell", mnems[i])
+			continue
+		}
+		body := amd64ExtProbe(t, stmt)
+		want, err := EncodeExtension(arch.AMD64, mnems[i], models[i]...)
+		if err != nil {
+			t.Errorf("%s: registry encode: %v", stmt, err)
+			continue
+		}
+		if !bytes.Equal(body, want) {
+			t.Errorf("%s:\n  got  %x\n  want %x (the registry encoding)", stmt, body, want)
+		}
+	}
+}