diff --git a/README.md b/README.md
index f65444b..b7d11b5 100644
--- a/README.md
+++ b/README.md
@@ -37,7 +37,6 @@ $ go get -u github.com/mmcloughlin/avo
 
 ```go
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/attr/make_textflag.go b/attr/make_textflag.go
index f5fc3ff..14f92fa 100644
--- a/attr/make_textflag.go
+++ b/attr/make_textflag.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/build/zinstructions_test.go b/build/zinstructions_test.go
index 2427ef9..3312b9c 100644
--- a/build/zinstructions_test.go
+++ b/build/zinstructions_test.go
@@ -1,7 +1,6 @@
 // Code generated by command: avogen -output zinstructions_test.go buildtest. DO NOT EDIT.
 
 //go:build !integration
-// +build !integration
 
 package build
 
diff --git a/buildtags/syntax_go116.go b/buildtags/syntax_go116.go
index 07c5068..e9d393d 100644
--- a/buildtags/syntax_go116.go
+++ b/buildtags/syntax_go116.go
@@ -1,5 +1,4 @@
 //go:build !go1.17
-// +build !go1.17
 
 package buildtags
 
diff --git a/buildtags/syntax_go117.go b/buildtags/syntax_go117.go
index a6dca4e..b624dee 100644
--- a/buildtags/syntax_go117.go
+++ b/buildtags/syntax_go117.go
@@ -1,5 +1,4 @@
 //go:build go1.17 && !go1.18
-// +build go1.17,!go1.18
 
 package buildtags
 
diff --git a/buildtags/syntax_go118.go b/buildtags/syntax_go118.go
index 7e70c72..d372f86 100644
--- a/buildtags/syntax_go118.go
+++ b/buildtags/syntax_go118.go
@@ -1,5 +1,4 @@
 //go:build go1.18
-// +build go1.18
 
 package buildtags
 
diff --git a/examples/add/README.md b/examples/add/README.md
index 68aefab..8625fa3 100644
--- a/examples/add/README.md
+++ b/examples/add/README.md
@@ -7,7 +7,6 @@ The [code generator](asm.go) is as follows:
 [embedmd]:# (asm.go)
 ```go
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/add/asm.go b/examples/add/asm.go
index 8c3d841..1f6496a 100644
--- a/examples/add/asm.go
+++ b/examples/add/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/args/asm.go b/examples/args/asm.go
index 69f87e4..f8fb8ef 100644
--- a/examples/args/asm.go
+++ b/examples/args/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/complex/asm.go b/examples/complex/asm.go
index e973565..6d14629 100644
--- a/examples/complex/asm.go
+++ b/examples/complex/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/data/asm.go b/examples/data/asm.go
index de55f32..b2ad1a2 100644
--- a/examples/data/asm.go
+++ b/examples/data/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/dot/asm.go b/examples/dot/asm.go
index da7ddf4..9e23d0a 100644
--- a/examples/dot/asm.go
+++ b/examples/dot/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/ext/asm.go b/examples/ext/asm.go
index d752b38..0c9d933 100644
--- a/examples/ext/asm.go
+++ b/examples/ext/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/fnv1a/asm.go b/examples/fnv1a/asm.go
index 809a1c9..1a59592 100644
--- a/examples/fnv1a/asm.go
+++ b/examples/fnv1a/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/geohash/asm.go b/examples/geohash/asm.go
index 81fbacd..f087303 100644
--- a/examples/geohash/asm.go
+++ b/examples/geohash/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/md5x16/asm.go b/examples/md5x16/asm.go
index 3d5e793..1919e20 100644
--- a/examples/md5x16/asm.go
+++ b/examples/md5x16/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/pragma/asm.go b/examples/pragma/asm.go
index 067b92f..141318b 100644
--- a/examples/pragma/asm.go
+++ b/examples/pragma/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/returns/asm.go b/examples/returns/asm.go
index 26121d3..0fd5dcd 100644
--- a/examples/returns/asm.go
+++ b/examples/returns/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/sha1/asm.go b/examples/sha1/asm.go
index 509aff3..05d75d5 100644
--- a/examples/sha1/asm.go
+++ b/examples/sha1/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/examples/stadtx/asm.go b/examples/stadtx/asm.go
index 8e849c6..e445e39 100644
--- a/examples/stadtx/asm.go
+++ b/examples/stadtx/asm.go
@@ -1,4 +1,4 @@
-// +build ignore
+//go:build ignore
 
 package main
 
diff --git a/examples/sum/asm.go b/examples/sum/asm.go
index 22963fe..50333b3 100644
--- a/examples/sum/asm.go
+++ b/examples/sum/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/internal/gen/buildtest.go b/internal/gen/buildtest.go
index 582b58e..ad24d26 100644
--- a/internal/gen/buildtest.go
+++ b/internal/gen/buildtest.go
@@ -20,7 +20,7 @@ func NewBuildTest(cfg printer.Config) Interface {
 
 func (b *buildtest) Generate(is []inst.Instruction) ([]byte, error) {
 	b.Printf("// %s\n\n", b.cfg.GeneratedWarning())
-	b.BuildTag("!integration")
+	b.BuildConstraint("!integration")
 	b.NL()
 	b.Printf("package build\n\n")
 	b.Printf("import (\n")
diff --git a/internal/gen/ctorstest.go b/internal/gen/ctorstest.go
index a3832c9..b271d2f 100644
--- a/internal/gen/ctorstest.go
+++ b/internal/gen/ctorstest.go
@@ -23,7 +23,7 @@ func NewCtorsTest(cfg printer.Config) Interface {
 
 func (c *ctorstest) Generate(is []inst.Instruction) ([]byte, error) {
 	c.Printf("// %s\n\n", c.cfg.GeneratedWarning())
-	c.BuildTag("!integration")
+	c.BuildConstraint("!integration")
 	c.NL()
 	c.Printf("package x86\n\n")
 	c.Printf("import (\n")
@@ -65,7 +65,7 @@ func NewCtorsStress(cfg printer.Config) Interface {
 
 func (c *ctorsstress) Generate(is []inst.Instruction) ([]byte, error) {
 	c.Printf("// %s\n\n", c.cfg.GeneratedWarning())
-	c.BuildTag("stress")
+	c.BuildConstraint("stress")
 	c.NL()
 	c.Printf("package x86\n\n")
 	c.Printf("import (\n")
@@ -112,7 +112,7 @@ func NewCtorsBench(cfg printer.Config) Interface {
 
 func (c *ctorsbench) Generate(is []inst.Instruction) ([]byte, error) {
 	c.Printf("// %s\n\n", c.cfg.GeneratedWarning())
-	c.BuildTag("stress")
+	c.BuildConstraint("stress")
 	c.NL()
 	c.Printf("package x86\n\n")
 	c.Printf("import (\n")
diff --git a/internal/inst/stress_test.go b/internal/inst/stress_test.go
index 193bc27..aa1b8be 100644
--- a/internal/inst/stress_test.go
+++ b/internal/inst/stress_test.go
@@ -2,7 +2,6 @@
 // stress-test only build.
 
 //go:build stress
-// +build stress
 
 package inst
 
diff --git a/internal/prnt/printer.go b/internal/prnt/printer.go
index cc9e89e..06d6e91 100644
--- a/internal/prnt/printer.go
+++ b/internal/prnt/printer.go
@@ -4,6 +4,7 @@ package prnt
 import (
 	"bytes"
 	"fmt"
+	"go/build/constraint"
 	"io"
 	"strings"
 )
@@ -75,9 +76,13 @@ func (g *Generator) Comment(lines ...string) {
 	}
 }
 
-// BuildTag outputs a build tag.
-func (g *Generator) BuildTag(tag string) {
-	g.Comment("+build " + tag)
+// BuildConstraint outputs a build constraint.
+func (g *Generator) BuildConstraint(expr string) {
+	line := fmt.Sprintf("//go:build %s", expr)
+	if _, err := constraint.Parse(line); err != nil {
+		g.AddError(err)
+	}
+	g.Linef(line)
 }
 
 // AddError records an error in code generation. The first non-nil error will
diff --git a/operand/make_const.go b/operand/make_const.go
index 4297530..be968c5 100644
--- a/operand/make_const.go
+++ b/operand/make_const.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/alloc/gp8/asm.go b/tests/alloc/gp8/asm.go
index 040f7e1..512cd10 100644
--- a/tests/alloc/gp8/asm.go
+++ b/tests/alloc/gp8/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/alloc/masks/asm.go b/tests/alloc/masks/asm.go
index ce4bf76..9c23f11 100644
--- a/tests/alloc/masks/asm.go
+++ b/tests/alloc/masks/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/alloc/upper32/asm.go b/tests/alloc/upper32/asm.go
index 817b8ee..4bd9d72 100644
--- a/tests/alloc/upper32/asm.go
+++ b/tests/alloc/upper32/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/alloc/zeroing/asm.go b/tests/alloc/zeroing/asm.go
index 9c7734e..a82c97b 100644
--- a/tests/alloc/zeroing/asm.go
+++ b/tests/alloc/zeroing/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/cast/asm.go b/tests/cast/asm.go
index 6362220..569d889 100644
--- a/tests/cast/asm.go
+++ b/tests/cast/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue100/allocfail/asm.go b/tests/fixedbugs/issue100/allocfail/asm.go
index b663962..a6c9aee 100644
--- a/tests/fixedbugs/issue100/allocfail/asm.go
+++ b/tests/fixedbugs/issue100/allocfail/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue100/minrepro/asm.go b/tests/fixedbugs/issue100/minrepro/asm.go
index 2af2b78..2aafcdc 100644
--- a/tests/fixedbugs/issue100/minrepro/asm.go
+++ b/tests/fixedbugs/issue100/minrepro/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue122/asm.go b/tests/fixedbugs/issue122/asm.go
index 26f48b1..f0acad4 100644
--- a/tests/fixedbugs/issue122/asm.go
+++ b/tests/fixedbugs/issue122/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue191/asm.go b/tests/fixedbugs/issue191/asm.go
index f6373d5..1aa6d22 100644
--- a/tests/fixedbugs/issue191/asm.go
+++ b/tests/fixedbugs/issue191/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue195/asm.go b/tests/fixedbugs/issue195/asm.go
index 9f1a10c..f8d15e5 100644
--- a/tests/fixedbugs/issue195/asm.go
+++ b/tests/fixedbugs/issue195/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue336/asm.go b/tests/fixedbugs/issue336/asm.go
index ff60018..d1e5f46 100644
--- a/tests/fixedbugs/issue336/asm.go
+++ b/tests/fixedbugs/issue336/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue50/asm.go b/tests/fixedbugs/issue50/asm.go
index 26b8556..00ac04d 100644
--- a/tests/fixedbugs/issue50/asm.go
+++ b/tests/fixedbugs/issue50/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue62/asm.go b/tests/fixedbugs/issue62/asm.go
index 0a9bd44..307a448 100644
--- a/tests/fixedbugs/issue62/asm.go
+++ b/tests/fixedbugs/issue62/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue65/asm.go b/tests/fixedbugs/issue65/asm.go
index edca6eb..1604da4 100644
--- a/tests/fixedbugs/issue65/asm.go
+++ b/tests/fixedbugs/issue65/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue68/asm.go b/tests/fixedbugs/issue68/asm.go
index 8ef87aa..c835cfe 100644
--- a/tests/fixedbugs/issue68/asm.go
+++ b/tests/fixedbugs/issue68/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue76/asm.go b/tests/fixedbugs/issue76/asm.go
index 7782277..7a0f660 100644
--- a/tests/fixedbugs/issue76/asm.go
+++ b/tests/fixedbugs/issue76/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fixedbugs/issue89/asm.go b/tests/fixedbugs/issue89/asm.go
index 2a6e487..0d4a402 100644
--- a/tests/fixedbugs/issue89/asm.go
+++ b/tests/fixedbugs/issue89/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/fmt/asm.go b/tests/fmt/asm.go
index 8ef6e58..dd2a617 100644
--- a/tests/fmt/asm.go
+++ b/tests/fmt/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/labels/asm.go b/tests/labels/asm.go
index 4db710e..532a70c 100644
--- a/tests/labels/asm.go
+++ b/tests/labels/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/signature/asm.go b/tests/signature/asm.go
index 996d411..4b8b98b 100644
--- a/tests/signature/asm.go
+++ b/tests/signature/asm.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/textflag/make_attrtest.go b/tests/textflag/make_attrtest.go
index f4e4914..8eced39 100644
--- a/tests/textflag/make_attrtest.go
+++ b/tests/textflag/make_attrtest.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/tests/thirdparty/make_workflow.go b/tests/thirdparty/make_workflow.go
index 39dcc56..669d627 100644
--- a/tests/thirdparty/make_workflow.go
+++ b/tests/thirdparty/make_workflow.go
@@ -1,5 +1,4 @@
 //go:build ignore
-// +build ignore
 
 package main
 
diff --git a/x86/stress_test.go b/x86/stress_test.go
index 50e96a2..0f1f7fb 100644
--- a/x86/stress_test.go
+++ b/x86/stress_test.go
@@ -2,7 +2,6 @@
 // are limited to stress-test mode.
 
 //go:build stress
-// +build stress
 
 package x86
 
diff --git a/x86/zctors_test.go b/x86/zctors_test.go
index 56e9def..6c44b33 100644
--- a/x86/zctors_test.go
+++ b/x86/zctors_test.go
@@ -1,7 +1,6 @@
 // Code generated by command: avogen -output zctors_test.go ctorstest. DO NOT EDIT.
 
 //go:build !integration
-// +build !integration
 
 package x86