diff --git a/src/cmd/asm/internal/arch/arch.go b/src/cmd/asm/internal/arch/arch.go
index 9110ca7c02..6f13a3a4db 100644
--- a/src/cmd/asm/internal/arch/arch.go
+++ b/src/cmd/asm/internal/arch/arch.go
@@ -171,6 +171,8 @@ func archX86(linkArch *obj.LinkArch) *Arch {
 	instructions["PSRLDQ"] = x86.APSRLO
 	instructions["PADDD"] = x86.APADDL
 
+	x86.InstInit()
+
 	return &Arch{
 		LinkArch:       linkArch,
 		Instructions:   instructions,
diff --git a/src/cmd/compile/internal/amd64/galign.go b/src/cmd/compile/internal/amd64/galign.go
index 90ee895364..59484b1537 100644
--- a/src/cmd/compile/internal/amd64/galign.go
+++ b/src/cmd/compile/internal/amd64/galign.go
@@ -27,4 +27,6 @@ func Init(arch *gc.Arch) {
 	arch.SSAMarkMoves = ssaMarkMoves
 	arch.SSAGenValue = ssaGenValue
 	arch.SSAGenBlock = ssaGenBlock
+
+	x86.InstInit()
 }
diff --git a/src/cmd/compile/internal/x86/galign.go b/src/cmd/compile/internal/x86/galign.go
index b5cf044bc7..5255e9c60e 100644
--- a/src/cmd/compile/internal/x86/galign.go
+++ b/src/cmd/compile/internal/x86/galign.go
@@ -33,4 +33,6 @@ func Init(arch *gc.Arch) {
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = ssaMarkMoves
+
+	x86.InstInit()
 }
diff --git a/src/cmd/internal/obj/x86/asm6.go b/src/cmd/internal/obj/x86/asm6.go
index 2fad88d67b..0e175e3760 100644
--- a/src/cmd/internal/obj/x86/asm6.go
+++ b/src/cmd/internal/obj/x86/asm6.go
@@ -883,8 +883,8 @@ var ymmxmm0f38 = []ytab{
  * two values match the Ytypes of the p->from and p->to operands.  The function
  * oclass in span.c computes the specific Ytype of an operand and then the set
  * of more general Ytypes that it satisfies is implied by the ycover table, set
- * up in instinit.  For example, oclass distinguishes the constants 0 and 1
- * from the more general 8-bit constants, but instinit says
+ * up in InstInit.  For example, oclass distinguishes the constants 0 and 1
+ * from the more general 8-bit constants, but InstInit says
  *
  *        ycover[Yi0*Ymax + Ys32] = 1;
  *        ycover[Yi1*Ymax + Ys32] = 1;
@@ -1770,7 +1770,7 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 	}
 
 	if ycover[0] == 0 {
-		instinit()
+		ctxt.Diag("x86 tables not initialized, call x86.InstInit first")
 	}
 
 	for p := ctxt.Cursym.Text; p != nil; p = p.Link {
@@ -1965,7 +1965,14 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 	}
 }
 
-func instinit() {
+func InstInit() {
+	if ycover[0] != 0 {
+		// Already initialized; stop now.
+		// This happens in the cmd/asm tests,
+		// each of which re-initializes the arch.
+		return
+	}
+
 	for i := 1; optab[i].as != 0; i++ {
 		c := optab[i].as
 		if opindex[c&obj.AMask] != nil {