cmd/internal/obj: eagerly initialize x86 assembler

Prior to this CL, instinit was called as needed.
This does not work well in a concurrent backend.
Initialization is very cheap; do it on startup instead.

Passes toolstash-check -all.
No compiler performance impact.

Updates #15756

Change-Id: Ifa5e82e8abf4504435e1b28766f5703a0555f42d
Reviewed-on: https://go-review.googlesource.com/38662
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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,

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()
 }

src/cmd/compile/internal/x86/galign.go

@@ -33,4 +33,6 @@ func Init(arch *gc.Arch) {
 	arch.Ginsnop = ginsnop
 
 	arch.SSAMarkMoves = ssaMarkMoves
+
+	x86.InstInit()
 }

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 {