cmd/internal/obj/arm64: simplify buildop

This code stems from the original 7l C code, where one way to determine
the end of a table is to put a sentinel entry, then scan for it. This is
now Go code and the length of an array is readily available.

Remove the sentinel and sentinel scan, then adjust the remaining code to
work accordingly.

Change-Id: I8964c787f5149f3548fa78bf8923aa7a93f9482e
Reviewed-on: https://go-review.googlesource.com/c/go/+/512536
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Run-TryBot: Joel Sing <[email protected]>

src/cmd/internal/obj/arm64/asm7.go

@@ -853,8 +853,6 @@ var optab = []Optab{
  {obj.ADUFFZERO, C_NONE, C_NONE, C_NONE, C_SBRA, 5, 4, 0, 0, 0}, // same as AB/ABL
  {obj.ADUFFCOPY, C_NONE, C_NONE, C_NONE, C_SBRA, 5, 4, 0, 0, 0}, // same as AB/ABL
  {obj.APCALIGN, C_LCON, C_NONE, C_NONE, C_NONE, 0, 0, 0, 0, 0}, // align code
-
- {obj.AXXX, C_NONE, C_NONE, C_NONE, C_NONE, 0, 4, 0, 0, 0},
 }
 
 // Valid pstate field values, and value to use in instruction.
@@ -2596,29 +2594,27 @@ func buildop(ctxt *obj.Link) {
  return
  }
 
- var n int
  for i := 0; i < C_GOK; i++ {
- for n = 0; n < C_GOK; n++ {
- if cmp(n, i) {
- xcmp[i][n] = true
+ for j := 0; j < C_GOK; j++ {
+ if cmp(j, i) {
+ xcmp[i][j] = true
  }
  }
  }
- for n = 0; optab[n].as != obj.AXXX; n++ {
- }
- sort.Sort(ocmp(optab[:n]))
- for i := 0; i < n; i++ {
- r := optab[i].as
- start := i
- for optab[i].as == r {
- i++
+
+ sort.Sort(ocmp(optab))
+ for i := 0; i < len(optab); i++ {
+  as, start := optab[i].as, i
+ for ; i < len(optab)-1; i++ {
+  if optab[i+1].as != as {
+  break
+ }
  }
- t := optab[start:i]
- i--
- oprangeset(r, t)
- switch r {
+ t := optab[start : i+1]
+ oprangeset(as, t)
+ switch as {
  default:
- ctxt.Diag("unknown op in build: %v", r)
+ ctxt.Diag("unknown op in build: %v", as)
  ctxt.DiagFlush()
  log.Fatalf("bad code")