runtime: use threads slice in parfor instead of unsafe pointer math

parfor originally used a tail array for its thread array.  This got
replaced with a slice allocation in the conversion to Go, but many of
its gnarlier effects remained.  Instead of keeping track of the
pointer to the first element of the slice and using unsafe pointer
math to get at the ith element, just keep the slice around and use
regular slice indexing.  There is no longer any need for padding to
64-bit align the tail array (there hasn't been since the Go
conversion), so remove this unnecessary padding from the parfor
struct.  Finally, since the slice tracks its own length, replace the
nthrmax field with len(thr).

Change-Id: I0020a1815849bca53e3613a8fa46ae4fbae67576
Reviewed-on: https://go-review.googlesource.com/3394
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/export_test.go

@@ -36,14 +36,13 @@ func LFStackPop(head *uint64) *LFNode {
 }
 
 type ParFor struct {
-	body    *byte
-	done    uint32
-	Nthr    uint32
-	nthrmax uint32
-	thrseq  uint32
-	Cnt     uint32
-	Ctx     *byte
-	wait    bool
+	body   *byte
+	done   uint32
+	Nthr   uint32
+	thrseq uint32
+	Cnt    uint32
+	Ctx    *byte
+	wait   bool
 }
 
 func NewParFor(nthrmax uint32) *ParFor {
@@ -69,7 +68,7 @@ func ParForDo(desc *ParFor) {
 
 func ParForIters(desc *ParFor, tid uint32) (uint32, uint32) {
 	desc1 := (*parfor)(unsafe.Pointer(desc))
-	pos := desc_thr_index(desc1, tid).pos
+	pos := desc1.thr[tid].pos
 	return uint32(pos), uint32(pos >> 32)
 }