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>

2 files changed, 24 insertions, 34 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 016938ed4e..3b13b7bb38 100644
--- a/src/runtime/export_test.go
+++ b/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)
 }
 
diff --git a/src/runtime/parfor.go b/src/runtime/parfor.go
index 1c28924096..fc5ebd887e 100644
--- a/src/runtime/parfor.go
+++ b/src/runtime/parfor.go
@@ -10,17 +10,17 @@ import "unsafe"
 
 // A parfor holds state for the parallel for operation.
 type parfor struct {
-	body    unsafe.Pointer // go func(*parfor, uint32), executed for each element
-	done    uint32         // number of idle threads
-	nthr    uint32         // total number of threads
-	nthrmax uint32         // maximum number of threads
-	thrseq  uint32         // thread id sequencer
-	cnt     uint32         // iteration space [0, cnt)
-	ctx     unsafe.Pointer // arbitrary user context
-	wait    bool           // if true, wait while all threads finish processing,
+	body   unsafe.Pointer // go func(*parfor, uint32), executed for each element
+	done   uint32         // number of idle threads
+	nthr   uint32         // total number of threads
+	thrseq uint32         // thread id sequencer
+	cnt    uint32         // iteration space [0, cnt)
+	ctx    unsafe.Pointer // arbitrary user context
+	wait   bool           // if true, wait while all threads finish processing,
 	// otherwise parfor may return while other threads are still working
-	thr *parforthread // array of thread descriptors
-	pad uint32        // to align parforthread.pos for 64-bit atomic operations
+
+	thr []parforthread // thread descriptors
+
 	// stats
 	nsteal     uint64
 	nstealcnt  uint64
@@ -42,14 +42,9 @@ type parforthread struct {
 	pad        [_CacheLineSize]byte
 }
 
-func desc_thr_index(desc *parfor, i uint32) *parforthread {
-	return (*parforthread)(add(unsafe.Pointer(desc.thr), uintptr(i)*unsafe.Sizeof(*desc.thr)))
-}
-
 func parforalloc(nthrmax uint32) *parfor {
 	return &parfor{
-		thr:     &make([]parforthread, nthrmax)[0],
-		nthrmax: nthrmax,
+		thr: make([]parforthread, nthrmax),
 	}
 }
 
@@ -66,7 +61,7 @@ func parforalloc(nthrmax uint32) *parfor {
 // The opaque user context ctx is recorded as desc.ctx and can be used by body.
 // TODO(austin): Remove ctx in favor of using a closure for body.
 func parforsetup(desc *parfor, nthr, n uint32, ctx unsafe.Pointer, wait bool, body func(*parfor, uint32)) {
-	if desc == nil || nthr == 0 || nthr > desc.nthrmax || body == nil {
+	if desc == nil || nthr == 0 || nthr > uint32(len(desc.thr)) || body == nil {
 		print("desc=", desc, " nthr=", nthr, " count=", n, " body=", body, "\n")
 		throw("parfor: invalid args")
 	}
@@ -84,14 +79,10 @@ func parforsetup(desc *parfor, nthr, n uint32, ctx unsafe.Pointer, wait bool, bo
 	desc.nosyield = 0
 	desc.nsleep = 0
 
-	for i := uint32(0); i < nthr; i++ {
+	for i := range desc.thr {
 		begin := uint32(uint64(n) * uint64(i) / uint64(nthr))
 		end := uint32(uint64(n) * uint64(i+1) / uint64(nthr))
-		pos := &desc_thr_index(desc, i).pos
-		if uintptr(unsafe.Pointer(pos))&7 != 0 {
-			throw("parforsetup: pos is not aligned")
-		}
-		*pos = uint64(begin) | uint64(end)<<32
+		desc.thr[i].pos = uint64(begin) | uint64(end)<<32
 	}
 }
 
@@ -112,7 +103,7 @@ func parfordo(desc *parfor) {
 		return
 	}
 
-	me := desc_thr_index(desc, tid)
+	me := &desc.thr[tid]
 	mypos := &me.pos
 	for {
 		for {
@@ -157,7 +148,7 @@ func parfordo(desc *parfor) {
 			if victim >= tid {
 				victim++
 			}
-			victimpos := &desc_thr_index(desc, victim).pos
+			victimpos := &desc.thr[victim].pos
 			for {
 				// See if it has any work.
 				pos := atomicload64(victimpos)