sync: simplify (*entry).tryStore

The only change to the go build -gcflags=-m=2 output was to remove
these two lines:

sync/map.go:178:26: &e.p escapes to heap
sync/map.go:178:26: 	from &e.p (passed to call[argument escapes]) at sync/map.go:178:25

Benchstat report for sync.Map benchmarks:

name                                            old time/op  new time/op  delta
LoadMostlyHits/*sync_test.DeepCopyMap-12        10.6ns ±11%  10.2ns ± 3%    ~     (p=0.299 n=10+8)
LoadMostlyHits/*sync_test.RWMutexMap-12         54.6ns ± 3%  54.6ns ± 2%    ~     (p=0.782 n=10+10)
LoadMostlyHits/*sync.Map-12                     10.1ns ± 1%  10.1ns ± 1%    ~     (p=1.127 n=10+8)
LoadMostlyMisses/*sync_test.DeepCopyMap-12      8.65ns ± 1%  8.77ns ± 5%  +1.39%  (p=0.017 n=9+10)
LoadMostlyMisses/*sync_test.RWMutexMap-12       53.6ns ± 2%  53.8ns ± 2%    ~     (p=0.408 n=10+9)
LoadMostlyMisses/*sync.Map-12                   7.37ns ± 1%  7.46ns ± 1%  +1.19%  (p=0.000 n=9+10)
LoadOrStoreBalanced/*sync_test.RWMutexMap-12     895ns ± 4%   906ns ± 3%    ~     (p=0.203 n=9+10)
LoadOrStoreBalanced/*sync.Map-12                 872ns ±10%   804ns ±12%  -7.75%  (p=0.014 n=10+10)
LoadOrStoreUnique/*sync_test.RWMutexMap-12      1.29µs ± 2%  1.28µs ± 1%    ~     (p=0.586 n=10+9)
LoadOrStoreUnique/*sync.Map-12                  1.30µs ± 7%  1.40µs ± 2%  +6.95%  (p=0.000 n=9+10)
LoadOrStoreCollision/*sync_test.DeepCopyMap-12  6.98ns ± 1%  6.91ns ± 1%  -1.10%  (p=0.000 n=10+10)
LoadOrStoreCollision/*sync_test.RWMutexMap-12    371ns ± 1%   372ns ± 2%    ~     (p=0.679 n=9+9)
LoadOrStoreCollision/*sync.Map-12               5.49ns ± 1%  5.49ns ± 1%    ~     (p=0.732 n=9+10)
Range/*sync_test.DeepCopyMap-12                 2.49µs ± 1%  2.50µs ± 0%    ~     (p=0.148 n=10+10)
Range/*sync_test.RWMutexMap-12                  54.7µs ± 1%  54.6µs ± 3%    ~     (p=0.549 n=9+10)
Range/*sync.Map-12                              2.74µs ± 1%  2.76µs ± 1%  +0.68%  (p=0.011 n=10+8)
AdversarialAlloc/*sync_test.DeepCopyMap-12      2.52µs ± 5%  2.54µs ± 7%    ~     (p=0.225 n=10+10)
AdversarialAlloc/*sync_test.RWMutexMap-12        108ns ± 1%   107ns ± 1%    ~     (p=0.101 n=10+9)
AdversarialAlloc/*sync.Map-12                    712ns ± 2%   714ns ± 3%    ~     (p=0.984 n=8+10)
AdversarialDelete/*sync_test.DeepCopyMap-12      581ns ± 3%   578ns ± 3%    ~     (p=0.781 n=9+9)
AdversarialDelete/*sync_test.RWMutexMap-12       126ns ± 2%   126ns ± 1%    ~     (p=0.883 n=10+10)
AdversarialDelete/*sync.Map-12                   155ns ± 8%   158ns ± 2%    ~     (p=0.158 n=10+9)

Change-Id: I1ed8e3109baca03087d0fad3df769fc7e38f6dbb
Reviewed-on: https://go-review.googlesource.com/137441Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sync/map.go

@@ -167,18 +167,14 @@ func (m *Map) Store(key, value interface{}) {
 // If the entry is expunged, tryStore returns false and leaves the entry
 // unchanged.
 func (e *entry) tryStore(i *interface{}) bool {
-	p := atomic.LoadPointer(&e.p)
-	if p == expunged {
-		return false
-	}
 	for {
-		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
-			return true
-		}
-		p = atomic.LoadPointer(&e.p)
+		p := atomic.LoadPointer(&e.p)
 		if p == expunged {
 			return false
 		}
+		if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
+			return true
+		}
 	}
 }