Reduce overhead of AlignedAtomic types

AlignedAtomicUint64 is 15 bytes and it takes 16 bytes in structures.  On
32-bit systems, variables and structure fields is guaranteed to be
32-bit aligned and this means that we need only 12 bytes to find 8
contiguous bytes.

1 files changed, 12 insertions, 10 deletions

diff --git a/pkg/atomicbitops/aligned_32bit_unsafe.go b/pkg/atomicbitops/aligned_32bit_unsafe.go
index 383f81ff2..a17d317cc 100644
--- a/pkg/atomicbitops/aligned_32bit_unsafe.go
+++ b/pkg/atomicbitops/aligned_32bit_unsafe.go
@@ -34,14 +34,15 @@ import (
 //
 // +stateify savable
 type AlignedAtomicInt64 struct {
-	value [15]byte
+	value   int64
+	value32 int32
 }
 
 func (aa *AlignedAtomicInt64) ptr() *int64 {
-	// In the 15-byte aa.value, there are guaranteed to be 8 contiguous
-	// bytes with 64-bit alignment. We find an address in this range by
-	// adding 7, then clear the 3 least significant bits to get its start.
-	return (*int64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value[0])) + 7) &^ 7))
+	// On 32-bit systems, aa.value is is guaranteed to be 32-bit aligned.
+	// It means that in the 12-byte aa.value, there are guaranteed to be 8
+	// contiguous bytes with 64-bit alignment.
+	return (*int64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value)) + 4) &^ 7))
 }
 
 // Load is analagous to atomic.LoadInt64.
@@ -71,14 +72,15 @@ func (aa *AlignedAtomicInt64) Add(v int64) int64 {
 //
 // +stateify savable
 type AlignedAtomicUint64 struct {
-	value [15]byte
+	value   uint64
+	value32 uint32
 }
 
 func (aa *AlignedAtomicUint64) ptr() *uint64 {
-	// In the 15-byte aa.value, there are guaranteed to be 8 contiguous
-	// bytes with 64-bit alignment. We find an address in this range by
-	// adding 7, then clear the 3 least significant bits to get its start.
-	return (*uint64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value[0])) + 7) &^ 7))
+	// On 32-bit systems, aa.value is is guaranteed to be 32-bit aligned.
+	// It means that in the 12-byte aa.value, there are guaranteed to be 8
+	// contiguous bytes with 64-bit alignment.
+	return (*uint64)(unsafe.Pointer((uintptr(unsafe.Pointer(&aa.value)) + 4) &^ 7))
 }
 
 // Load is analagous to atomic.LoadUint64.