compiler,runtime: use the size hint when creating a new map

It defaults to hint/8 number of buckets. This number may be tuned in the
future.

compiler/compiler.go

@@ -1425,7 +1425,16 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		valueSize := c.targetData.TypeAllocSize(llvmValueType)
 		llvmKeySize := llvm.ConstInt(c.ctx.Int8Type(), keySize, false)
 		llvmValueSize := llvm.ConstInt(c.ctx.Int8Type(), valueSize, false)
-		hashmap := c.createRuntimeCall("hashmapMake", []llvm.Value{llvmKeySize, llvmValueSize}, "")
+		sizeHint := llvm.ConstInt(c.uintptrType, 8, false)
+		if expr.Reserve != nil {
+			sizeHint = c.getValue(frame, expr.Reserve)
+			var err error
+			sizeHint, err = c.parseConvert(expr.Reserve.Type(), types.Typ[types.Uintptr], sizeHint, expr.Pos())
+			if err != nil {
+				return llvm.Value{}, err
+			}
+		}
+		hashmap := c.createRuntimeCall("hashmapMake", []llvm.Value{llvmKeySize, llvmValueSize, sizeHint}, "")
 		return hashmap, nil
 	case *ssa.MakeSlice:
 		sliceLen := c.getValue(frame, expr.Len)

src/runtime/hashmap.go

@@ -60,14 +60,20 @@ func hashmapTopHash(hash uint32) uint8 {
 }
 
 // Create a new hashmap with the given keySize and valueSize.
-func hashmapMake(keySize, valueSize uint8) *hashmap {
+func hashmapMake(keySize, valueSize uint8, sizeHint uintptr) *hashmap {
+	numBuckets := sizeHint / 8
+	bucketBits := uint8(0)
+	for numBuckets != 0 {
+		numBuckets /= 2
+		bucketBits++
+	}
 	bucketBufSize := unsafe.Sizeof(hashmapBucket{}) + uintptr(keySize)*8 + uintptr(valueSize)*8
-	bucket := alloc(bucketBufSize)
+	buckets := alloc(bucketBufSize * (1 << bucketBits))
 	return &hashmap{
-		buckets:    bucket,
+		buckets:    buckets,
 		keySize:    keySize,
 		valueSize:  valueSize,
-		bucketBits: 0,
+		bucketBits: bucketBits,
 	}
 }
 

testdata/map.go

@@ -47,6 +47,18 @@ func main() {
 	println(testMapArrayKey[arrKey])
 	testMapArrayKey[arrKey] = 5555
 	println(testMapArrayKey[arrKey])
+
+	// test preallocated map
+	squares := make(map[int]int, 200)
+	for i := 0; i < 100; i++ {
+		squares[i] = i*i
+		for j := 0; j <= i; j++ {
+			if v := squares[j]; v != j*j {
+				println("unexpected value read back from squares map:", j, v)
+			}
+		}
+	}
+	println("tested preallocated map")
 }
 
 func readMap(m map[string]int, key string) {
@@ -56,6 +68,7 @@ func readMap(m map[string]int, key string) {
 		println(" ", k, "=", v)
 	}
 }
+
 func lookup(m map[string]int, key string) {
 	value, ok := m[key]
 	println("lookup with comma-ok:", key, value, ok)

testdata/map.txt

@@ -54,3 +54,4 @@ true false 0
 42
 4321
 5555
+tested preallocated map