compiler: implement clear builtin for maps

Author: aykevl

compiler/compiler.go

@@ -1631,6 +1631,10 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 			}, "")
 			call.AddCallSiteAttribute(1, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("align"), uint64(elementAlign)))
 
+			return llvm.Value{}, nil
+		case *types.Map:
+			m := argValues[0]
+			b.createMapClear(m)
 			return llvm.Value{}, nil
 		default:
 			return llvm.Value{}, b.makeError(pos, "unsupported type in clear builtin: "+typ.String())

compiler/map.go

@@ -185,6 +185,11 @@ func (b *builder) createMapDelete(keyType types.Type, m, key llvm.Value, pos tok
 	}
 }
 
+// Clear the given map.
+func (b *builder) createMapClear(m llvm.Value) {
+	b.createRuntimeCall("hashmapClear", []llvm.Value{m}, "")
+}
+
 // createMapIteratorNext lowers the *ssa.Next instruction for iterating over a
 // map. It returns a tuple of {bool, key, value} with the result of the
 // iteration.

compiler/testdata/go1.21.go

@@ -59,3 +59,7 @@ func clearSlice(s []int) {
 func clearZeroSizedSlice(s []struct{}) {
 	clear(s)
 }
+
+func clearMap(m map[string]int) {
+	clear(m)
+}

compiler/testdata/go1.21.ll

@@ -147,6 +147,15 @@ entry:
   ret void
 }
 
+; Function Attrs: nounwind
+define hidden void @main.clearMap(ptr dereferenceable_or_null(40) %m, ptr %context) unnamed_addr #2 {
+entry:
+  call void @runtime.hashmapClear(ptr %m, ptr undef) #5
+  ret void
+}
+
+declare void @runtime.hashmapClear(ptr dereferenceable_or_null(40), ptr) #1
+
 ; Function Attrs: nocallback nofree nosync nounwind readnone speculatable willreturn
 declare i32 @llvm.smin.i32(i32, i32) #4
 

src/runtime/hashmap.go

@@ -91,6 +91,35 @@ func hashmapMakeUnsafePointer(keySize, valueSize uintptr, sizeHint uintptr, alg
 	return (unsafe.Pointer)(hashmapMake(keySize, valueSize, sizeHint, alg))
 }
 
+// Remove all entries from the map, without actually deallocating the space for
+// it. This is used for the clear builtin, and can be used to reuse a map (to
+// avoid extra heap allocations).
+func hashmapClear(m *hashmap) {
+	if m == nil {
+		// Nothing to do. According to the spec:
+		// > If the map or slice is nil, clear is a no-op.
+		return
+	}
+
+	m.count = 0
+	numBuckets := uintptr(1) << m.bucketBits
+	bucketSize := hashmapBucketSize(m)
+	for i := uintptr(0); i < numBuckets; i++ {
+		bucket := hashmapBucketAddr(m, m.buckets, i)
+		for bucket != nil {
+			// Clear the tophash, to mark these keys/values as removed.
+			bucket.tophash = [8]uint8{}
+
+			// Clear the keys and values in the bucket so that the GC won't pin
+			// these allocations.
+			memzero(unsafe.Add(unsafe.Pointer(bucket), unsafe.Sizeof(hashmapBucket{})), bucketSize-unsafe.Sizeof(hashmapBucket{}))
+
+			// Move on to the next bucket in the chain.
+			bucket = bucket.next
+		}
+	}
+}
+
 func hashmapKeyEqualAlg(alg hashmapAlgorithm) func(x, y unsafe.Pointer, n uintptr) bool {
 	switch alg {
 	case hashmapAlgorithmBinary:

testdata/go1.21.go

@@ -15,4 +15,15 @@ func main() {
 	s := []int{1, 2, 3, 4, 5}
 	clear(s[:3])
 	println("cleared s[:3]:", s[0], s[1], s[2], s[3], s[4])
+
+	// The clear builtin, for maps.
+	m := map[int]string{
+		1: "one",
+		2: "two",
+		3: "three",
+	}
+	clear(m)
+	println("cleared map:", m[1], m[2], m[3], len(m))
+	m[4] = "four"
+	println("added to cleared map:", m[1], m[2], m[3], m[4], len(m))
 }

testdata/go1.21.txt

@@ -1,3 +1,5 @@
 min/max: -3 5
 min/max: -3.000000e+000 +5.000000e+000
 cleared s[:3]: 0 0 0 4 5
+cleared map:    0
+added to cleared map:    four 1