add noescape annotations

Author: mhr3

gozstd.go

@@ -147,9 +147,22 @@ func compress(cctx, cctxDict *cctxWrapper, dst, src []byte, cd *CDict, compressi
 	return dst
 }
 
+// noescape hides a pointer from escape analysis. It is the identity function
+// but escape analysis doesn't think the output depends on the input.
+// noescape is inlined and currently compiles down to zero instructions.
+// This is copied from go's strings.Builder. Allows us to use stack-allocated
+// slices.
+//go:nosplit
+//go:nocheckptr
+func noescape(p unsafe.Pointer) unsafe.Pointer {
+	x := uintptr(p)
+	return unsafe.Pointer(x ^ 0)
+}
+
 func compressInternal(cctx, cctxDict *cctxWrapper, dst, src []byte, cd *CDict, compressionLevel int, mustSucceed bool) C.size_t {
-	dstHdr := (*reflect.SliceHeader)(unsafe.Pointer(&dst))
-	srcHdr := (*reflect.SliceHeader)(unsafe.Pointer(&src))
+	// using noescape will allow this to work with stack-allocated slices
+	dstHdr := (*reflect.SliceHeader)(noescape(unsafe.Pointer(&dst)))
+	srcHdr := (*reflect.SliceHeader)(noescape(unsafe.Pointer(&src)))
 
 	if cd != nil {
 		result := C.ZSTD_compress_usingCDict_wrapper(
@@ -180,6 +193,7 @@ func compressInternal(cctx, cctxDict *cctxWrapper, dst, src []byte, cd *CDict, c
 	if mustSucceed {
 		ensureNoError("ZSTD_compressCCtx", result)
 	}
+
 	return result
 }
 
@@ -258,7 +272,7 @@ func decompress(dctx, dctxDict *dctxWrapper, dst, src []byte, dd *DDict) ([]byte
 	}
 
 	// Slow path - resize dst to fit decompressed data.
-	srcHdr := (*reflect.SliceHeader)(unsafe.Pointer(&src))
+	srcHdr := (*reflect.SliceHeader)(noescape(unsafe.Pointer(&src)))
 	contentSize := C.ZSTD_getFrameContentSize_wrapper(unsafe.Pointer(srcHdr.Data), C.size_t(len(src)))
 	switch {
 	case contentSize == C.ZSTD_CONTENTSIZE_UNKNOWN || contentSize > maxFrameContentSize:
@@ -290,8 +304,8 @@ func decompress(dctx, dctxDict *dctxWrapper, dst, src []byte, dd *DDict) ([]byte
 
 func decompressInternal(dctx, dctxDict *dctxWrapper, dst, src []byte, dd *DDict) C.size_t {
 	var (
-		dstHdr = (*reflect.SliceHeader)(unsafe.Pointer(&dst))
-		srcHdr = (*reflect.SliceHeader)(unsafe.Pointer(&src))
+		dstHdr = (*reflect.SliceHeader)(noescape(unsafe.Pointer(&dst)))
+		srcHdr = (*reflect.SliceHeader)(noescape(unsafe.Pointer(&src)))
 		n      C.size_t
 	)
 	if dd != nil {

gozstd_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/hex"
 	"fmt"
+	"io"
 	"math/rand"
 	"runtime"
 	"strings"
@@ -54,6 +55,14 @@ func TestDecompressSmallBlockWithoutSingleSegmentFlag(t *testing.T) {
 	})
 }
 
+func TestCompressEmpty(t *testing.T) {
+	var dst [64]byte
+	res := Compress(dst[:0], nil)
+	if len(res) > 0 {
+		t.Fatalf("unexpected non-empty compressed frame: %X", res)
+	}
+}
+
 func TestDecompressTooLarge(t *testing.T) {
 	src := []byte{40, 181, 47, 253, 228, 122, 118, 105, 67, 140, 234, 85, 20, 159, 67}
 	_, err := Decompress(nil, src)
@@ -70,6 +79,48 @@ func mustUnhex(dataHex string) []byte {
 	return data
 }
 
+func TestCompressWithStackMove(t *testing.T) {
+	var srcBuf [96]byte
+
+	n, err := io.ReadFull(rand.New(rand.NewSource(time.Now().Unix())), srcBuf[:])
+	if err != nil {
+		t.Fatalf("cannot fill srcBuf with random data: %s", err)
+	}
+
+	// We're running this twice, because the first run will allocate
+	// objects in sync.Pool, calls to which extend the stack, and the second
+	// run can skip those allocations and extend the stack right before
+	// the CGO call.
+	// Note that this test might require some go:nosplit annotations
+	// to force the stack move to happen exactly before the CGO call.
+	for i := 0; i < 2; i++ {
+		ch := make(chan struct{})
+		go func() {
+			defer close(ch)
+
+			var dstBuf [1416]byte
+
+			res := Compress(dstBuf[:0], srcBuf[:n])
+
+			// make a copy of the result, so the original can remain on the stack
+			compressedCpy := make([]byte, len(res))
+			copy(compressedCpy, res)
+
+			orig, err := Decompress(nil, compressedCpy)
+			if err != nil {
+				panic(fmt.Errorf("cannot decompress: %s", err))
+			}
+			if !bytes.Equal(orig, srcBuf[:n]) {
+				panic(fmt.Errorf("unexpected decompressed data; got %q; want %q", orig, srcBuf[:n]))
+			}
+		}()
+		// wait for the goroutine to finish
+		<-ch
+	}
+
+	runtime.GC()
+}
+
 func TestCompressDecompressDistinctConcurrentDicts(t *testing.T) {
 	// Build multiple distinct dicts.
 	var cdicts []*CDict