runtime/coverage: use unsafe.Slice, not reflect.SliceHeader

Change-Id: I59c4757df83c12b4c8b85cdd523552c5e5e7bf95
Reviewed-on: https://go-review.googlesource.com/c/go/+/508977
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: ianlancetaylor

src/runtime/coverage/apis.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"internal/coverage"
 	"io"
-	"reflect"
 	"sync/atomic"
 	"unsafe"
 )
@@ -158,13 +157,8 @@ func ClearCounters() error {
 	// inconsistency when reading the counter array from the thread
 	// running ClearCounters.
 
-	var sd []atomic.Uint32
-
-	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
 	for _, c := range cl {
-		bufHdr.Data = uintptr(unsafe.Pointer(c.Counters))
-		bufHdr.Len = int(c.Len)
-		bufHdr.Cap = int(c.Len)
+		sd := unsafe.Slice((*atomic.Uint32)(unsafe.Pointer(c.Counters)), int(c.Len))
 		for i := 0; i < len(sd); i++ {
 			// Skip ahead until the next non-zero value.
 			sdi := sd[i].Load()

src/runtime/coverage/emit.go

@@ -14,7 +14,6 @@ import (
 	"io"
 	"os"
 	"path/filepath"
-	"reflect"
 	"runtime"
 	"strconv"
 	"sync/atomic"
@@ -447,26 +446,16 @@ func (s *emitState) needMetaDataFile() bool {
 func writeMetaData(w io.Writer, metalist []rtcov.CovMetaBlob, cmode coverage.CounterMode, gran coverage.CounterGranularity, finalHash [16]byte) error {
 	mfw := encodemeta.NewCoverageMetaFileWriter("<io.Writer>", w)
 
-	// Note: "sd" is re-initialized on each iteration of the loop
-	// below, and would normally be declared inside the loop, but
-	// placed here escape analysis since we capture it in bufHdr.
-	var sd []byte
-	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
-
 	var blobs [][]byte
 	for _, e := range metalist {
-		bufHdr.Data = uintptr(unsafe.Pointer(e.P))
-		bufHdr.Len = int(e.Len)
-		bufHdr.Cap = int(e.Len)
+		sd := unsafe.Slice(e.P, int(e.Len))
 		blobs = append(blobs, sd)
 	}
 	return mfw.Write(finalHash, blobs, cmode, gran)
 }
 
 func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
-	var sd []atomic.Uint32
 	var tcounters []uint32
-	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
 
 	rdCounters := func(actrs []atomic.Uint32, ctrs []uint32) []uint32 {
 		ctrs = ctrs[:0]
@@ -478,9 +467,7 @@ func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
 
 	dpkg := uint32(0)
 	for _, c := range s.counterlist {
-		bufHdr.Data = uintptr(unsafe.Pointer(c.Counters))
-		bufHdr.Len = int(c.Len)
-		bufHdr.Cap = int(c.Len)
+		sd := unsafe.Slice((*atomic.Uint32)(unsafe.Pointer(c.Counters)), int(c.Len))
 		for i := 0; i < len(sd); i++ {
 			// Skip ahead until the next non-zero value.
 			sdi := sd[i].Load()