cmd/compile: clean up slice and string offsets/sizes

Minor cleanup:

* Modernize comments.
* Change from int to int64 to avoid conversions.
* Use idiomatic names.

Passes toolstash-check.

Change-Id: I93560c81926c0f4e00f33129cb4846b53bea99e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/227548
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/align.go

@@ -319,10 +319,10 @@ func dowidth(t *types.Type) {
 		Fatalf("dowidth any")
 
 	case TSTRING:
-		if sizeof_String == 0 {
+		if sizeofString == 0 {
 			Fatalf("early dowidth string")
 		}
-		w = int64(sizeof_String)
+		w = sizeofString
 		t.Align = uint8(Widthptr)
 
 	case TARRAY:
@@ -344,7 +344,7 @@ func dowidth(t *types.Type) {
 		if t.Elem() == nil {
 			break
 		}
-		w = int64(sizeof_Slice)
+		w = sizeofSlice
 		checkwidth(t.Elem())
 		t.Align = uint8(Widthptr)
 

src/cmd/compile/internal/gc/go.go

@@ -64,32 +64,30 @@ const (
 	_ = uint((1 << 3) - iota) // static assert for iota <= (1 << 3)
 )
 
-// note this is the runtime representation
-// of the compilers slices.
+// Slices in the runtime are represented by three components:
 //
-// typedef	struct
-// {				// must not move anything
-// 	uchar	array[8];	// pointer to data
-// 	uchar	nel[4];		// number of elements
-// 	uchar	cap[4];		// allocated number of elements
-// } Slice;
-var slice_array int // runtime offsetof(Slice,array) - same for String
-
-var slice_nel int // runtime offsetof(Slice,nel) - same for String
-
-var slice_cap int // runtime offsetof(Slice,cap)
-
-var sizeof_Slice int // runtime sizeof(Slice)
-
-// note this is the runtime representation
-// of the compilers strings.
+// type slice struct {
+// 	ptr unsafe.Pointer
+// 	len int
+// 	cap int
+// }
 //
-// typedef	struct
-// {				// must not move anything
-// 	uchar	array[8];	// pointer to data
-// 	uchar	nel[4];		// number of elements
-// } String;
-var sizeof_String int // runtime sizeof(String)
+// Strings in the runtime are represented by two components:
+//
+// type string struct {
+// 	ptr unsafe.Pointer
+// 	len int
+// }
+//
+// These variables are the offsets of fields and sizes of these structs.
+var (
+	slicePtrOffset int64
+	sliceLenOffset int64
+	sliceCapOffset int64
+
+	sizeofSlice  int64
+	sizeofString int64
+)
 
 var pragcgobuf [][]string
 

src/cmd/compile/internal/gc/sinit.go

@@ -130,11 +130,11 @@ func (s *InitSchedule) staticcopy(l *Node, r *Node) bool {
 		a := s.inittemps[r]
 
 		n := l.copy()
-		n.Xoffset = l.Xoffset + int64(slice_array)
+		n.Xoffset = l.Xoffset + slicePtrOffset
 		gdata(n, nod(OADDR, a, nil), Widthptr)
-		n.Xoffset = l.Xoffset + int64(slice_nel)
+		n.Xoffset = l.Xoffset + sliceLenOffset
 		gdata(n, r.Right, Widthptr)
-		n.Xoffset = l.Xoffset + int64(slice_cap)
+		n.Xoffset = l.Xoffset + sliceCapOffset
 		gdata(n, r.Right, Widthptr)
 		return true
 
@@ -227,11 +227,11 @@ func (s *InitSchedule) staticassign(l *Node, r *Node) bool {
 		a := staticname(ta)
 		s.inittemps[r] = a
 		n := l.copy()
-		n.Xoffset = l.Xoffset + int64(slice_array)
+		n.Xoffset = l.Xoffset + slicePtrOffset
 		gdata(n, nod(OADDR, a, nil), Widthptr)
-		n.Xoffset = l.Xoffset + int64(slice_nel)
+		n.Xoffset = l.Xoffset + sliceLenOffset
 		gdata(n, r.Right, Widthptr)
-		n.Xoffset = l.Xoffset + int64(slice_cap)
+		n.Xoffset = l.Xoffset + sliceCapOffset
 		gdata(n, r.Right, Widthptr)
 
 		// Fall through to init underlying array.
@@ -615,11 +615,11 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
 		v.Type = types.Types[TINT]
 		setintconst(&v, t.NumElem())
 
-		nam.Xoffset += int64(slice_array)
+		nam.Xoffset += slicePtrOffset
 		gdata(&nam, nod(OADDR, vstat, nil), Widthptr)
-		nam.Xoffset += int64(slice_nel) - int64(slice_array)
+		nam.Xoffset += sliceLenOffset - slicePtrOffset
 		gdata(&nam, &v, Widthptr)
-		nam.Xoffset += int64(slice_cap) - int64(slice_nel)
+		nam.Xoffset += sliceCapOffset - sliceLenOffset
 		gdata(&nam, &v, Widthptr)
 
 		return

src/cmd/compile/internal/gc/ssa.go

@@ -2830,7 +2830,7 @@ func (s *state) append(n *Node, inplace bool) *ssa.Value {
 			// Tell liveness we're about to build a new slice
 			s.vars[&memVar] = s.newValue1A(ssa.OpVarDef, types.TypeMem, sn, s.mem())
 		}
-		capaddr := s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.IntPtr, int64(slice_cap), addr)
+		capaddr := s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.IntPtr, sliceCapOffset, addr)
 		s.store(types.Types[TINT], capaddr, r[2])
 		s.store(pt, addr, r[0])
 		// load the value we just stored to avoid having to spill it
@@ -2851,7 +2851,7 @@ func (s *state) append(n *Node, inplace bool) *ssa.Value {
 	if inplace {
 		l = s.variable(&lenVar, types.Types[TINT]) // generates phi for len
 		nl = s.newValue2(s.ssaOp(OADD, types.Types[TINT]), types.Types[TINT], l, s.constInt(types.Types[TINT], nargs))
-		lenaddr := s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.IntPtr, int64(slice_nel), addr)
+		lenaddr := s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.IntPtr, sliceLenOffset, addr)
 		s.store(types.Types[TINT], lenaddr, nl)
 	}
 

src/cmd/compile/internal/gc/universe.go

@@ -342,13 +342,13 @@ func typeinit() {
 	simtype[TFUNC] = TPTR
 	simtype[TUNSAFEPTR] = TPTR
 
-	slice_array = int(Rnd(0, int64(Widthptr)))
-	slice_nel = int(Rnd(int64(slice_array)+int64(Widthptr), int64(Widthptr)))
-	slice_cap = int(Rnd(int64(slice_nel)+int64(Widthptr), int64(Widthptr)))
-	sizeof_Slice = int(Rnd(int64(slice_cap)+int64(Widthptr), int64(Widthptr)))
+	slicePtrOffset = 0
+	sliceLenOffset = Rnd(slicePtrOffset+int64(Widthptr), int64(Widthptr))
+	sliceCapOffset = Rnd(sliceLenOffset+int64(Widthptr), int64(Widthptr))
+	sizeofSlice = Rnd(sliceCapOffset+int64(Widthptr), int64(Widthptr))
 
 	// string is same as slice wo the cap
-	sizeof_String = int(Rnd(int64(slice_nel)+int64(Widthptr), int64(Widthptr)))
+	sizeofString = Rnd(sliceLenOffset+int64(Widthptr), int64(Widthptr))
 
 	dowidth(types.Types[TSTRING])
 	dowidth(types.Idealstring)