cmd/compile: change the way SSA does slice zero-cap detection

There is a special case for slicing s[i:j] when the resulting
slice has zero capacity, to prevent pointing to the next object
in memory.

Change this special case code from:
  rptr := rcap == 0 ? ptr : ptr+i*elemsize
to
  rptr := ptr + (rcap == 0 ? 0 : i) * elemsize

This change leads to slightly smaller generated code, replacing
a load with a register zero.

old:
	0x002e 00046 (slice.go:8)	CMPQ	BX, $0
	0x0032 00050 (slice.go:8)	JEQ	$0, 78
	0x0034 00052 (slice.go:8)	MOVQ	"".a+8(FP), BP
	0x0039 00057 (slice.go:8)	LEAQ	(BP)(CX*8), AX
	0x003e 00062 ... rest of function ...

	0x004e 00078 (slice.go:7)	MOVQ	"".a+8(FP), AX
	0x0053 00083 (slice.go:8)	JMP	62

new:
	0x002e 00046 (slice.go:8)	CMPQ	BX, $0
	0x0032 00050 (slice.go:8)	JEQ	$0, 78
	0x0034 00052 (slice.go:8)	MOVQ	"".a+8(FP), BP
	0x0039 00057 (slice.go:8)	LEAQ	(BP)(CX*8), AX
	0x003e 00062 ... rest of function...

	0x004e 00078 (slice.go:8)	MOVQ	$0, CX
	0x0050 00080 (slice.go:8)	JMP	52

Change-Id: I2a396616b0d7b090c226a47c92a7ba14b128401f
Reviewed-on: https://go-review.googlesource.com/20994Reviewed-by: David Chase <drchase@google.com>

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

@@ -335,6 +335,7 @@ var (
 	typVar   = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "typ"}}
 	idataVar = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "idata"}}
 	okVar    = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "ok"}}
+	deltaVar = Node{Op: ONAME, Class: Pxxx, Sym: &Sym{Name: "delta"}}
 )
 
 // startBlock sets the current block we're generating code in to b.
@@ -3104,15 +3105,16 @@ func (s *state) slice(t *Type, v, i, j, k *ssa.Value) (p, l, c *ssa.Value) {
 	// Generate the following code assuming that indexes are in bounds.
 	// The conditional is to make sure that we don't generate a slice
 	// that points to the next object in memory.
-	// rlen = (Sub64 j i)
-	// rcap = (Sub64 k i)
-	// p = ptr
-	// if rcap != 0 {
-	//    p = (AddPtr ptr (Mul64 low (Const64 size)))
+	// rlen = j-i
+	// rcap = k-i
+	// delta = i*elemsize
+	// if rcap == 0 {
+	//    delta = 0
 	// }
-	// result = (SliceMake p size)
+	// rptr = p+delta
+	// result = (SliceMake rptr rlen rcap)
 	subOp := s.ssaOp(OSUB, Types[TINT])
-	neqOp := s.ssaOp(ONE, Types[TINT])
+	eqOp := s.ssaOp(OEQ, Types[TINT])
 	mulOp := s.ssaOp(OMUL, Types[TINT])
 	rlen := s.newValue2(subOp, Types[TINT], j, i)
 	var rcap *ssa.Value
@@ -3128,36 +3130,38 @@ func (s *state) slice(t *Type, v, i, j, k *ssa.Value) (p, l, c *ssa.Value) {
 		rcap = s.newValue2(subOp, Types[TINT], k, i)
 	}
 
-	s.vars[&ptrVar] = ptr
+	// delta = # of elements to offset pointer by.
+	s.vars[&deltaVar] = i
 
-	// Generate code to test the resulting slice length.
-	cmp := s.newValue2(neqOp, Types[TBOOL], rcap, s.constInt(Types[TINT], 0))
+	// Generate code to set delta=0 if the resulting capacity is zero.
+	if !((i.Op == ssa.OpConst64 && i.AuxInt == 0) ||
+		(i.Op == ssa.OpConst32 && int32(i.AuxInt) == 0)) {
+		cmp := s.newValue2(eqOp, Types[TBOOL], rcap, zero)
 
-	b := s.endBlock()
-	b.Kind = ssa.BlockIf
-	b.Likely = ssa.BranchLikely
-	b.SetControl(cmp)
+		b := s.endBlock()
+		b.Kind = ssa.BlockIf
+		b.Likely = ssa.BranchUnlikely
+		b.SetControl(cmp)
 
-	// Generate code for non-zero length slice case.
-	nz := s.f.NewBlock(ssa.BlockPlain)
-	b.AddEdgeTo(nz)
-	s.startBlock(nz)
-	var inc *ssa.Value
-	if elemtype.Width == 1 {
-		inc = i
-	} else {
-		inc = s.newValue2(mulOp, Types[TINT], i, s.constInt(Types[TINT], elemtype.Width))
+		// Generate block which zeros the delta variable.
+		nz := s.f.NewBlock(ssa.BlockPlain)
+		b.AddEdgeTo(nz)
+		s.startBlock(nz)
+		s.vars[&deltaVar] = zero
+		s.endBlock()
+
+		// All done.
+		merge := s.f.NewBlock(ssa.BlockPlain)
+		b.AddEdgeTo(merge)
+		nz.AddEdgeTo(merge)
+		s.startBlock(merge)
+
+		// TODO: use conditional moves somehow?
 	}
-	s.vars[&ptrVar] = s.newValue2(ssa.OpAddPtr, ptrtype, ptr, inc)
-	s.endBlock()
 
-	// All done.
-	merge := s.f.NewBlock(ssa.BlockPlain)
-	b.AddEdgeTo(merge)
-	nz.AddEdgeTo(merge)
-	s.startBlock(merge)
-	rptr := s.variable(&ptrVar, ptrtype)
-	delete(s.vars, &ptrVar)
+	// Compute rptr = ptr + delta * elemsize
+	rptr := s.newValue2(ssa.OpAddPtr, ptrtype, ptr, s.newValue2(mulOp, Types[TINT], s.variable(&deltaVar, Types[TINT]), s.constInt(Types[TINT], elemtype.Width)))
+	delete(s.vars, &deltaVar)
 	return rptr, rlen, rcap
 }