cmd/compile: omit some temp panicdivide calls

When the divisor is known to be a constant
non-zero, don't insert panicdivide calls
that will just be eliminated later.

The main benefit here is readability of the SSA
form for compiler developers.

Change-Id: Icb7d07fc996941fbaff84524ac3e4b53d8e75fda
Reviewed-on: https://go-review.googlesource.com/28530
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1857,19 +1857,12 @@ func (s *state) expr(n *Node) *ssa.Value {
 		}
 		if n.Type.IsFloat() {
 			return s.newValue2(s.ssaOp(n.Op, n.Type), a.Type, a, b)
-		} else {
-			// do a size-appropriate check for zero
-			cmp := s.newValue2(s.ssaOp(ONE, n.Type), Types[TBOOL], b, s.zeroVal(n.Type))
-			s.check(cmp, panicdivide)
-			return s.newValue2(s.ssaOp(n.Op, n.Type), a.Type, a, b)
 		}
+		return s.intDivide(n, a, b)
 	case OMOD:
 		a := s.expr(n.Left)
 		b := s.expr(n.Right)
-		// do a size-appropriate check for zero
-		cmp := s.newValue2(s.ssaOp(ONE, n.Type), Types[TBOOL], b, s.zeroVal(n.Type))
-		s.check(cmp, panicdivide)
-		return s.newValue2(s.ssaOp(n.Op, n.Type), a.Type, a, b)
+		return s.intDivide(n, a, b)
 	case OADD, OSUB:
 		a := s.expr(n.Left)
 		b := s.expr(n.Right)
@@ -3230,6 +3223,22 @@ func (s *state) check(cmp *ssa.Value, fn *Node) {
 	s.startBlock(bNext)
 }
 
+func (s *state) intDivide(n *Node, a, b *ssa.Value) *ssa.Value {
+	needcheck := true
+	switch b.Op {
+	case ssa.OpConst8, ssa.OpConst16, ssa.OpConst32, ssa.OpConst64:
+		if b.AuxInt != 0 {
+			needcheck = false
+		}
+	}
+	if needcheck {
+		// do a size-appropriate check for zero
+		cmp := s.newValue2(s.ssaOp(ONE, n.Type), Types[TBOOL], b, s.zeroVal(n.Type))
+		s.check(cmp, panicdivide)
+	}
+	return s.newValue2(s.ssaOp(n.Op, n.Type), a.Type, a, b)
+}
+
 // rtcall issues a call to the given runtime function fn with the listed args.
 // Returns a slice of results of the given result types.
 // The call is added to the end of the current block.