cmd/compile: optimize unsafe.Slice generated code

We don't need a multiply when the element type is size 0 or 1.

The panic functions don't return, so we don't need any post-call
code (register restores, etc.).

Change-Id: I0dcea5df56d29d7be26554ddca966b3903c672e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/419754
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -165,6 +165,8 @@
 
 // Convert x * 1 to x.
 (Mul(8|16|32|64)  (Const(8|16|32|64)  [1]) x) => x
+(Select0 (Mul(32|64)uover (Const(32|64) [1]) x)) => x
+(Select1 (Mul(32|64)uover (Const(32|64) [1]) x)) => (ConstBool [false])
 
 // Convert x * -1 to -x.
 (Mul(8|16|32|64)  (Const(8|16|32|64)  [-1]) x) => (Neg(8|16|32|64)  x)
@@ -531,6 +533,8 @@
 (Add(64|32|16|8) (Const(64|32|16|8) [0]) x) => x
 (Sub(64|32|16|8) x x) => (Const(64|32|16|8) [0])
 (Mul(64|32|16|8) (Const(64|32|16|8) [0]) _) => (Const(64|32|16|8) [0])
+(Select0 (Mul(64|32)uover (Const(64|32) [0]) x)) => (Const(64|32) [0])
+(Select1 (Mul(64|32)uover (Const(64|32) [0]) x)) => (ConstBool [false])
 
 (Com(64|32|16|8) (Com(64|32|16|8)  x)) => x
 (Com(64|32|16|8) (Const(64|32|16|8) [c])) => (Const(64|32|16|8) [^c])

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

@@ -1421,7 +1421,7 @@ func (s *state) stmt(n ir.Node) {
 		s.callResult(n, callNormal)
 		if n.Op() == ir.OCALLFUNC && n.X.Op() == ir.ONAME && n.X.(*ir.Name).Class == ir.PFUNC {
 			if fn := n.X.Sym().Name; base.Flag.CompilingRuntime && fn == "throw" ||
-				n.X.Sym().Pkg == ir.Pkgs.Runtime && (fn == "throwinit" || fn == "gopanic" || fn == "panicwrap" || fn == "block" || fn == "panicmakeslicelen" || fn == "panicmakeslicecap") {
+				n.X.Sym().Pkg == ir.Pkgs.Runtime && (fn == "throwinit" || fn == "gopanic" || fn == "panicwrap" || fn == "block" || fn == "panicmakeslicelen" || fn == "panicmakeslicecap" || fn == "panicunsafeslicelen" || fn == "panicunsafeslicenilptr") {
 				m := s.mem()
 				b := s.endBlock()
 				b.Kind = ssa.BlockExit

test/codegen/slices.go

@@ -6,6 +6,8 @@
 
 package codegen
 
+import "unsafe"
+
 // This file contains code generation tests related to the handling of
 // slice types.
 
@@ -368,3 +370,16 @@ func SliceWithSubtractBound(a []int, b int) []int {
 	// ppc64:"SUBC",-"NEG"
 	return a[(3 - b):]
 }
+
+// --------------------------------------- //
+//   Code generation for unsafe.Slice      //
+// --------------------------------------- //
+
+func Slice1(p *byte, i int) []byte {
+	// amd64:-"MULQ"
+	return unsafe.Slice(p, i)
+}
+func Slice0(p *struct{}, i int) []struct{} {
+	// amd64:-"MULQ"
+	return unsafe.Slice(p, i)
+}