[dev.ssa] cmd/compile: short-circuit empty blocks

Empty blocks are introduced to remove critical edges.
After regalloc, we can remove any of the added blocks
that are still empty.

Change-Id: I0b40e95ac3a6cc1e632a479443479532b6c5ccd9
Reviewed-on: https://go-review.googlesource.com/18833
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -4183,23 +4183,36 @@ func (s *genState) genValue(v *ssa.Value) {
 	case ssa.OpAMD64LoweredNilCheck:
 		// Optimization - if the subsequent block has a load or store
 		// at the same address, we don't need to issue this instruction.
+		mem := v.Args[1]
 		for _, w := range v.Block.Succs[0].Values {
+			if w.Op == ssa.OpPhi {
+				if w.Type.IsMemory() {
+					mem = w
+				}
+				continue
+			}
 			if len(w.Args) == 0 || !w.Args[len(w.Args)-1].Type.IsMemory() {
 				// w doesn't use a store - can't be a memory op.
 				continue
 			}
-			if w.Args[len(w.Args)-1] != v.Args[1] {
+			if w.Args[len(w.Args)-1] != mem {
 				v.Fatalf("wrong store after nilcheck v=%s w=%s", v, w)
 			}
 			switch w.Op {
 			case ssa.OpAMD64MOVQload, ssa.OpAMD64MOVLload, ssa.OpAMD64MOVWload, ssa.OpAMD64MOVBload,
 				ssa.OpAMD64MOVQstore, ssa.OpAMD64MOVLstore, ssa.OpAMD64MOVWstore, ssa.OpAMD64MOVBstore:
 				if w.Args[0] == v.Args[0] && w.Aux == nil && w.AuxInt >= 0 && w.AuxInt < minZeroPage {
+					if Debug_checknil != 0 && int(v.Line) > 1 {
+						Warnl(int(v.Line), "removed nil check")
+					}
 					return
 				}
 			case ssa.OpAMD64MOVQstoreconst, ssa.OpAMD64MOVLstoreconst, ssa.OpAMD64MOVWstoreconst, ssa.OpAMD64MOVBstoreconst:
 				off := ssa.StoreConst(v.AuxInt).Off()
 				if w.Args[0] == v.Args[0] && w.Aux == nil && off >= 0 && off < minZeroPage {
+					if Debug_checknil != 0 && int(v.Line) > 1 {
+						Warnl(int(v.Line), "removed nil check")
+					}
 					return
 				}
 			}

src/cmd/compile/internal/ssa/TODO

@@ -42,7 +42,6 @@ Optimizations (better compiled code)
   (all instructions, really)
 - combine LEAQs
 - store followed by load to same address
-- short circuit blocks which are just a jump (undo critical edge processing when no instructions are put in it by regalloc)
 - (CMPconst [0] (AND x y)) -> (TEST x y)
 - more (LOAD (ADDQ )) -> LOADIDX
 - CMPL/SETEQ/TESTB/JEQ -> CMPL/JEQ

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

@@ -18,6 +18,7 @@ func checkFunc(f *Func) {
 			f.Fatalf("%s.Func=%s, want %s", b, b.Func.Name, f.Name)
 		}
 
+		if f.RegAlloc == nil {
 			for i, c := range b.Succs {
 				for j, d := range b.Succs {
 					if i != j && c == d {
@@ -25,6 +26,7 @@ func checkFunc(f *Func) {
 					}
 				}
 			}
+		}
 		// Note: duplicate successors are hard in the following case:
 		//      if(...) goto x else goto x
 		//   x: v = phi(a, b)
@@ -34,6 +36,7 @@ func checkFunc(f *Func) {
 		// all successors are distinct.  They will need to be distinct
 		// anyway for register allocation (duplicate successors implies
 		// the existence of critical edges).
+		// After regalloc we can allow non-distinct predecessors.
 
 		for _, p := range b.Preds {
 			var found bool

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

@@ -105,7 +105,8 @@ var passes = [...]pass{
 	{"layout", layout},       // schedule blocks
 	{"schedule", schedule},   // schedule values
 	{"flagalloc", flagalloc}, // allocate flags register
-	{"regalloc", regalloc},
+	{"regalloc", regalloc},   // allocate int & float registers
+	{"trim", trim},           // remove empty blocks
 }
 
 // Double-check phase ordering constraints.
@@ -148,6 +149,8 @@ var passOrder = [...]constraint{
 	{"schedule", "flagalloc"},
 	// regalloc needs flags to be allocated first.
 	{"flagalloc", "regalloc"},
+	// trim needs regalloc to be done first.
+	{"regalloc", "trim"},
 }
 
 func init() {

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

@@ -0,0 +1,37 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssa
+
+// trim removes blocks with no code in them.
+// These blocks were inserted to remove critical edges.
+func trim(f *Func) {
+	i := 0
+	for _, b := range f.Blocks {
+		if b.Kind != BlockPlain || len(b.Values) != 0 || len(b.Preds) != 1 {
+			f.Blocks[i] = b
+			i++
+			continue
+		}
+		// TODO: handle len(b.Preds)>1 case.
+
+		// Splice b out of the graph.
+		pred := b.Preds[0]
+		succ := b.Succs[0]
+		for j, s := range pred.Succs {
+			if s == b {
+				pred.Succs[j] = succ
+			}
+		}
+		for j, p := range succ.Preds {
+			if p == b {
+				succ.Preds[j] = pred
+			}
+		}
+	}
+	for j := i; j < len(f.Blocks); j++ {
+		f.Blocks[j] = nil
+	}
+	f.Blocks = f.Blocks[:i]
+}

test/nilptr3_ssa.go

@@ -156,7 +156,7 @@ func f4(x *[10]int) {
 	// and the offset is small enough that if x is nil, the address will still be
 	// in the first unmapped page of memory.
 
-	_ = x[9] // ERROR "generated nil check" // bug would like to remove before indirect
+	_ = x[9] // ERROR "removed nil check"
 
 	for {
 		if x[9] != 0 { // ERROR "removed nil check"