Commit 3c26c0db authored by Keith Randall's avatar Keith Randall

[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: 's avatarDavid Chase <drchase@google.com>
parent 7730880f
......@@ -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
}
}
......
......@@ -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
......
......@@ -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
......
......@@ -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() {
......
// 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]
}
......@@ -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"
......
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