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cmd/compile: tweak OIF construction for binarySearch

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When emitting base cases, previously we would emit:

    if c1 { s1 }
    if c2 { s2 }
    if c3 { s3 }

With this CL, we instead emit:

    if c1 { s1 }
    else if c2 { s2 }
    else if c3 { s3 }

Most of the time, this doesn't make a difference, because s1/s2/s3 are
typically "goto" statements. But for type switches, we currently emit:

    if hash == 271 { if _, ok := iface.(T1); ok { goto t1case } }
    if hash == 314 { if _, ok := iface.(T2); ok { goto t2case } }

That is, the if bodies can fallthrough, even though it's impossible
for them to match any of the subsequent cases.

Change-Id: I453d424d0b5e40060a703738bbb374523f1c403c
Reviewed-on: https://go-review.googlesource.com/c/go/+/195339
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Matthew Dempsky 2019-09-13 15:13:22 -07:00
parent 04fb929a5b
commit 1b2c794896
1 changed files with 16 additions and 18 deletions
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34
src/cmd/compile/internal/gc/swt.go
View File

@ -397,14 +397,10 @@ func (s *exprSwitch) flush() {
}
return le
},
func(i int, out *Nodes) {
func(i int, nif *Node) {
c := &cc[i]
nif := nodl(c.pos, OIF, c.test(s.exprname), nil)
nif.Left = typecheck(nif.Left, ctxExpr)
nif.Left = defaultlit(nif.Left, nil)
nif.Left = c.test(s.exprname)
nif.Nbody.Set1(c.jmp)
out.Append(nif)
},
)
}
@ -521,8 +517,8 @@ func walkTypeSwitch(sw *Node) {
singleType := ncase.List.Len() == 1 && ncase.List.First().Op == OTYPE
label := autolabel(".s")
jmp := npos(ncase.Pos, nodSym(OGOTO, nil, label))
if ncase.List.Len() == 0 { // default:
if defaultGoto != nil {
Fatalf("duplicate default case not detected during typechecking")
@ -672,16 +668,12 @@ func (s *typeSwitch) flush() {
func(i int) *Node {
return nod(OLE, s.hashname, nodintconst(int64(cc[i-1].hash)))
},
func(i int, out *Nodes) {
func(i int, nif *Node) {
// TODO(mdempsky): Omit hash equality check if
// there's only one type.
c := cc[i]
a := nod(OIF, nil, nil)
a.Left = nod(OEQ, s.hashname, nodintconst(int64(c.hash)))
a.Left = typecheck(a.Left, ctxExpr)
a.Left = defaultlit(a.Left, nil)
a.Nbody.AppendNodes(&c.body)
out.Append(a)
nif.Left = nod(OEQ, s.hashname, nodintconst(int64(c.hash)))
nif.Nbody.AppendNodes(&c.body)
},
)
}
@ -691,10 +683,11 @@ func (s *typeSwitch) flush() {
// switch statements.
//
// less(i) should return a boolean expression. If it evaluates true,
// then cases [0, i) will be tested; otherwise, cases [i, n).
// then cases before i will be tested; otherwise, cases i and later.
//
// base(i, out) should append statements to out to test the i'th case.
func binarySearch(n int, out *Nodes, less func(i int) *Node, base func(i int, out *Nodes)) {
// base(i, nif) should setup nif (an OIF node) to test case i. In
// particular, it should set nif.Left and nif.Nbody.
func binarySearch(n int, out *Nodes, less func(i int) *Node, base func(i int, nif *Node)) {
const binarySearchMin = 4 // minimum number of cases for binary search
var do func(lo, hi int, out *Nodes)
@ -702,7 +695,12 @@ func binarySearch(n int, out *Nodes, less func(i int) *Node, base func(i int, ou
n := hi - lo
if n < binarySearchMin {
for i := lo; i < hi; i++ {
base(i, out)
nif := nod(OIF, nil, nil)
base(i, nif)
nif.Left = typecheck(nif.Left, ctxExpr)
nif.Left = defaultlit(nif.Left, nil)
out.Append(nif)
out = &nif.Rlist
}
return
}