[dev.ssa] cmd/compile/internal/ssa: simplify how exit blocks are used
Move to implicit (mostly) instead of explicit exit blocks.
RET and RETJMP have no outgoing edges - they implicitly exit.
CALL only has one outgoing edge, as its exception edge is
implicit as well.
Exit blocks are only used for unconditionally panicking code,
like the failed branches of nil and bounds checks.
There may now be more than one exit block. No merges happen
at exit blocks.
The only downside is it is harder to find all the places code
can exit the method. See the reverse dominator code for an
example.
Change-Id: I42e2fd809a4bf81301ab993e29ad9f203ce48eb0
Reviewed-on: https://go-review.googlesource.com/14462
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index b29c328..35d9e6a 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -74,9 +74,6 @@
// Allocate starting block
s.f.Entry = s.f.NewBlock(ssa.BlockPlain)
- // Allocate exit block
- s.exit = s.f.NewBlock(ssa.BlockExit)
-
// Allocate starting values
s.vars = map[*Node]*ssa.Value{}
s.labels = map[string]*ssaLabel{}
@@ -121,14 +118,8 @@
b := s.endBlock()
b.Kind = ssa.BlockRet
b.Control = m
- b.AddEdgeTo(s.exit)
}
- // Finish up exit block
- s.startBlock(s.exit)
- s.exit.Control = s.mem()
- s.endBlock()
-
// Check that we used all labels
for name, lab := range s.labels {
if !lab.used() && !lab.reported {
@@ -181,9 +172,6 @@
// function we're building
f *ssa.Func
- // exit block that "return" jumps to (and panics jump to)
- exit *ssa.Block
-
// labels and labeled control flow nodes (OFOR, OSWITCH, OSELECT) in f
labels map[string]*ssaLabel
labeledNodes map[*Node]*ssaLabel
@@ -582,7 +570,6 @@
b := s.endBlock()
b.Kind = ssa.BlockRet
b.Control = m
- b.AddEdgeTo(s.exit)
case ORETJMP:
s.stmtList(n.List)
m := s.mem()
@@ -590,7 +577,6 @@
b.Kind = ssa.BlockRetJmp
b.Aux = n.Left.Sym
b.Control = m
- b.AddEdgeTo(s.exit)
case OCONTINUE, OBREAK:
var op string
@@ -776,7 +762,6 @@
b.Kind = ssa.BlockCall
b.Control = r
b.AddEdgeTo(bNext)
- b.AddEdgeTo(s.exit)
s.startBlock(bNext)
default:
@@ -1859,7 +1844,6 @@
b.Kind = ssa.BlockCall
b.Control = call
b.AddEdgeTo(bNext)
- b.AddEdgeTo(s.exit)
// read result from stack at the start of the fallthrough block
s.startBlock(bNext)
@@ -2154,11 +2138,12 @@
bPanic := s.f.NewBlock(ssa.BlockPlain)
b.AddEdgeTo(bNext)
b.AddEdgeTo(bPanic)
- bPanic.AddEdgeTo(s.exit)
s.startBlock(bPanic)
// TODO: implicit nil checks somehow?
- s.vars[&memvar] = s.newValue2(ssa.OpPanicNilCheck, ssa.TypeMem, ptr, s.mem())
+ chk := s.newValue2(ssa.OpPanicNilCheck, ssa.TypeMem, ptr, s.mem())
s.endBlock()
+ bPanic.Kind = ssa.BlockExit
+ bPanic.Control = chk
s.startBlock(bNext)
}
@@ -2200,12 +2185,13 @@
bPanic := s.f.NewBlock(ssa.BlockPlain)
b.AddEdgeTo(bNext)
b.AddEdgeTo(bPanic)
- bPanic.AddEdgeTo(s.exit)
s.startBlock(bPanic)
// The panic check takes/returns memory to ensure that the right
// memory state is observed if the panic happens.
- s.vars[&memvar] = s.newValue1(panicOp, ssa.TypeMem, s.mem())
+ chk := s.newValue1(panicOp, ssa.TypeMem, s.mem())
s.endBlock()
+ bPanic.Kind = ssa.BlockExit
+ bPanic.Control = chk
s.startBlock(bNext)
}
@@ -3492,18 +3478,8 @@
func (s *genState) genBlock(b, next *ssa.Block) {
lineno = b.Line
- // after a panic call, don't emit any branch code
- if len(b.Values) > 0 {
- switch b.Values[len(b.Values)-1].Op {
- case ssa.OpAMD64LoweredPanicNilCheck,
- ssa.OpAMD64LoweredPanicIndexCheck,
- ssa.OpAMD64LoweredPanicSliceCheck:
- return
- }
- }
-
switch b.Kind {
- case ssa.BlockPlain:
+ case ssa.BlockPlain, ssa.BlockCall:
if b.Succs[0] != next {
p := Prog(obj.AJMP)
p.To.Type = obj.TYPE_BRANCH
@@ -3520,12 +3496,6 @@
p.To.Type = obj.TYPE_MEM
p.To.Name = obj.NAME_EXTERN
p.To.Sym = Linksym(b.Aux.(*Sym))
- case ssa.BlockCall:
- if b.Succs[0] != next {
- p := Prog(obj.AJMP)
- p.To.Type = obj.TYPE_BRANCH
- s.branches = append(s.branches, branch{p, b.Succs[0]})
- }
case ssa.BlockAMD64EQF:
genFPJump(s, b, next, &eqfJumps)
