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ssa: terminate empty defer dispatch blocks #2213

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ef2722d
ssa: terminate empty defer dispatch blocks
zhouguangyuan0718 Jul 29, 2026
8784fb9
test: cover empty defer dispatch
zhouguangyuan0718 Jul 29, 2026
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4 changes: 4 additions & 0 deletions ssa/eh.go
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Original file line number Diff line number Diff line change
Expand Up @@ -591,6 +591,10 @@ func (p Function) endDefer(b Builder) {
b.Jump(rethNext)
}
}
// With no defer statements, the loop does not position the builder.
if n == 0 {

@fennoai fennoai Bot Jul 29, 2026

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The n == 0 branch encodes a subtle, load-bearing invariant with no comment: the n >= 1 loop above implicitly leaves the builder positioned on rethsNext[1] (which equals procBlk when n == 1, since the final i == 0 iteration skips its Jump), so the trailing SetThreadDefer / IndirectJump land in procBlk. When n == 0 the loop never runs and nothing sets the insert point, so this branch is required for those instructions to be emitted into procBlk.

Consider a one-line comment, e.g.:

// No defer stmts: the loop above never runs, so position the builder
// on procBlk directly before emitting the tail (SetThreadDefer/IndirectJump).
if n == 0 {
    b.SetBlockEx(procBlk, AtEnd, true)
}

This matches the inline-comment style already used nearby (e.g. eh.go:601) and prevents a future maintainer from mistaking the branch for dead/redundant code.

b.SetBlockEx(procBlk, AtEnd, true)
}
link := b.getField(b.Load(self.data), deferLink)
b.Call(b.Pkg.rtFunc("SetThreadDefer"), link)
b.IndirectJump(b.Load(rundPtr), nexts)
Expand Down
12 changes: 9 additions & 3 deletions ssa/eh_defer_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ import (

"github.com/goplus/llgo/ssa"
"github.com/goplus/llgo/ssa/ssatest"
"github.com/xgo-dev/llvm"
)

func TestExplicitDeferStackIR(t *testing.T) {
Expand Down Expand Up @@ -67,16 +68,18 @@ func TestExplicitDeferStackFallbackAndNilBuiltin(t *testing.T) {
}
}

func TestExplicitDeferStackDrainWithoutLoopCases(t *testing.T) {
func TestExplicitDeferStackWithoutDeferredActions(t *testing.T) {
prog := ssatest.NewProgram(t, nil)
pkg := prog.NewPackage("foo", "foo")

fn := pkg.NewFunc("main", ssa.NoArgsNoRet, ssa.InGo)
b := fn.MakeBody(1)
fn.SetRecover(fn.MakeBlock())
recoverBlock := fn.MakeBlock()
fn.SetRecover(recoverBlock)
b.SetBlock(recoverBlock).Return()
b.SetBlock(fn.Block(0))

_ = b.BuiltinCall("ssa:deferstack")
b.DeferStackDrain()
b.RunDefers()
b.Return()
b.EndBuild()
Expand All @@ -88,6 +91,9 @@ func TestExplicitDeferStackDrainWithoutLoopCases(t *testing.T) {
if !strings.Contains(ir, "sigsetjmp") && !strings.Contains(ir, "setjmp") {
t.Fatalf("expected defer stack setup with recover, got:\n%s", ir)
}
if err := llvm.VerifyModule(pkg.Module(), llvm.ReturnStatusAction); err != nil {
t.Fatalf("explicit defer stack without loop cases produced invalid IR: %v\n%s", err, ir)
}
}

func TestExplicitDeferStackDrainWithoutRecoverNoop(t *testing.T) {
Expand Down
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