[llgo] Elide alloca for unused received values in select
Summary: If a receive case in a select statement is not assigned to a named variable, then we can eliminate the alloca and copy at runtime.
Test Plan: lit test added
Reviewers: pcc
Reviewed By: pcc
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D6785
llvm-svn: 225033
GitOrigin-RevId: 75f34af99c413d4573826ca355b53f67eb5626d7
diff --git a/irgen/channels.go b/irgen/channels.go
index dc8f2c0..f0f4aab 100644
--- a/irgen/channels.go
+++ b/irgen/channels.go
@@ -14,6 +14,7 @@
package irgen
import (
+ "llvm.org/llgo/third_party/go.tools/go/ssa"
"llvm.org/llgo/third_party/go.tools/go/types"
"llvm.org/llvm/bindings/go/llvm"
)
@@ -60,16 +61,9 @@
fr.runtime.builtinClose.call(fr, ch.value)
}
-// selectState is equivalent to ssa.SelectState
-type selectState struct {
- Dir types.ChanDir
- Chan *govalue
- Send *govalue
-}
-
-func (fr *frame) chanSelect(states []selectState, blocking bool) (index, recvOk *govalue, recvElems []*govalue) {
- n := uint64(len(states))
- if !blocking {
+func (fr *frame) chanSelect(sel *ssa.Select) (index, recvOk *govalue, recvElems []*govalue) {
+ n := uint64(len(sel.States))
+ if !sel.Blocking {
// non-blocking means there's a default case
n++
}
@@ -77,17 +71,21 @@
selectp := fr.runtime.newSelect.call(fr, size)[0]
// Allocate stack for the values to send and receive.
- //
- // TODO(axw) check if received elements have any users, and
- // elide stack allocation if not (pass nil to recv2 instead.)
- ptrs := make([]llvm.Value, len(states))
- for i, state := range states {
+ ptrs := make([]llvm.Value, len(sel.States))
+ for i, state := range sel.States {
chantyp := state.Chan.Type().Underlying().(*types.Chan)
elemtyp := fr.types.ToLLVM(chantyp.Elem())
- ptrs[i] = fr.allocaBuilder.CreateAlloca(elemtyp, "")
if state.Dir == types.SendOnly {
- fr.builder.CreateStore(state.Send.value, ptrs[i])
+ ptrs[i] = fr.allocaBuilder.CreateAlloca(elemtyp, "")
+ fr.builder.CreateStore(fr.llvmvalue(state.Send), ptrs[i])
} else {
+ // Only allocate stack space if the received value is used.
+ used := chanSelectStateUsed(sel, len(recvElems))
+ if used {
+ ptrs[i] = fr.allocaBuilder.CreateAlloca(elemtyp, "")
+ } else {
+ ptrs[i] = llvm.ConstNull(llvm.PointerType(llvm.Int8Type(), 0))
+ }
recvElems = append(recvElems, newValue(ptrs[i], chantyp.Elem()))
}
}
@@ -97,12 +95,12 @@
if len(recvElems) > 0 {
receivedp = fr.allocaBuilder.CreateAlloca(fr.types.ToLLVM(types.Typ[types.Bool]), "")
}
- if !blocking {
+ if !sel.Blocking {
// If the default case is chosen, the index must be -1.
fr.runtime.selectdefault.call(fr, selectp, llvm.ConstAllOnes(llvm.Int32Type()))
}
- for i, state := range states {
- ch := state.Chan.value
+ for i, state := range sel.States {
+ ch := fr.llvmvalue(state.Chan)
index := llvm.ConstInt(llvm.Int32Type(), uint64(i), false)
if state.Dir == types.SendOnly {
fr.runtime.selectsend.call(fr, selectp, ch, ptrs[i], index)
@@ -121,3 +119,16 @@
}
return index, recvOk, recvElems
}
+
+func chanSelectStateUsed(sel *ssa.Select, recvIndex int) bool {
+ for _, instr := range *sel.Referrers() {
+ extract, ok := instr.(*ssa.Extract)
+ if !ok || extract.Index != (recvIndex+2) {
+ continue
+ }
+ if len(*extract.Referrers()) > 0 {
+ return true
+ }
+ }
+ return false
+}
diff --git a/irgen/ssa.go b/irgen/ssa.go
index 1b09b71..1991ddb 100644
--- a/irgen/ssa.go
+++ b/irgen/ssa.go
@@ -1152,15 +1152,7 @@
fr.runDefers()
case *ssa.Select:
- states := make([]selectState, len(instr.States))
- for i, state := range instr.States {
- states[i] = selectState{
- Dir: state.Dir,
- Chan: fr.value(state.Chan),
- Send: fr.value(state.Send),
- }
- }
- index, recvOk, recvElems := fr.chanSelect(states, instr.Blocking)
+ index, recvOk, recvElems := fr.chanSelect(instr)
tuple := append([]*govalue{index, recvOk}, recvElems...)
fr.tuples[instr] = tuple
diff --git a/test/irgen/select.go b/test/irgen/select.go
new file mode 100644
index 0000000..748277a
--- /dev/null
+++ b/test/irgen/select.go
@@ -0,0 +1,18 @@
+// RUN: llgo -S -emit-llvm -o - %s | FileCheck %s
+
+package foo
+
+// CHECK-NOT: alloca [1024 x i8]
+// CHECK-NOT: alloca [2048 x i8]
+// CHECK: alloca [4096 x i8]
+func F() {
+ ch1 := make(chan [1024]byte)
+ ch2 := make(chan [2048]byte)
+ ch3 := make(chan [4096]byte)
+ select {
+ case <-ch1:
+ case _ = <-ch2:
+ case x := <-ch3:
+ _ = x[0]
+ }
+}
