[llvm] r357252 - Switch lowering: exploit unreachable fall-through when lowering case range cluster
Hans Wennborg via llvm-commits
llvm-commits at lists.llvm.org
Fri Mar 29 06:40:05 PDT 2019
Author: hans
Date: Fri Mar 29 06:40:05 2019
New Revision: 357252
URL: http://llvm.org/viewvc/llvm-project?rev=357252&view=rev
Log:
Switch lowering: exploit unreachable fall-through when lowering case range cluster
In the example below, we would previously emit two range checks, one for cases
1--3 and one for 4--6. This patch makes us exploit the fact that the
fall-through is unreachable and only one range check is necessary.
switch i32 %i, label %default [
i32 1, label %bb1
i32 2, label %bb1
i32 3, label %bb1
i32 4, label %bb2
i32 5, label %bb2
i32 6, label %bb2
]
default: unreachable
Modified:
llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll
llvm/trunk/test/CodeGen/X86/switch.ll
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=357252&r1=357251&r2=357252&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Fri Mar 29 06:40:05 2019
@@ -2283,6 +2283,17 @@ void SelectionDAGBuilder::visitSwitchCas
SDValue CondLHS = getValue(CB.CmpLHS);
SDLoc dl = CB.DL;
+ if (CB.CC == ISD::SETTRUE) {
+ // Branch or fall through to TrueBB.
+ addSuccessorWithProb(SwitchBB, CB.TrueBB, CB.TrueProb);
+ SwitchBB->normalizeSuccProbs();
+ if (CB.TrueBB != NextBlock(SwitchBB)) {
+ DAG.setRoot(DAG.getNode(ISD::BR, dl, MVT::Other, getControlRoot(),
+ DAG.getBasicBlock(CB.TrueBB)));
+ }
+ return;
+ }
+
// Build the setcc now.
if (!CB.CmpMHS) {
// Fold "(X == true)" to X and "(X == false)" to !X to
@@ -10303,10 +10314,13 @@ void SelectionDAGBuilder::lowerWorkItem(
MachineBasicBlock *CurMBB = W.MBB;
for (CaseClusterIt I = W.FirstCluster, E = W.LastCluster; I <= E; ++I) {
+ bool FallthroughUnreachable = false;
MachineBasicBlock *Fallthrough;
if (I == W.LastCluster) {
// For the last cluster, fall through to the default destination.
Fallthrough = DefaultMBB;
+ FallthroughUnreachable = isa<UnreachableInst>(
+ DefaultMBB->getBasicBlock()->getFirstNonPHIOrDbg());
} else {
Fallthrough = CurMF->CreateMachineBasicBlock(CurMBB->getBasicBlock());
CurMF->insert(BBI, Fallthrough);
@@ -10343,9 +10357,7 @@ void SelectionDAGBuilder::lowerWorkItem(
}
}
- if (Fallthrough == DefaultMBB &&
- isa<UnreachableInst>(
- DefaultMBB->getBasicBlock()->getFirstNonPHIOrDbg())) {
+ if (FallthroughUnreachable) {
// Skip the range check if the fallthrough block is unreachable.
JTH->OmitRangeCheck = true;
}
@@ -10368,6 +10380,8 @@ void SelectionDAGBuilder::lowerWorkItem(
break;
}
case CC_BitTests: {
+ // FIXME: If Fallthrough is unreachable, skip the range check.
+
// FIXME: Optimize away range check based on pivot comparisons.
BitTestBlock *BTB = &BitTestCases[I->BTCasesIndex];
@@ -10412,6 +10426,10 @@ void SelectionDAGBuilder::lowerWorkItem(
RHS = I->High;
}
+ // If Fallthrough is unreachable, fold away the comparison.
+ if (FallthroughUnreachable)
+ CC = ISD::SETTRUE;
+
// The false probability is the sum of all unhandled cases.
CaseBlock CB(CC, LHS, RHS, MHS, I->MBB, Fallthrough, CurMBB,
getCurSDLoc(), I->Prob, UnhandledProbs);
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h?rev=357252&r1=357251&r2=357252&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h Fri Mar 29 06:40:05 2019
@@ -226,6 +226,8 @@ private:
/// multi-case switch statements.
struct CaseBlock {
// The condition code to use for the case block's setcc node.
+ // Besides the integer condition codes, this can also be SETTRUE, in which
+ // case no comparison gets emitted.
ISD::CondCode CC;
// The LHS/MHS/RHS of the comparison to emit.
Modified: llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll?rev=357252&r1=357251&r2=357252&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll (original)
+++ llvm/trunk/test/CodeGen/AArch64/switch-unreachable-default.ll Fri Mar 29 06:40:05 2019
@@ -74,17 +74,15 @@ entry:
i32 64, label %bb5
]
-; The switch is lowered with a jump table for cases 1--32 and case 64 checked
-; separately. Even though the default of switch is unreachable, the fall-through
-; for the jump table *is* reachable so the range check must be emitted.
+; The switch is lowered with a jump table for cases 1--32 and case 64 handled
+; separately. Even though the default of the switch is unreachable, the
+; fall-through for the jump table *is* reachable so the range check must be
+; emitted.
;
; CHECK-LABEL: reachable_fallthrough
-; CHECK: sub
-; CHECK: cmp
+; CHECK: sub [[REG:w[0-9]+]], w0, #1
+; CHECK: cmp [[REG]], #31
; CHECK: b.hi
-;
-; TODO: Drop the cmp for the 64 case since its fallthrough is unreachable.
-
def: unreachable
bb1: br label %return
@@ -94,6 +92,6 @@ bb4: br label %return
bb5: br label %return
return:
- %p = phi i32 [ 3, %bb1 ], [ 2, %bb2 ], [ 1, %bb3 ], [ 0, %bb4 ], [ 0, %bb5 ]
+ %p = phi i32 [ 3, %bb1 ], [ 2, %bb2 ], [ 1, %bb3 ], [ 0, %bb4 ], [ 42, %bb5 ]
ret i32 %p
}
Modified: llvm/trunk/test/CodeGen/X86/switch.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/switch.ll?rev=357252&r1=357251&r2=357252&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/switch.ll (original)
+++ llvm/trunk/test/CodeGen/X86/switch.ll Fri Mar 29 06:40:05 2019
@@ -778,3 +778,30 @@ end:
; CHECK: btl
; CHECK-NOT: btl
}
+
+
+define void @range_with_unreachable_fallthrough(i32 %i) {
+entry:
+ switch i32 %i, label %default [
+ i32 1, label %bb1
+ i32 2, label %bb1
+ i32 3, label %bb1
+ i32 4, label %bb2
+ i32 5, label %bb2
+ i32 6, label %bb2
+ ]
+bb1: tail call void @g(i32 0) br label %return
+bb2: tail call void @g(i32 1) br label %return
+default: unreachable
+
+return:
+ ret void
+
+; CHECK-LABEL: range_with_unreachable_fallthrough:
+; Since the default is unreachable, either the 1--3 or the 4--6 cluster will be
+; reached. Only one comparison should be emitted.
+; CHECK: cmpl
+; CHECK-NOT: cmpl
+; CHECK: jmp g
+; CHECK: jmp g
+}
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