[llvm] 6227f02 - [SimpleLoopUnswitch] Update DefaultExit condition to check unreachable is not empty.

[Alina Sbirlea via llvm-commits]

Author: Alina Sbirlea
Date: 2020-05-07T13:48:30-07:00
New Revision: 6227f021ad400cc604ca2d15d3639eabfd66a2d9

URL: https://github.com/llvm/llvm-project/commit/6227f021ad400cc604ca2d15d3639eabfd66a2d9
DIFF: https://github.com/llvm/llvm-project/commit/6227f021ad400cc604ca2d15d3639eabfd66a2d9.diff

LOG: [SimpleLoopUnswitch] Update DefaultExit condition to check unreachable is not empty.

Summary:
Update the check for the default exit block to not only check that the
terminator is not unreachable, but also check that unreachable block has
*only* the unreachable instruction.

Reviewers: chandlerc

Subscribers: hiraditya, uabelho, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78277

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
    llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index 1085a2922d46..6d63277e85b7 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -598,6 +598,23 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
 
   auto *ParentBB = SI.getParent();
 
+  auto IsTriviallyUnswitchableExitBlock = [&](BasicBlock &BBToCheck) {
+    // BBToCheck is not an exit block if it is inside loop L.
+    if (L.contains(&BBToCheck))
+      return false;
+    // BBToCheck is not trivial to unswitch if its phis aren't loop invariant.
+    if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, BBToCheck))
+      return false;
+    // We do not unswitch a block that only has an unreachable statement, as
+    // it's possible this is a previously unswitched block. Only unswitch if
+    // either the terminator is not unreachable, or, if it is, it's not the only
+    // instruction in the block.
+    auto *TI = BBToCheck.getTerminator();
+    bool isUnreachable = isa<UnreachableInst>(TI);
+    return !isUnreachable ||
+           (isUnreachable && (BBToCheck.getFirstNonPHIOrDbg() != TI));
+  };
+
   SmallVector<int, 4> ExitCaseIndices;
   for (auto Case : SI.cases()) {
     auto *SuccBB = Case.getCaseSuccessor();
@@ -608,9 +625,7 @@ static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
   BasicBlock *DefaultExitBB = nullptr;
   SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
       SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0);
-  if (!L.contains(SI.getDefaultDest()) &&
-      areLoopExitPHIsLoopInvariant(L, *ParentBB, *SI.getDefaultDest()) &&
-      !isa<UnreachableInst>(SI.getDefaultDest()->getTerminator())) {
+  if (IsTriviallyUnswitchableExitBlock(*SI.getDefaultDest())) {
     DefaultExitBB = SI.getDefaultDest();
   } else if (ExitCaseIndices.empty())
     return false;

diff  --git a/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch.ll b/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch.ll
index 4f01fd517444..713743db6500 100644
--- a/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch.ll
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch.ll
@@ -1243,3 +1243,52 @@ loopexit:
 ; CHECK:       loopexit:
 ; CHECK-NEXT:    ret
 }
+
+declare void @f()
+declare void @g()
+define void @test_unswitch_switch_with_nonempty_unreachable() {
+; CHECK-LABEL: @test_unswitch_switch_with_nonempty_unreachable()
+entry:
+  br label %loop
+
+loop:
+  %cleanup.dest.slot.0 = select i1 undef, i32 5, i32 undef
+  br label %for.cond
+
+for.cond:
+  switch i32 %cleanup.dest.slot.0, label %NonEmptyUnreachableBlock [
+    i32 0, label %for.cond
+    i32 1, label %NonEmptyUnreachableBlock
+    i32 2, label %loop.loopexit
+  ]
+
+loop.loopexit:
+  unreachable
+
+NonEmptyUnreachableBlock:
+  call void @f()
+  call void @g()
+  unreachable
+
+; CHECK:loop:
+; CHECK-NEXT:  %cleanup.dest.slot.0 = select i1 undef, i32 5, i32 undef
+; CHECK-NEXT:  switch i32 %cleanup.dest.slot.0, label %NonEmptyUnreachableBlock [
+; CHECK-NEXT:    i32 1, label %NonEmptyUnreachableBlock
+; CHECK-NEXT:    i32 2, label %loop.loopexit
+; CHECK-NEXT:    i32 0, label %loop.split
+; CHECK-NEXT:  ]
+
+; CHECK:loop.split:
+; CHECK-NEXT:  br label %for.cond
+
+; CHECK:for.cond:
+; CHECK-NEXT:  br label %for.cond
+
+; CHECK:loop.loopexit:
+; CHECK-NEXT:  unreachable
+
+; CHECK:NonEmptyUnreachableBlock:
+; CHECK-NEXT:  call void @f()
+; CHECK-NEXT:  call void @g()
+; CHECK-NEXT:  unreachable
+}