From: Nick Lewycky <nicholas@mxc.ca>
Date: Fri, 3 Jun 2011 06:27:15 +0000 (+0000)
Subject: Bail on unswitching a switch statement for a case with a critical edge. We name
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=444f297a782047fbe6255a3543bc3ccd025c0691;p=oota-llvm.git

Bail on unswitching a switch statement for a case with a critical edge. We name
which edge to split by pred/succ pair, which means that we can end up splitting
the wrong edge (by case value) in the switch statement entirely. Fixes PR10031!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132535 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Transforms/Scalar/LoopUnswitch.cpp b/lib/Transforms/Scalar/LoopUnswitch.cpp
index 01a99af1439..e05f29c3e13 100644
--- a/lib/Transforms/Scalar/LoopUnswitch.cpp
+++ b/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -258,6 +258,7 @@ bool LoopUnswitch::processCurrentLoop() {
       if (LoopCond && SI->getNumCases() > 1) {
         // Find a value to unswitch on:
         // FIXME: this should chose the most expensive case!
+        // FIXME: scan for a case with a non-critical edge?
         Constant *UnswitchVal = SI->getCaseValue(1);
         // Do not process same value again and again.
         if (!UnswitchedVals.insert(UnswitchVal))
@@ -560,6 +561,8 @@ void LoopUnswitch::SplitExitEdges(Loop *L,
     BasicBlock *ExitBlock = ExitBlocks[i];
     SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
                                        pred_end(ExitBlock));
+    // Although SplitBlockPredecessors doesn't preserve loop-simplify in
+    // general, if we call it on all predecessors of all exits then it does.
     SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
                            ".us-lcssa", this);
   }
@@ -915,10 +918,11 @@ void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
     // Found a dead case value.  Don't remove PHI nodes in the 
     // successor if they become single-entry, those PHI nodes may
     // be in the Users list.
-        
+
     BasicBlock *Switch = SI->getParent();
     BasicBlock *SISucc = SI->getSuccessor(DeadCase);
     BasicBlock *Latch = L->getLoopLatch();
+    if (!SI->findCaseDest(SISucc)) continue;  // Edge is critical.
     // If the DeadCase successor dominates the loop latch, then the
     // transformation isn't safe since it will delete the sole predecessor edge
     // to the latch.
diff --git a/test/Transforms/LoopUnswitch/2011-06-02-CritSwitch.ll b/test/Transforms/LoopUnswitch/2011-06-02-CritSwitch.ll
new file mode 100644
index 00000000000..61c54ddb156
--- /dev/null
+++ b/test/Transforms/LoopUnswitch/2011-06-02-CritSwitch.ll
@@ -0,0 +1,28 @@
+; RUN: opt -loop-unswitch -disable-output
+; PR10031
+
+define i32 @test(i32 %command) {
+entry:
+  br label %tailrecurse
+
+tailrecurse:                                      ; preds = %if.then14, %tailrecurse, %entry
+  br i1 undef, label %if.then, label %tailrecurse
+
+if.then:                                          ; preds = %tailrecurse
+  switch i32 %command, label %sw.bb [
+    i32 2, label %land.lhs.true
+    i32 0, label %land.lhs.true
+  ]
+
+land.lhs.true:                                    ; preds = %if.then, %if.then
+  br i1 undef, label %sw.bb, label %if.then14
+
+if.then14:                                        ; preds = %land.lhs.true
+  switch i32 %command, label %tailrecurse [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb
+  ]
+
+sw.bb:                                            ; preds = %if.then14
+  unreachable
+}