[PATCH] D23722: [InstCombine] Allow sinking from unique predecessor with multiple edges

[Jun Bum Lim via llvm-commits]

junbuml created this revision.
junbuml added reviewers: mcrosier, majnemer, spatel.
junbuml added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

We can allow sinking if the single user block has only one unique predecessor, regardless of the number of edges. Note that a switch statement with multiple cases can have the same destination.

https://reviews.llvm.org/D23722

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/sink_instruction.ll

Index: test/Transforms/InstCombine/sink_instruction.ll
===================================================================
--- test/Transforms/InstCombine/sink_instruction.ll
+++ test/Transforms/InstCombine/sink_instruction.ll
@@ -54,3 +54,26 @@
 }
 
 declare i32 @bar()
+
+define i32 @test3(i32* nocapture readonly %P, i32 %i) {
+entry:
+  %idxprom = sext i32 %i to i64
+  %arrayidx = getelementptr inbounds i32, i32* %P, i64 %idxprom
+  %0 = load i32, i32* %arrayidx, align 4
+  switch i32 %i, label %sw.epilog [
+    i32 5, label %sw.bb
+    i32 2, label %sw.bb
+  ]
+
+sw.bb:                                            ; preds = %entry, %entry
+; CHECK-LABEL: sw.bb:
+; CHECK: %idxprom = sext i32 %i to i64
+; CHECK: %arrayidx = getelementptr inbounds i32, i32* %P, i64 %idxprom
+; CHECK: %0 = load i32, i32* %arrayidx, align 4
+  %add = add nsw i32 %0, %i
+  br label %sw.epilog
+
+sw.epilog:                                        ; preds = %entry, %sw.bb
+  %sum.0 = phi i32 [ %add, %sw.bb ], [ 0, %entry ]
+  ret i32 %sum.0
+}
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2903,7 +2903,7 @@
         // If the user is one of our immediate successors, and if that successor
         // only has us as a predecessors (we'd have to split the critical edge
         // otherwise), we can keep going.
-        if (UserIsSuccessor && UserParent->getSinglePredecessor()) {
+        if (UserIsSuccessor && UserParent->getUniquePredecessor()) {
           // Okay, the CFG is simple enough, try to sink this instruction.
           if (TryToSinkInstruction(I, UserParent)) {
             DEBUG(dbgs() << "IC: Sink: " << *I << '\n');


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