[PATCH] D48182: [JumpThreading] Make sure UNDEF does not make us think a block can have multiple possible destinations

[Xin Tong via Phabricator via llvm-commits]

trentxintong created this revision.
trentxintong added a reviewer: davide.

Make sure UNDEF does not make us think a block can have multiple possible destinations. This can prevent
folding.


Repository:
  rL LLVM

https://reviews.llvm.org/D48182

Files:
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/fold-not-thread.ll


Index: test/Transforms/JumpThreading/fold-not-thread.ll
===================================================================
--- test/Transforms/JumpThreading/fold-not-thread.ll
+++ test/Transforms/JumpThreading/fold-not-thread.ll
@@ -244,3 +244,50 @@
   call void @f3()
   ret void
 }
+
+; Make sure we can fold this branch ... We will not be able to thread it as
+; L0 is too big to duplicate. LX is the unreachable block here, and it feeds
+; UNDEF to L2. We want to make sure the UNDEF does not make jump threading think
+; L2 has multiple destinations. 
+; CHECK-LABEL: @test_br_folding_not_threading_multiple_preds_with_undef(
+; CHECK: L3:
+; CHECK: call i32 @f2()
+; CHECK: call void @f3()
+; CHECK-NEXT: ret void
+define void @test_br_folding_not_threading_multiple_preds_with_undef(i1 %condx, i1 %cond) nounwind {
+L0:
+  br i1 %condx, label %L2, label %L1
+
+L1:
+  br i1 %cond, label %L2, label %L5
+
+LX:
+  br label %L2
+
+L2:
+  %p = phi i1 [ %condx, %L0 ], [ %cond, %L1 ], [ undef, %LX]
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  call i32 @f2()
+  br i1 %p, label %L3, label %L4
+
+L3:
+  call void @f3()
+  ret void
+L4:
+  call void @f3()
+  ret void
+L5:
+  call void @f3()
+  ret void
+}
Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -1620,16 +1620,19 @@
     }
 
     // If we have exactly one destination, remember it for efficiency below.
-    if (PredToDestList.empty()) {
-      OnlyDest = DestBB;
-      OnlyVal = Val;
-    } else {
-      if (OnlyDest != DestBB)
-        OnlyDest = MultipleDestSentinel;
-      // It possible we have same destination, but different value, e.g. default
-      // case in switchinst.
-      if (Val != OnlyVal)
-        OnlyVal = MultipleVal;
+    // and we can ignore null destination.
+    if (DestBB) {
+      if (PredToDestList.empty()) {
+        OnlyDest = DestBB;
+        OnlyVal = Val;
+      } else {
+        if (OnlyDest != DestBB)
+          OnlyDest = MultipleDestSentinel;
+        // It possible we have same destination, but different value, e.g.
+        // default case in switchinst.
+        if (Val != OnlyVal)
+          OnlyVal = MultipleVal;
+      }
     }
 
     // We know where this predecessor is going.


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