[PATCH] D30869: [JumpThread] In case all predecessor go to a single successor of current BB. We want to fold (not thread).

[Xin Tong via Phabricator via llvm-commits]

trentxintong created this revision.

In case all predecessor go to a single successor of current BB. We want to fold (not thread).


https://reviews.llvm.org/D30869

Files:
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/basic.ll


Index: test/Transforms/JumpThreading/basic.ll
===================================================================
--- test/Transforms/JumpThreading/basic.ll
+++ test/Transforms/JumpThreading/basic.ll
@@ -4,6 +4,46 @@
 declare i32 @f2()
 declare void @f3()
 
+; Make sure we can fold this branch ... We will not be able to thread it as
+; L0 is too big to duplicate.
+;
+; CHECK-LABEL: @test_br_folding_not_threading(
+; CHECK: L1:
+; CHECK: call i32 @f2()
+; CHECK: call void @f3()
+; CHECK-NEXT: ret void
+; CHECK-NOT: br
+; CHECK: L3:
+define void @test_br_folding_not_threading(i1 %cond, i1 %value) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L3 
+L0:
+  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 %cond, label %L1, label %L2 
+
+L1:
+  call void @f3()
+  ret void
+L2:
+  call void @f3()
+  ret void
+L3:
+  call void @f3()
+  ret void
+}
+
 define i32 @test1(i1 %cond) {
 ; CHECK-LABEL: @test1(
 
Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -1265,6 +1265,33 @@
   if (PredToDestList.empty())
     return false;
 
+  // If all the predecessors go to a single known successor, we want to fold,
+  // not thread. By doing so, we do not need to duplicate the current block and
+  // also miss potential opportunities in case we dont/cant duplicate.
+  if (OnlyDest && OnlyDest != MultipleDestSentinel) {
+    if (PredToDestList.size() ==
+        (unsigned)std::distance(pred_begin(BB), pred_end(BB))) {
+      for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
+        if (SuccBB == OnlyDest)
+          continue;
+        // This is unreachable successor.
+        SuccBB->removePredecessor(BB, true);
+      }
+
+      // Finally update the terminator.
+      TerminatorInst *Term = BB->getTerminator();
+      BranchInst::Create(OnlyDest, Term);
+      Term->eraseFromParent();
+
+      // If the condition is now dead due to the removal of the old terminator,
+      // erase it.
+      Instruction *CondInst = dyn_cast<Instruction>(Cond);
+      if (CondInst && CondInst->use_empty())
+        CondInst->eraseFromParent();
+      return true;
+    }
+  }
+
   // Determine which is the most common successor.  If we have many inputs and
   // this block is a switch, we want to start by threading the batch that goes
   // to the most popular destination first.  If we only know about one


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