[PATCH] D68032: Handle successor's PHI node correctly when flattening CFG merges two if-regions

[Jaebaek Seo via Phabricator via llvm-commits]

jaebaek created this revision.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

FlattenCFG merges two 'if' basicblocks by inserting one basicblock
to another basicblock. The inserted basicblock can have a successor
that contains a PHI node whoes incoming basicblock is the inserted
basicblock. Since the existing code does not handle it, it becomes
a badref.

if (cond1)

  statement

if (cond2)

  statement

successor - contains PHI node whose predecessor is cond2

-->
if (cond1 || cond2)

  statement

(BB for cond2 was deleted)
successor - contains PHI node whose predecessor is cond2 --> bad ref!


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D68032

Files:
  llvm/lib/Transforms/Utils/FlattenCFG.cpp
  llvm/test/Transforms/Util/flattencfg.ll


Index: llvm/test/Transforms/Util/flattencfg.ll
===================================================================
--- llvm/test/Transforms/Util/flattencfg.ll
+++ llvm/test/Transforms/Util/flattencfg.ll
@@ -54,3 +54,33 @@
   br i1 %1, label %bb4, label %bb3
 }
 
+; CHECK-LABEL: @test_not_crash3
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    %a_eq_0 = icmp eq i32 %a, 0
+; CHECK-NEXT:    %a_eq_1 = icmp eq i32 %a, 1
+; CHECK-NEXT:    [[COND:%[a-z0-9]+]] = or i1 %a_eq_0, %a_eq_1
+; CHECK-NEXT:    br i1 [[COND]], label %bb2, label %bb3
+; CHECK:       bb2:
+; CHECK-NEXT:    br label %bb3
+; CHECK:       bb3:
+; CHECK-NEXT:    %check_badref = phi i32 [ 17, %entry ], [ 11, %bb2 ]
+; CHECK-NEXT:    ret void
+define void @test_not_crash3(i32 %a) #0 {
+entry:
+  %a_eq_0 = icmp eq i32 %a, 0
+  br i1 %a_eq_0, label %bb0, label %bb1
+
+bb0:                                              ; preds = %entry
+  br label %bb1
+
+bb1:                                              ; preds = %bb0, %entry
+  %a_eq_1 = icmp eq i32 %a, 1
+  br i1 %a_eq_1, label %bb2, label %bb3
+
+bb2:                                              ; preds = %bb1
+  br label %bb3
+
+bb3:                                              ; preds = %bb2, %bb1
+  %check_badref = phi i32 [ 17, %bb1 ], [ 11, %bb2 ]
+  ret void
+}
Index: llvm/lib/Transforms/Utils/FlattenCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/FlattenCFG.cpp
+++ llvm/lib/Transforms/Utils/FlattenCFG.cpp
@@ -67,7 +67,7 @@
 /// Before:
 ///   ......
 ///   %cmp10 = fcmp une float %tmp1, %tmp2
-///   br i1 %cmp1, label %if.then, label %lor.rhs
+///   br i1 %cmp10, label %if.then, label %lor.rhs
 ///
 /// lor.rhs:
 ///   ......
@@ -453,6 +453,18 @@
   PBI->replaceUsesOfWith(CC, NC);
   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
 
+  // Handle PHI node to replace its predecessors to FirstEntryBlock.
+  for (unsigned int i = 0; i < PBI->getNumSuccessors(); ++i) {
+    BasicBlock *Successor = PBI->getSuccessor(i);
+    PHINode *SomePHI = dyn_cast<PHINode>(Successor->begin());
+    if (SomePHI) {
+      for (unsigned int j = 0; j < SomePHI->getNumIncomingValues(); ++j) {
+        if (SomePHI->getIncomingBlock(j) == SecondEntryBlock)
+          SomePHI->setIncomingBlock(j, FirstEntryBlock);
+      }
+    }
+  }
+
   // Remove IfTrue1
   if (IfTrue1 != FirstEntryBlock) {
     IfTrue1->dropAllReferences();


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