[PATCH] D66892: [BasicBlockUtils] Add metadata fixing in SplitBlockPredecessors.

[Alexander Us via Phabricator via llvm-commits]

aus_intel created this revision.
aus_intel added reviewers: lattner, llvm-commits, Orlando.
aus_intel added a project: LLVM.
Herald added a subscriber: hiraditya.

In case when BB is header of some loop and predecessor is latch of
this loop, metadata was not attached to newly created basic block.
This led to loss of loop metadata for other passes.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D66892

Files:
  llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
  llvm/test/Transforms/LoopSimplify/loop_metadata.ll


Index: llvm/test/Transforms/LoopSimplify/loop_metadata.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopSimplify/loop_metadata.ll
@@ -0,0 +1,36 @@
+; RUN: opt -S -loop-simplify < %s | FileCheck %s
+
+; CHECK: for.cond.loopexit:
+; CHECK: br label %for.cond, !llvm.loop !0
+; CHECK: br i1 %cmp1, label %for.body1, label %for.cond.loopexit
+
+define void @foo() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond1, %entry
+  %j = phi i32 [ 0, %entry ], [ %add, %for.cond1 ]
+  %cmp = icmp ult i32 %j, 8
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %dummy1 = add i32 1, 1
+  %add = add nuw nsw i32 %j, 1
+  br label %for.cond1
+
+for.cond1:                                      ; preds = %for.body1, %for.body
+  %i.0 = phi i32 [ 1, %for.body ], [ %inc, %for.body1 ]
+  %cmp1 = icmp ult i32 %i.0, 8
+  br i1 %cmp1, label %for.body1, label %for.cond, !llvm.loop !0
+
+for.body1:                                      ; preds = %for.cond1
+  %dummy2 = add i32 1, 1
+  %inc = add nuw nsw i32 %i.0, 1
+  br label %for.cond1
+
+for.end:                                       ; preds = %for.cond
+  ret void
+}
+
+!0 = distinct !{!0, !1}
+!1 = !{!"llvm.loop.unroll.full"}
Index: llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
===================================================================
--- llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -579,24 +579,33 @@
 
   // The new block unconditionally branches to the old block.
   BranchInst *BI = BranchInst::Create(BB, NewBB);
+  bool IsBBHeader = LI && LI->isLoopHeader(BB);
+  Loop *BBLoop = LI ? LI->getLoopFor(BB) : nullptr;
   // Splitting the predecessors of a loop header creates a preheader block.
-  if (LI && LI->isLoopHeader(BB))
+  if (IsBBHeader)
     // Using the loop start line number prevents debuggers stepping into the
     // loop body for this instruction.
-    BI->setDebugLoc(LI->getLoopFor(BB)->getStartLoc());
+    BI->setDebugLoc(BBLoop->getStartLoc());
   else
     BI->setDebugLoc(BB->getFirstNonPHIOrDbg()->getDebugLoc());
 
   // Move the edges from Preds to point to NewBB instead of BB.
-  for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
+  for (BasicBlock *Pred : Preds) {
+    Instruction *PI = Pred->getTerminator();
     // This is slightly more strict than necessary; the minimum requirement
     // is that there be no more than one indirectbr branching to BB. And
     // all BlockAddress uses would need to be updated.
-    assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
+    assert(!isa<IndirectBrInst>(PI) &&
            "Cannot split an edge from an IndirectBrInst");
-    assert(!isa<CallBrInst>(Preds[i]->getTerminator()) &&
-           "Cannot split an edge from a CallBrInst");
-    Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
+    assert(!isa<CallBrInst>(PI) && "Cannot split an edge from a CallBrInst");
+    if (IsBBHeader && BBLoop->contains(Pred) && BBLoop->isLoopLatch(Pred)) {
+      // Update loop metadata if it exists.
+      if (MDNode *LoopMD = PI->getMetadata(LLVMContext::MD_loop)) {
+        BI->setMetadata(LLVMContext::MD_loop, LoopMD);
+        PI->setMetadata(LLVMContext::MD_loop, nullptr);
+      }
+    }
+    PI->replaceUsesOfWith(BB, NewBB);
   }
 
   // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI


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