[AMDGPU][FixIrreducible][UnifyLoopExits] Support callbr with inline-asm

First batch of changes to add support for basic inline-asm callbr for
the AMDGPU backend.

Author: ro-i

llvm/include/llvm/Support/GenericLoopInfoImpl.h

@@ -355,7 +355,7 @@ void LoopBase<BlockT, LoopT>::verifyLoop() const {
     if (BB == getHeader()) {
       assert(!OutsideLoopPreds.empty() && "Loop is unreachable!");
     } else if (!OutsideLoopPreds.empty()) {
-      // A non-header loop shouldn't be reachable from outside the loop,
+      // A non-header loop block shouldn't be reachable from outside the loop,
       // though it is permitted if the predecessor is not itself actually
       // reachable.
       BlockT *EntryBB = &BB->getParent()->front();

llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h

@@ -607,10 +607,17 @@ LLVM_ABI bool SplitIndirectBrCriticalEdges(Function &F,
 // successors
 LLVM_ABI void InvertBranch(BranchInst *PBI, IRBuilderBase &Builder);
 
-// Check whether the function only has simple terminator:
+template <typename... TermInst>
+LLVM_ABI bool hasOnlyGivenTerminators(const Function &F);
+
+// Check whether the function only has blocks with simple terminators:
 // br/brcond/unreachable/ret
 LLVM_ABI bool hasOnlySimpleTerminator(const Function &F);
 
+// Check whether the function only has blocks with simple terminators
+// (br/brcond/unreachable/ret) or callbr.
+LLVM_ABI bool hasOnlySimpleTerminatorOrCallBr(const Function &F);
+
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H

llvm/include/llvm/Transforms/Utils/ControlFlowUtils.h

@@ -15,10 +15,13 @@
 
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/IR/CycleInfo.h"
 
 namespace llvm {
 
 class BasicBlock;
+class CallBrInst;
+class LoopInfo;
 class DomTreeUpdater;
 
 /// Given a set of branch descriptors [BB, Succ0, Succ1], create a "hub" such
@@ -104,7 +107,8 @@ struct ControlFlowHub {
         : BB(BB), Succ0(Succ0), Succ1(Succ1) {}
   };
 
-  void addBranch(BasicBlock *BB, BasicBlock *Succ0, BasicBlock *Succ1) {
+  void addBranch(BasicBlock *BB, BasicBlock *Succ0,
+                 BasicBlock *Succ1 = nullptr) {
     assert(BB);
     assert(Succ0 || Succ1);
     Branches.emplace_back(BB, Succ0, Succ1);
@@ -118,6 +122,35 @@ struct ControlFlowHub {
            std::optional<unsigned> MaxControlFlowBooleans = std::nullopt);
 
   SmallVector<BranchDescriptor> Branches;
+
+  /**
+   * \brief Create a new intermediate target block for a callbr edge.
+   *
+   * This function creates a new basic block (the "target block") that sits
+   * between a callbr instruction and one of its successors. The callbr's
+   * successor is rewired to this new block, and the new block unconditionally
+   * branches to the original successor. This is useful for normalizing control
+   * flow, e.g., when transforming irreducible loops.
+   *
+   * \param CallBr         The callbr instruction whose edge is to be split.
+   * \param Succ           The original successor basic block to be reached.
+   * \param SuccIdx        The index of the successor in the callbr instruction.
+   * \param AttachToCallBr If true, the new block is associated with the
+   * callbr's parent for loop/cycle info. If false, the new block is associated
+   * with the callbr's successor for loop/cycle info. \param CI Optional
+   * CycleInfo for updating cycle membership. \param DTU            Optional
+   * DomTreeUpdater for updating the dominator tree. \param LI Optional LoopInfo
+   * for updating loop membership.
+   *
+   * \returns The newly created intermediate target block.
+   *
+   * \note This function updates PHI nodes, dominator tree, loop info, and cycle
+   * info as needed.
+   */
+  static BasicBlock *
+  createCallBrTarget(CallBrInst *CallBr, BasicBlock *Succ, unsigned SuccIdx,
+                     bool AttachToCallBr = true, CycleInfo *CI = nullptr,
+                     DomTreeUpdater *DTU = nullptr, LoopInfo *LI = nullptr);
 };
 
 } // end namespace llvm

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -1766,12 +1766,21 @@ void llvm::InvertBranch(BranchInst *PBI, IRBuilderBase &Builder) {
   PBI->swapSuccessors();
 }
 
-bool llvm::hasOnlySimpleTerminator(const Function &F) {
+template <typename... TermInst>
+bool llvm::hasOnlyGivenTerminators(const Function &F) {
   for (auto &BB : F) {
     auto *Term = BB.getTerminator();
-    if (!(isa<ReturnInst>(Term) || isa<UnreachableInst>(Term) ||
-          isa<BranchInst>(Term)))
+    if (!(isa<TermInst>(Term) || ...))
       return false;
   }
   return true;
 }
+
+bool llvm::hasOnlySimpleTerminator(const Function &F) {
+  return hasOnlyGivenTerminators<ReturnInst, UnreachableInst, BranchInst>(F);
+}
+
+bool llvm::hasOnlySimpleTerminatorOrCallBr(const Function &F) {
+  return hasOnlyGivenTerminators<ReturnInst, UnreachableInst, BranchInst,
+                                 CallBrInst>(F);
+}

llvm/lib/Transforms/Utils/ControlFlowUtils.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/Analysis/DomTreeUpdater.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/ValueHandle.h"
@@ -282,7 +283,9 @@ std::pair<BasicBlock *, bool> ControlFlowHub::finalize(
 
   for (auto [BB, Succ0, Succ1] : Branches) {
 #ifndef NDEBUG
-    assert(Incoming.insert(BB).second && "Duplicate entry for incoming block.");
+    assert(
+        (Incoming.insert(BB).second || isa<CallBrInst>(BB->getTerminator())) &&
+        "Duplicate entry for incoming block.");
 #endif
     if (Succ0)
       Outgoing.insert(Succ0);
@@ -342,3 +345,55 @@ std::pair<BasicBlock *, bool> ControlFlowHub::finalize(
 
   return {FirstGuardBlock, true};
 }
+
+BasicBlock *ControlFlowHub::createCallBrTarget(
+    CallBrInst *CallBr, BasicBlock *Succ, unsigned SuccIdx, bool AttachToCallBr,
+    CycleInfo *CI, DomTreeUpdater *DTU, LoopInfo *LI) {
+  BasicBlock *CallBrBlock = CallBr->getParent();
+  BasicBlock *CallBrTarget =
+      BasicBlock::Create(CallBrBlock->getContext(),
+                         CallBrBlock->getName() + ".target." + Succ->getName(),
+                         CallBrBlock->getParent());
+  // Rewire control flow from callbr to the new target block.
+  Succ->replacePhiUsesWith(CallBrBlock, CallBrTarget);
+  CallBr->setSuccessor(SuccIdx, CallBrTarget);
+  // Jump from the new target block to the original successor.
+  BranchInst::Create(Succ, CallBrTarget);
+  if (LI) {
+    if (Loop *L = LI->getLoopFor(AttachToCallBr ? CallBrBlock : Succ); L) {
+      bool AddToLoop = true;
+      if (AttachToCallBr) {
+        // Check if the loops are disjoint. In that case, we do not add the
+        // intermediate target to any loop.
+        if (auto *LL = LI->getLoopFor(Succ);
+            LL && !L->contains(LL) && !LL->contains(L))
+          AddToLoop = false;
+      }
+      if (AddToLoop)
+        L->addBasicBlockToLoop(CallBrTarget, *LI);
+    }
+  }
+  if (CI) {
+    if (auto *C = CI->getCycle(AttachToCallBr ? CallBrBlock : Succ); C) {
+      bool AddToCycle = true;
+      if (AttachToCallBr) {
+        // Check if the cycles are disjoint. In that case, we do not add the
+        // intermediate target to any cycle.
+        if (auto *CC = CI->getCycle(Succ); CC) {
+          auto *CommonC = CI->getSmallestCommonCycle(C, CC);
+          if (CommonC != C && CommonC != CC)
+            AddToCycle = false;
+        }
+      }
+      if (AddToCycle)
+        CI->addBlockToCycle(CallBrTarget, C);
+    }
+  }
+  if (DTU) {
+    DTU->applyUpdates({{DominatorTree::Insert, CallBrBlock, CallBrTarget}});
+    if (DTU->getDomTree().dominates(CallBrBlock, Succ))
+      DTU->applyUpdates({{DominatorTree::Delete, CallBrBlock, Succ},
+                         {DominatorTree::Insert, CallBrTarget, Succ}});
+  }
+  return CallBrTarget;
+}

llvm/lib/Transforms/Utils/FixIrreducible.cpp

@@ -79,6 +79,53 @@
 // Limitation: The pass cannot handle switch statements and indirect
 //             branches. Both must be lowered to plain branches first.
 //
+// CallBr support: CallBr is handled as a more general branch instruction which
+// can have multiple successors. The pass redirects the edges to intermediate
+// target blocks that unconditionally branch to the original callbr target
+// blocks. This allows the control flow hub to know to which of the original
+// target blocks to jump to.
+// Example input CFG:
+//                        Entry (callbr)
+//                       /     \
+//                      v       v
+//                      H ----> B
+//                      ^      /|
+//                       `----' |
+//                              v
+//                             Exit
+//
+// becomes:
+//                        Entry (callbr)
+//                       /     \
+//                      v       v
+//                 target.H   target.B
+//                      |       |
+//                      v       v
+//                      H ----> B
+//                      ^      /|
+//                       `----' |
+//                              v
+//                             Exit
+//
+// Note
+// OUTPUT CFG: Converted to a natural loop with a new header N.
+//
+//                        Entry (callbr)
+//                       /     \
+//                      v       v
+//                 target.H   target.B
+//                      \       /
+//                       \     /
+//                        v   v
+//                          N <---.
+//                         / \     \
+//                        /   \     |
+//                       v     v    /
+//                       H --> B --'
+//                             |
+//                             v
+//                            Exit
+//
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Utils/FixIrreducible.h"
@@ -231,6 +278,7 @@ static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
     return false;
   LLVM_DEBUG(dbgs() << "Processing cycle:\n" << CI.print(&C) << "\n";);
 
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
   ControlFlowHub CHub;
   SetVector<BasicBlock *> Predecessors;
 
@@ -242,18 +290,33 @@ static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
   }
 
   for (BasicBlock *P : Predecessors) {
-    auto *Branch = cast<BranchInst>(P->getTerminator());
-    // Exactly one of the two successors is the header.
-    BasicBlock *Succ0 = Branch->getSuccessor(0) == Header ? Header : nullptr;
-    BasicBlock *Succ1 = Succ0 ? nullptr : Header;
-    if (!Succ0)
-      assert(Branch->getSuccessor(1) == Header);
-    assert(Succ0 || Succ1);
-    CHub.addBranch(P, Succ0, Succ1);
-
-    LLVM_DEBUG(dbgs() << "Added internal branch: " << P->getName() << " -> "
-                      << (Succ0 ? Succ0->getName() : "") << " "
-                      << (Succ1 ? Succ1->getName() : "") << "\n");
+    if (BranchInst *Branch = dyn_cast<BranchInst>(P->getTerminator()); Branch) {
+      // Exactly one of the two successors is the header.
+      BasicBlock *Succ0 = Branch->getSuccessor(0) == Header ? Header : nullptr;
+      BasicBlock *Succ1 = Succ0 ? nullptr : Header;
+      if (!Succ0)
+        assert(Branch->getSuccessor(1) == Header);
+      assert(Succ0 || Succ1);
+      CHub.addBranch(P, Succ0, Succ1);
+
+      LLVM_DEBUG(dbgs() << "Added internal branch: " << P->getName() << " -> "
+                        << (Succ0 ? Succ0->getName() : "") << " "
+                        << (Succ1 ? Succ1->getName() : "") << "\n");
+    } else if (CallBrInst *CallBr = dyn_cast<CallBrInst>(P->getTerminator());
+               CallBr) {
+      for (unsigned I = 0; I < CallBr->getNumSuccessors(); ++I) {
+        BasicBlock *Succ = CallBr->getSuccessor(I);
+        if (Succ != Header)
+          continue;
+        BasicBlock *NewSucc = llvm::ControlFlowHub::createCallBrTarget(
+            CallBr, Succ, I, false, &CI, &DTU, LI);
+        CHub.addBranch(NewSucc, Succ);
+        LLVM_DEBUG(dbgs() << "Added internal branch: " << NewSucc->getName()
+                          << " -> " << Succ->getName() << "\n");
+      }
+    } else {
+      llvm_unreachable("Unsupported block terminator.");
+    }
   }
 
   // Redirect external incoming edges. This includes the edges on the header.
@@ -266,17 +329,32 @@ static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
   }
 
   for (BasicBlock *P : Predecessors) {
-    auto *Branch = cast<BranchInst>(P->getTerminator());
-    BasicBlock *Succ0 = Branch->getSuccessor(0);
-    Succ0 = C.contains(Succ0) ? Succ0 : nullptr;
-    BasicBlock *Succ1 =
-        Branch->isUnconditional() ? nullptr : Branch->getSuccessor(1);
-    Succ1 = Succ1 && C.contains(Succ1) ? Succ1 : nullptr;
-    CHub.addBranch(P, Succ0, Succ1);
-
-    LLVM_DEBUG(dbgs() << "Added external branch: " << P->getName() << " -> "
-                      << (Succ0 ? Succ0->getName() : "") << " "
-                      << (Succ1 ? Succ1->getName() : "") << "\n");
+    if (BranchInst *Branch = dyn_cast<BranchInst>(P->getTerminator()); Branch) {
+      BasicBlock *Succ0 = Branch->getSuccessor(0);
+      Succ0 = C.contains(Succ0) ? Succ0 : nullptr;
+      BasicBlock *Succ1 =
+          Branch->isUnconditional() ? nullptr : Branch->getSuccessor(1);
+      Succ1 = Succ1 && C.contains(Succ1) ? Succ1 : nullptr;
+      CHub.addBranch(P, Succ0, Succ1);
+
+      LLVM_DEBUG(dbgs() << "Added external branch: " << P->getName() << " -> "
+                        << (Succ0 ? Succ0->getName() : "") << " "
+                        << (Succ1 ? Succ1->getName() : "") << "\n");
+    } else if (CallBrInst *CallBr = dyn_cast<CallBrInst>(P->getTerminator());
+               CallBr) {
+      for (unsigned I = 0; I < CallBr->getNumSuccessors(); ++I) {
+        BasicBlock *Succ = CallBr->getSuccessor(I);
+        if (!C.contains(Succ))
+          continue;
+        BasicBlock *NewSucc = llvm::ControlFlowHub::createCallBrTarget(
+            CallBr, Succ, I, true, &CI, &DTU, LI);
+        CHub.addBranch(NewSucc, Succ);
+        LLVM_DEBUG(dbgs() << "Added external branch: " << NewSucc->getName()
+                          << " -> " << Succ->getName() << "\n");
+      }
+    } else {
+      llvm_unreachable("Unsupported block terminator.");
+    }
   }
 
   // Redirect all the backedges through a "hub" consisting of a series
@@ -292,7 +370,6 @@ static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
   SetVector<BasicBlock *> Entries;
   Entries.insert(C.entry_rbegin(), C.entry_rend());
 
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
   CHub.finalize(&DTU, GuardBlocks, "irr");
 #if defined(EXPENSIVE_CHECKS)
   assert(DT.verify(DominatorTree::VerificationLevel::Full));
@@ -325,7 +402,7 @@ static bool FixIrreducibleImpl(Function &F, CycleInfo &CI, DominatorTree &DT,
   LLVM_DEBUG(dbgs() << "===== Fix irreducible control-flow in function: "
                     << F.getName() << "\n");
 
-  assert(hasOnlySimpleTerminator(F) && "Unsupported block terminator.");
+  assert(hasOnlySimpleTerminatorOrCallBr(F) && "Unsupported block terminator.");
 
   bool Changed = false;
   for (Cycle *TopCycle : CI.toplevel_cycles()) {