Fix OpLoopMerge instruction placement

Author: pvelesko

lib/SPIRV/libSPIRV/SPIRVModule.cpp

@@ -1858,20 +1858,46 @@ SPIRVInstruction *SPIRVModuleImpl::addSelectionMergeInst(
 SPIRVInstruction *SPIRVModuleImpl::addLoopMergeInst(
     SPIRVId MergeBlock, SPIRVId ContinueTarget, SPIRVWord LoopControl,
     std::vector<SPIRVWord> LoopControlParameters, SPIRVBasicBlock *BB) {
-  return addInstruction(
-      new SPIRVLoopMerge(MergeBlock, ContinueTarget, LoopControl,
-                         LoopControlParameters, BB),
-      BB, const_cast<SPIRVInstruction *>(BB->getTerminateInstr()));
+  SPIRVInstruction *TermInst =
+      const_cast<SPIRVInstruction *>(BB->getTerminateInstr());
+  // OpLoopMerge must be the second-to-last instruction in the block,
+  // immediately preceding the branch instruction (OpBranch or
+  // OpBranchConditional)
+  if (TermInst && (TermInst->getOpCode() == OpBranch ||
+                   TermInst->getOpCode() == OpBranchConditional)) {
+    return addInstruction(new SPIRVLoopMerge(MergeBlock, ContinueTarget,
+                                             LoopControl, LoopControlParameters,
+                                             BB),
+                          BB, TermInst);
+  }
+  // If terminator doesn't exist yet or is not a branch, add at end
+  // (it will be before terminator when added)
+  return addInstruction(new SPIRVLoopMerge(MergeBlock, ContinueTarget,
+                                           LoopControl, LoopControlParameters,
+                                           BB),
+                        BB);
 }
 
 SPIRVInstruction *SPIRVModuleImpl::addLoopControlINTELInst(
     SPIRVWord LoopControl, std::vector<SPIRVWord> LoopControlParameters,
     SPIRVBasicBlock *BB) {
   addCapability(CapabilityUnstructuredLoopControlsINTEL);
   addExtension(ExtensionID::SPV_INTEL_unstructured_loop_controls);
+  SPIRVInstruction *TermInst =
+      const_cast<SPIRVInstruction *>(BB->getTerminateInstr());
+  // OpLoopControlINTEL must be the second-to-last instruction in the block,
+  // immediately preceding the branch instruction (OpBranch or
+  // OpBranchConditional)
+  if (TermInst && (TermInst->getOpCode() == OpBranch ||
+                   TermInst->getOpCode() == OpBranchConditional)) {
+    return addInstruction(
+        new SPIRVLoopControlINTEL(LoopControl, LoopControlParameters, BB), BB,
+        TermInst);
+  }
+  // If terminator doesn't exist yet or is not a branch, add at end
+  // (it will be before terminator when added)
   return addInstruction(
-      new SPIRVLoopControlINTEL(LoopControl, LoopControlParameters, BB), BB,
-      const_cast<SPIRVInstruction *>(BB->getTerminateInstr()));
+      new SPIRVLoopControlINTEL(LoopControl, LoopControlParameters, BB), BB);
 }
 
 SPIRVInstruction *SPIRVModuleImpl::addFixedPointIntelInst(

test/OpLoopMerge_loopMerge.ll

@@ -1,6 +1,10 @@
 ; RUN: llvm-as %s -o %t.bc
 ; RUN: llvm-spirv %t.bc -o %t.spv
 ; RUN: spirv-val %t.spv
+; RUN: llvm-spirv --to-text %t.spv -o - | FileCheck %s
+
+; CHECK: LoopMerge
+; CHECK-NEXT: BranchConditional
 
 target triple = "spir64"