NVVMReflectDCE option

Author: YonahGoldberg

llvm/lib/Target/NVPTX/NVVMReflect.cpp

@@ -88,7 +88,7 @@ ModulePass *llvm::createNVVMReflectPass(unsigned SmVersion) {
 }
 
 static cl::opt<bool>
-    NVVMReflectEnabled("nvvm-reflect-enable", cl::init(true), cl::Hidden,
+    NVVMReflectEnabled("nvvm-reflect-enable", cl::init(false), cl::Hidden,
                        cl::desc("NVVM reflection, enabled by default"));
 
 char NVVMReflectLegacyPass::ID = 0;
@@ -105,6 +105,10 @@ static cl::list<std::string> ReflectList(
     cl::desc("A key=value pair. Replace __nvvm_reflect(name) with value."),
     cl::ValueRequired);
 
+static cl::opt<bool> NVVMReflectDCE("nvvm-reflect-dce", cl::init(false),
+                                    cl::Hidden,
+                                    cl::desc("Delete dead blocks introduced by reflect call elimination"));
+
 // Set the ReflectMap with, first, the value of __CUDA_FTZ from module metadata,
 // and then the key/value pairs from the command line.
 void NVVMReflect::populateReflectMap(Module &M) {
@@ -241,8 +245,9 @@ void NVVMReflect::replaceReflectCalls(
 
   for (auto &[Call, NewValue] : ReflectReplacements)
     ReplaceInstructionWithConst(Call, NewValue);
-
-  // Alternate between constant folding/propagation and dead block elimination.
+  
+  // Constant fold reflect results. If NVVMReflectDCE is enabled, we will
+  // alternate between constant folding/propagation and dead block elimination.
   // Terminator folding may create new dead blocks. When those dead blocks are
   // deleted, their live successors may have PHIs that can be simplified, which
   // may yield more work for folding/propagation.
@@ -256,11 +261,12 @@ void NVVMReflect::replaceReflectCalls(
         BasicBlock *BB = I->getParent();
         SmallVector<BasicBlock *, 8> Succs(successors(BB));
         // Some blocks may become dead if the terminator is folded because
-        // a conditional branch is turned into a direct branch.
+        // a conditional branch is turned into a direct branch. Add those dead blocks
+        // to the dead blocks set if NVVMReflectDCE is enabled.
         if (ConstantFoldTerminator(BB)) {
           for (BasicBlock *Succ : Succs) {
             if (pred_empty(Succ) &&
-                Succ != &Succ->getParent()->getEntryBlock()) {
+                Succ != &Succ->getParent()->getEntryBlock() && NVVMReflectDCE) {
               SetVector<BasicBlock *> TransitivelyDead =
                   findTransitivelyDeadBlocks(Succ);
               DeadBlocks.insert(TransitivelyDead.begin(),