Revert "[NVVMReflect] Force dead branch elimination in NVVMReflect (#81189)"

This reverts commit 9211e67.

Summary:
This seemed to crash one one of the CUDA math tests. Revert until it can
be fixed.

Author: jhuber6

llvm/docs/NVPTXUsage.rst

@@ -296,11 +296,6 @@ pipeline, immediately after the link stage. The ``internalize`` pass is also
 recommended to remove unused math functions from the resulting PTX. For an
 input IR module ``module.bc``, the following compilation flow is recommended:
 
-The ``NVVMReflect`` pass will attempt to remove dead code even without
-optimizations. This allows potentially incompatible instructions to be avoided
-at all optimizations levels. This currently only works for simple conditionals
-like the above example.
-
 1. Save list of external functions in ``module.bc``
 2. Link ``module.bc`` with ``libdevice.compute_XX.YY.bc``
 3. Internalize all functions not in list from (1)

llvm/lib/Target/NVPTX/NVVMReflect.cpp

@@ -20,7 +20,6 @@
 
 #include "NVPTX.h"
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
@@ -37,8 +36,6 @@
 #include "llvm/Support/raw_os_ostream.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/BasicBlockUtils.h"
-#include "llvm/Transforms/Utils/Local.h"
 #include <sstream>
 #include <string>
 #define NVVM_REFLECT_FUNCTION "__nvvm_reflect"
@@ -90,7 +87,6 @@ static bool runNVVMReflect(Function &F, unsigned SmVersion) {
   }
 
   SmallVector<Instruction *, 4> ToRemove;
-  SmallVector<ICmpInst *, 4> ToSimplify;
 
   // Go through the calls in this function.  Each call to __nvvm_reflect or
   // llvm.nvvm.reflect should be a CallInst with a ConstantArray argument.
@@ -175,71 +171,13 @@ static bool runNVVMReflect(Function &F, unsigned SmVersion) {
     } else if (ReflectArg == "__CUDA_ARCH") {
       ReflectVal = SmVersion * 10;
     }
-
-    // If the immediate user is a simple comparison we want to simplify it.
-    // TODO: This currently does not handle switch instructions.
-    for (User *U : Call->users())
-      if (ICmpInst *I = dyn_cast<ICmpInst>(U))
-        ToSimplify.push_back(I);
-
     Call->replaceAllUsesWith(ConstantInt::get(Call->getType(), ReflectVal));
     ToRemove.push_back(Call);
   }
 
   for (Instruction *I : ToRemove)
     I->eraseFromParent();
 
-  // The code guarded by __nvvm_reflect may be invalid for the target machine.
-  // We need to do some basic dead code elimination to trim invalid code before
-  // it reaches the backend at all optimization levels.
-  SmallVector<BranchInst *> Simplified;
-  for (ICmpInst *Cmp : ToSimplify) {
-    Constant *LHS = dyn_cast<Constant>(Cmp->getOperand(0));
-    Constant *RHS = dyn_cast<Constant>(Cmp->getOperand(1));
-
-    if (!LHS || !RHS)
-      continue;
-
-    // If the comparison is a compile time constant we simply propagate it.
-    Constant *C = ConstantFoldCompareInstOperands(
-        Cmp->getPredicate(), LHS, RHS, Cmp->getModule()->getDataLayout());
-
-    if (!C)
-      continue;
-
-    for (User *U : Cmp->users())
-      if (BranchInst *I = dyn_cast<BranchInst>(U))
-        Simplified.push_back(I);
-
-    Cmp->replaceAllUsesWith(C);
-    Cmp->eraseFromParent();
-  }
-
-  // Each instruction here is a conditional branch off of a constant true or
-  // false value. Simply replace it with an unconditional branch to the
-  // appropriate basic block and delete the rest if it is trivially dead.
-  DenseSet<Instruction *> Removed;
-  for (BranchInst *Branch : Simplified) {
-    if (Removed.contains(Branch))
-      continue;
-
-    ConstantInt *C = dyn_cast<ConstantInt>(Branch->getCondition());
-    if (!C || (!C->isOne() && !C->isZero()))
-      continue;
-
-    BasicBlock *TrueBB =
-        C->isOne() ? Branch->getSuccessor(0) : Branch->getSuccessor(1);
-    BasicBlock *FalseBB =
-        C->isOne() ? Branch->getSuccessor(1) : Branch->getSuccessor(0);
-
-    ReplaceInstWithInst(Branch, BranchInst::Create(TrueBB));
-    if (FalseBB->use_empty() && FalseBB->hasNPredecessors(0) &&
-        FalseBB->getFirstNonPHIOrDbg()) {
-      Removed.insert(FalseBB->getFirstNonPHIOrDbg());
-      changeToUnreachable(FalseBB->getFirstNonPHIOrDbg());
-    }
-  }
-
   return ToRemove.size() > 0;
 }
 

llvm/test/CodeGen/NVPTX/nvvm-reflect-arch-O0.ll

@@ -18,3 +18,4 @@ define i32 @foo(float %a, float %b) {
 ; SM35: ret i32 350  
   ret i32 %reflect
 }
+