Remove unused PHI node cycles

Sometimes PHI nodes form a cyclic dependency, which uses some values but
not used anywhere. Such cycles are not removed by DCE and, what is much
worse, often prevent used instructions from being bailed

IGC/VectorCompiler/lib/GenXCodeGen/GenXGEPLowering.cpp

@@ -137,13 +137,8 @@ Value *GenXGEPLowering::visitPtrToIntInst(PtrToIntInst &PTI) {
       // The `addrspacecast` is just no-op so it can be eliminated
       Src = Cast->getOperand(0);
     } else if (isa<BitCastInst>(Cast)) {
-      auto *Arg = Cast->getOperand(0);
-      auto *ArgTy = Arg->getType();
-      auto *CastTy = Cast->getType();
-      if (ArgTy->isVectorTy() != CastTy->isVectorTy())
-        break;
       // The `bitcast` is just no-op so it can be eliminated
-      Src = Arg;
+      Src = Cast->getOperand(0);
     } else {
       // We found `inttoptr`, getting rid of `ptrtoint`
       if (isa<IntToPtrInst>(Cast))

IGC/VectorCompiler/lib/GenXCodeGen/GenXPatternMatch.cpp

@@ -151,6 +151,8 @@ class GenXPatternMatch : public FunctionPass,
 
   void visitICmpInst(ICmpInst &I);
 
+  void visitPHINode(PHINode &I);
+
   void visitSRem(BinaryOperator &I);
 
   void visitSDiv(BinaryOperator &I);
@@ -3669,3 +3671,37 @@ bool GenXPatternMatch::extendMask(BinaryOperator *BO) {
 
   return true;
 }
+
+void GenXPatternMatch::visitPHINode(PHINode &I) {
+  if (Kind != PatternMatchKind::PreLegalization)
+    return;
+
+  std::unordered_set<PHINode *> Visited;
+  SmallVector<PHINode *, 8> Stack;
+  Stack.push_back(&I);
+  bool Cycle = false;
+  while (!Stack.empty()) {
+    auto *Phi = Stack.pop_back_val();
+    if (Visited.find(Phi) != Visited.end()) {
+      Cycle = true;
+      continue;
+    }
+    Visited.insert(Phi);
+    for (auto U : Phi->users()) {
+      auto *PhiUser = dyn_cast<PHINode>(U);
+      if (!PhiUser)
+        return;
+      Stack.push_back(PhiUser);
+    }
+  }
+
+  if (!Cycle)
+    return;
+
+  for (auto *Inst : Visited) {
+    Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
+    Inst->eraseFromParent();
+  }
+
+  Changed = true;
+}

IGC/VectorCompiler/test/GenXGEPLowering/fold-ptrtoint.ll

@@ -63,14 +63,3 @@ label:                                                ; preds = %entry
   %splat = shufflevector <16 x i64> %ins, <16 x i64> undef, <16 x i32> zeroinitializer
   ret void
 }
-
-; CHECK-LABEL: @test_bitcast_vector_to_scalar
-define i64 @test_bitcast_vector_to_scalar(<1 x i8*> %vptr) {
-  %ptr = bitcast <1 x i8*> %vptr to i8*
-  %gep = getelementptr i8, i8* %ptr, i32 123
-  %addr = ptrtoint i8* %gep to i64
-  ; CHECK: [[PTI:%[^ ]+]] = ptrtoint i8* %ptr to i64
-  ; CHECK: [[ADD:%[^ ]+]] = add i64 [[PTI]], 123
-  ; CHECK: ret i64 [[ADD]]
-  ret i64 %addr
-}

IGC/VectorCompiler/test/PatternMatch/phi_cycle.ll

@@ -0,0 +1,41 @@
+;=========================== begin_copyright_notice ============================
+;
+; Copyright (C) 2023 Intel Corporation
+;
+; SPDX-License-Identifier: MIT
+;
+;============================ end_copyright_notice =============================
+;
+; RUN: opt %use_old_pass_manager% -GenXPatternMatch -march=genx64 -mcpu=Gen9 -mtriple=spir64-unknown-unknown -S < %s | FileCheck %s
+; CHECK-NOT: %bad1 = phi i32 [ undef, %entry ], [ %bad2, %loop.end ]
+; CHECK-NOT: %bad2 = phi i32 [ %bad1, %true ], [ %mul, %false ]
+
+define i32 @test(i32 %a, i32 %n) {
+entry:
+  br label %loop
+
+loop:
+  %bad1 = phi i32 [ undef, %entry ], [ %bad2, %loop.end ]
+  %res = phi i32 [ 0, %entry ], [ %res.next, %loop.end ]
+  %i = phi i32 [ 0, %entry ], [ %i.next, %loop.end ]
+  %if.cond = icmp slt i32 %i, 10
+  br i1 %if.cond, label %true, label %false
+
+true:
+  %add = add i32 %res, %a
+  br label %loop.end
+
+false:
+  %mul = mul i32 %res, %a
+  br label %loop.end
+
+loop.end:
+  %bad2 = phi i32 [ %bad1, %true ], [ %mul, %false ]
+  %res.next = phi i32 [ %add, %true ], [ %mul, %false ]
+  %i.next = add i32 %i, 1
+  %loop.cond = icmp slt i32 %i.next, %n
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret i32 %res.next
+}