[mlir][llvm] Implement ConstantLike for ZeroOp, UndefOp, PoisonOp

These act as constants and should be propagated whenever possible.
It is safe to do so for mlir.undef and mlir.poison because they remain
"dirty" through out their lifetime and can be duplicated, merged, etc.
per the LangRef.

Signed-off-by: Guy David <guy.david@nextsilicon.com>

Author: Guy David

mlir/include/mlir/Dialect/LLVMIR/LLVMAttrDefs.td

@@ -1037,4 +1037,28 @@ def LLVM_TargetFeaturesAttr : LLVM_Attr<"TargetFeatures", "target_features">
   let genVerifyDecl = 1;
 }
 
+//===----------------------------------------------------------------------===//
+// UndefAttr
+//===----------------------------------------------------------------------===//
+
+/// Folded into from LLVM::UndefOp.
+def LLVM_UndefAttr : LLVM_Attr<"Undef", "undef"> {
+}
+
+//===----------------------------------------------------------------------===//
+// PoisonAttr
+//===----------------------------------------------------------------------===//
+
+/// Folded into from LLVM::PoisonOp.
+def LLVM_PoisonAttr : LLVM_Attr<"Poison", "poison"> {
+}
+
+//===----------------------------------------------------------------------===//
+// ZeroAttr
+//===----------------------------------------------------------------------===//
+
+/// Folded into from LLVM::ZeroOp.
+def LLVM_ZeroAttr : LLVM_Attr<"Zero", "zero"> {
+}
+
 #endif // LLVMIR_ATTRDEFS

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -1522,7 +1522,7 @@ def LLVM_NoneTokenOp
   let assemblyFormat = "attr-dict `:` type($res)";
 }
 
-def LLVM_UndefOp : LLVM_Op<"mlir.undef", [Pure]>,
+def LLVM_UndefOp : LLVM_Op<"mlir.undef", [Pure, ConstantLike]>,
                    LLVM_Builder<"$res = llvm::UndefValue::get($_resultType);"> {
   let summary = "Creates an undefined value of LLVM dialect type.";
   let description = [{
@@ -1541,9 +1541,10 @@ def LLVM_UndefOp : LLVM_Op<"mlir.undef", [Pure]>,
   let results = (outs LLVM_Type:$res);
   let builders = [LLVM_OneResultOpBuilder];
   let assemblyFormat = "attr-dict `:` type($res)";
+  let hasFolder = 1;
 }
 
-def LLVM_PoisonOp : LLVM_Op<"mlir.poison", [Pure]>,
+def LLVM_PoisonOp : LLVM_Op<"mlir.poison", [Pure, ConstantLike]>,
                     LLVM_Builder<"$res = llvm::PoisonValue::get($_resultType);"> {
   let summary = "Creates a poison value of LLVM dialect type.";
   let description = [{
@@ -1563,10 +1564,11 @@ def LLVM_PoisonOp : LLVM_Op<"mlir.poison", [Pure]>,
   let results = (outs LLVM_Type:$res);
   let builders = [LLVM_OneResultOpBuilder];
   let assemblyFormat = "attr-dict `:` type($res)";
+  let hasFolder = 1;
 }
 
 def LLVM_ZeroOp
-    : LLVM_Op<"mlir.zero", [Pure]>,
+    : LLVM_Op<"mlir.zero", [Pure, ConstantLike]>,
       LLVM_Builder<"$res = llvm::Constant::getNullValue($_resultType);">
 {
   let summary = "Creates a zero-initialized value of LLVM dialect type.";
@@ -1588,6 +1590,7 @@ def LLVM_ZeroOp
   let builders = [LLVM_OneResultOpBuilder];
   let assemblyFormat = "attr-dict `:` type($res)";
   let hasVerifier = 1;
+  let hasFolder = 1;
 }
 
 def LLVM_ConstantOp

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -2555,6 +2555,24 @@ Region *LLVMFuncOp::getCallableRegion() {
   return &getBody();
 }
 
+//===----------------------------------------------------------------------===//
+// UndefOp.
+//===----------------------------------------------------------------------===//
+
+/// Fold an undef operation to a dedicated undef attribute.
+OpFoldResult LLVM::UndefOp::fold(FoldAdaptor) {
+  return mlir::LLVM::UndefAttr::get(getContext());
+}
+
+//===----------------------------------------------------------------------===//
+// PoisonOp.
+//===----------------------------------------------------------------------===//
+
+/// Fold a poison operation to a dedicated poison attribute.
+OpFoldResult LLVM::PoisonOp::fold(FoldAdaptor) {
+  return mlir::LLVM::PoisonAttr::get(getContext());
+}
+
 //===----------------------------------------------------------------------===//
 // ZeroOp.
 //===----------------------------------------------------------------------===//
@@ -2568,6 +2586,15 @@ LogicalResult LLVM::ZeroOp::verify() {
   return success();
 }
 
+/// Fold a zero operation to a builtin zero attribute when possible and fall
+/// back to a dedicated zero attribute.
+OpFoldResult LLVM::ZeroOp::fold(FoldAdaptor) {
+  OpFoldResult result = Builder(getContext()).getZeroAttr(getType());
+  if (result)
+    return result;
+  return mlir::LLVM::ZeroAttr::get(getContext());
+}
+
 //===----------------------------------------------------------------------===//
 // ConstantOp.
 //===----------------------------------------------------------------------===//
@@ -3271,11 +3298,18 @@ LogicalResult LLVMDialect::verifyRegionResultAttribute(Operation *op,
 
 Operation *LLVMDialect::materializeConstant(OpBuilder &builder, Attribute value,
                                             Type type, Location loc) {
-  // If this was folded from an llvm.mlir.addressof operation, it should be
-  // materialized as such.
+  // If this was folded from an operation other than llvm.mlir.constant, it
+  // should be materialized as such. Note that an llvm.mlir.zero may fold into
+  // a builtin zero attribute and thus will materialize as a llvm.mlir.constant.
   if (auto symbol = dyn_cast<FlatSymbolRefAttr>(value))
     if (isa<LLVM::LLVMPointerType>(type))
       return builder.create<LLVM::AddressOfOp>(loc, type, symbol);
+  if (isa<LLVM::UndefAttr>(value))
+    return builder.create<LLVM::UndefOp>(loc, type);
+  if (isa<LLVM::PoisonAttr>(value))
+    return builder.create<LLVM::PoisonOp>(loc, type);
+  if (isa<LLVM::ZeroAttr>(value))
+    return builder.create<LLVM::ZeroOp>(loc, type);
   // Otherwise try materializing it as a regular llvm.mlir.constant op.
   return LLVM::ConstantOp::materialize(builder, value, type, loc);
 }