[mlir][llvm] Add poison constant.

This patch introduces the poison constant from LLVM in the LLVM IR dialect. It also adds import and export support for it, along with roundtrip tests.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D146631

mlir/docs/Dialects/LLVM.md

@@ -105,6 +105,7 @@ values for thread-safety and concept parsimony reasons. Instead, regular values
 are produced by dedicated operations that have the corresponding semantics:
 [`llvm.mlir.constant`](#llvmmlirconstant-mlirllvmconstantop),
 [`llvm.mlir.undef`](#llvmmlirundef-mlirllvmundefop),
+[`llvm.mlir.poison`](#llvmmlirpoison-mlirllvmpoisonop),
 [`llvm.mlir.null`](#llvmmlirnull-mlirllvmnullop). Note how these operations are
 prefixed with `mlir.` to indicate that they don't belong to LLVM IR but are only
 necessary to model it in MLIR. The values produced by these operations are

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

@@ -1652,7 +1652,7 @@ def LLVM_UndefOp : LLVM_Op<"mlir.undef", [Pure]>,
     Unlike LLVM IR, MLIR does not have first-class undefined values. Such values
     must be created as SSA values using `llvm.mlir.undef`. This operation has no
     operands or attributes. It creates an undefined value of the specified LLVM
-    IR dialect type wrapping an LLVM IR structure type.
+    IR dialect type.
 
     Example:
 
@@ -1666,6 +1666,28 @@ def LLVM_UndefOp : LLVM_Op<"mlir.undef", [Pure]>,
   let assemblyFormat = "attr-dict `:` type($res)";
 }
 
+def LLVM_PoisonOp : LLVM_Op<"mlir.poison", [Pure]>,
+                    LLVM_Builder<"$res = llvm::PoisonValue::get($_resultType);"> {
+  let summary = "Creates a poison value of LLVM dialect type.";
+  let description = [{
+    Unlike LLVM IR, MLIR does not have first-class poison values. Such values
+    must be created as SSA values using `llvm.mlir.poison`. This operation has
+    no operands or attributes. It creates a poison value of the specified LLVM
+    IR dialect type.
+
+    Example:
+
+    ```mlir
+    // Create a poison value for a structure with a 32-bit integer followed
+    // by a float.
+    %0 = llvm.mlir.poison : !llvm.struct<(i32, f32)>
+    ```
+  }];
+  let results = (outs LLVM_Type:$res);
+  let builders = [LLVM_OneResultOpBuilder];
+  let assemblyFormat = "attr-dict `:` type($res)";
+}
+
 def LLVM_ConstantOp
     : LLVM_Op<"mlir.constant", [Pure, ConstantLike]>,
       LLVM_Builder<[{$res = getLLVMConstant($_resultType, $value, $_location,

mlir/lib/Target/LLVMIR/ModuleImport.cpp

@@ -1036,6 +1036,12 @@ FailureOr<Value> ModuleImport::convertConstant(llvm::Constant *constant) {
     return builder.create<NullOp>(loc, type).getResult();
   }
 
+  // Convert poison.
+  if (auto *poisonVal = dyn_cast<llvm::PoisonValue>(constant)) {
+    Type type = convertType(poisonVal->getType());
+    return builder.create<PoisonOp>(loc, type).getResult();
+  }
+
   // Convert undef.
   if (auto *undefVal = dyn_cast<llvm::UndefValue>(constant)) {
     Type type = convertType(undefVal->getType());

mlir/test/Dialect/LLVMIR/roundtrip.mlir

@@ -438,10 +438,14 @@ llvm.func @invokeLandingpad() -> i32 attributes { personality = @__gxx_personali
 func.func @useFreezeOp(%arg0: i32) {
   // CHECK:  = llvm.freeze %[[ARG0:.*]] : i32
   %0 = llvm.freeze %arg0 : i32
-  // CHECK: %[[x:.*]] = llvm.mlir.undef : i8
+  // CHECK: %[[UNDEF:.*]] = llvm.mlir.undef : i8
   %1 = llvm.mlir.undef : i8
-  // CHECK:  = llvm.freeze %[[x]] : i8
+  // CHECK:  = llvm.freeze %[[UNDEF]] : i8
   %2 = llvm.freeze %1 : i8
+  // CHECK: %[[POISON:.*]] = llvm.mlir.poison : i8
+  %3 = llvm.mlir.poison : i8
+  // CHECK:  = llvm.freeze %[[POISON]] : i8
+  %4 = llvm.freeze %3 : i8
   return
 }
 

mlir/test/Target/LLVMIR/Import/instructions.ll

@@ -523,10 +523,13 @@ define void @gep_dynamic_idx(ptr %ptr, i32 %idx) {
 ; CHECK-SAME:  %[[ARG1:[a-zA-Z0-9]+]]
 define void @freeze(i32 %arg1) {
   ; CHECK:  %[[UNDEF:[0-9]+]] = llvm.mlir.undef : i64
+  ; CHECK:  %[[POISON:[0-9]+]] = llvm.mlir.poison : i16
   ; CHECK:  llvm.freeze %[[ARG1]] : i32
   ; CHECK:  llvm.freeze %[[UNDEF]] : i64
+  ; CHECK:  llvm.freeze %[[POISON]] : i16
   %1 = freeze i32 %arg1
   %2 = freeze i64 undef
+  %3 = freeze i16 poison
   ret void
 }
 

mlir/test/Target/LLVMIR/llvmir.mlir

@@ -1618,6 +1618,9 @@ llvm.func @callFreezeOp(%x : i32) {
   %1 = llvm.mlir.undef : i32
   // CHECK: freeze i32 undef
   %2 = llvm.freeze %1 : i32
+  %3 = llvm.mlir.poison : i32
+  // CHECK: freeze i32 poison
+  %4 = llvm.freeze %3 : i32
   llvm.return
 }