[mlir][ArmSME] Add arm_sme.streaming_vl operation (#77321)

This operation provides a convenient way to query the streaming vector
length regardless of the streaming mode. This most useful for functions
that call/pass data to streaming functions, but are not streaming
themselves.

Example:
```mlir
%svl_w = arm_sme.streaming_vl <word>
```

Created based on discussion here:
#76086 (comment)

mlir/include/mlir/Dialect/ArmSME/IR/ArmSMEOps.td

@@ -223,6 +223,21 @@ def ArmSME_CombiningKindAttr : EnumAttr<ArmSME_Dialect, CombiningKind,
   let defaultValue = "CombiningKind::Add";
 }
 
+def TypeSize : I32EnumAttr<"TypeSize", "Size of a vector element type", [
+  I32EnumAttrCase<"Byte"  , 0, "byte">,
+  I32EnumAttrCase<"Half"  , 1, "half">,
+  I32EnumAttrCase<"Word"  , 2, "word">,
+  I32EnumAttrCase<"Double", 3, "double">,
+]> {
+  let cppNamespace = "::mlir::arm_sme";
+  let genSpecializedAttr = 0;
+}
+
+def ArmSME_TypeSizeAttr : EnumAttr<ArmSME_Dialect, TypeSize,
+                                   "type_size"> {
+  let assemblyFormat = "`<` $value `>`";
+}
+
 //===----------------------------------------------------------------------===//
 // ArmSME op definitions
 //===----------------------------------------------------------------------===//
@@ -768,4 +783,33 @@ let arguments = (ins
   }];
 }
 
+def StreamingVLOp : ArmSME_Op<"streaming_vl", [Pure]>
+{
+  let summary = "Query the streaming vector length";
+
+  let description = [{
+    This operation returns the streaming vector length (SVL) for a given type
+    size. Unlike `vector.vscale` the value returned is invariant to the
+    streaming mode.
+
+    Example:
+    ```mlir
+    // Streaming vector length in:
+    // - bytes (8-bit, SVL.B)
+    %svl_b = arm_sme.streaming_vl <byte>
+    // - half words (16-bit, SVL.H)
+    %svl_h = arm_sme.streaming_vl <half>
+    // - words (32-bit, SVL.W)
+    %svl_w = arm_sme.streaming_vl <word>
+    // - double words (64-bit, SVL.D)
+    %svl_d = arm_sme.streaming_vl <double>
+    ```
+  }];
+
+  let arguments = (ins ArmSME_TypeSizeAttr: $type_size);
+  let results = (outs Index);
+
+  let assemblyFormat = "$type_size attr-dict";
+}
+
 #endif // ARMSME_OPS

mlir/lib/Conversion/ArmSMEToLLVM/ArmSMEToLLVM.cpp

@@ -518,6 +518,45 @@ struct OuterProductOpConversion
   }
 };
 
+/// Lower `arm_sme.streaming_vl` to SME CNTS intrinsics.
+///
+/// Example:
+///
+///   %0 = arm_sme.streaming_vl <half>
+///
+/// is converted to:
+///
+///   %cnt = "arm_sme.intr.cntsh"() : () -> i64
+///   %0 = arith.index_cast %cnt : i64 to index
+///
+struct StreamingVLOpConversion
+    : public ConvertOpToLLVMPattern<arm_sme::StreamingVLOp> {
+  using ConvertOpToLLVMPattern<arm_sme::StreamingVLOp>::ConvertOpToLLVMPattern;
+
+  LogicalResult
+  matchAndRewrite(arm_sme::StreamingVLOp streamingVlOp,
+                  arm_sme::StreamingVLOp::Adaptor adaptor,
+                  ConversionPatternRewriter &rewriter) const override {
+    auto loc = streamingVlOp.getLoc();
+    auto i64Type = rewriter.getI64Type();
+    auto *intrOp = [&]() -> Operation * {
+      switch (streamingVlOp.getTypeSize()) {
+      case arm_sme::TypeSize::Byte:
+        return rewriter.create<arm_sme::aarch64_sme_cntsb>(loc, i64Type);
+      case arm_sme::TypeSize::Half:
+        return rewriter.create<arm_sme::aarch64_sme_cntsh>(loc, i64Type);
+      case arm_sme::TypeSize::Word:
+        return rewriter.create<arm_sme::aarch64_sme_cntsw>(loc, i64Type);
+      case arm_sme::TypeSize::Double:
+        return rewriter.create<arm_sme::aarch64_sme_cntsd>(loc, i64Type);
+      }
+    }();
+    rewriter.replaceOpWithNewOp<arith::IndexCastOp>(
+        streamingVlOp, rewriter.getIndexType(), intrOp->getResult(0));
+    return success();
+  }
+};
+
 } // namespace
 
 namespace {
@@ -555,7 +594,9 @@ void mlir::configureArmSMEToLLVMConversionLegality(ConversionTarget &target) {
       arm_sme::aarch64_sme_st1w_vert, arm_sme::aarch64_sme_st1d_vert,
       arm_sme::aarch64_sme_st1q_vert, arm_sme::aarch64_sme_read_horiz,
       arm_sme::aarch64_sme_read_vert, arm_sme::aarch64_sme_write_horiz,
-      arm_sme::aarch64_sme_write_vert, arm_sme::aarch64_sme_mopa>();
+      arm_sme::aarch64_sme_write_vert, arm_sme::aarch64_sme_mopa,
+      arm_sme::aarch64_sme_cntsb, arm_sme::aarch64_sme_cntsh,
+      arm_sme::aarch64_sme_cntsw, arm_sme::aarch64_sme_cntsd>();
   target.addLegalDialect<arith::ArithDialect>();
   target.addLegalOp<UnrealizedConversionCastOp>();
 }
@@ -572,8 +613,8 @@ void mlir::populateArmSMEToLLVMConversionPatterns(LLVMTypeConverter &converter,
 
   patterns.add<LoadTileSliceConversion, MoveTileSliceToVectorConversion,
                MoveVectorToTileSliceConversion, StoreTileSliceConversion,
-               OuterProductOpConversion, ZeroOpConversion, GetTileConversion>(
-      converter);
+               OuterProductOpConversion, ZeroOpConversion, GetTileConversion,
+               StreamingVLOpConversion>(converter);
 }
 
 std::unique_ptr<Pass> mlir::createConvertArmSMEToLLVMPass() {

mlir/test/Conversion/ArmSMEToLLVM/arm-sme-to-llvm.mlir

@@ -559,3 +559,45 @@ func.func @arm_sme_move_tile_slice_to_vector_ver_i128(%tile_slice_index : index)
   %slice = arm_sme.move_tile_slice_to_vector %tile[%tile_slice_index] layout<vertical> : vector<[1]xi128> from vector<[1]x[1]xi128>
   return %slice : vector<[1]xi128>
 }
+
+//===----------------------------------------------------------------------===//
+// arm_sme.streaming_vl
+//===----------------------------------------------------------------------===//
+
+// -----
+
+// CHECK-LABEL: @arm_sme_streaming_vl_bytes
+// CHECK: %[[COUNT:.*]] = "arm_sme.intr.cntsb"() : () -> i64
+// CHECK: %[[INDEX_COUNT:.*]] = arith.index_cast %[[COUNT]] : i64 to index
+// CHECK: return %[[INDEX_COUNT]] : index
+func.func @arm_sme_streaming_vl_bytes() -> index {
+  %svl_b = arm_sme.streaming_vl <byte>
+  return %svl_b : index
+}
+
+// -----
+
+// CHECK-LABEL: @arm_sme_streaming_vl_half_words
+// CHECK: "arm_sme.intr.cntsh"() : () -> i64
+func.func @arm_sme_streaming_vl_half_words() -> index {
+  %svl_h = arm_sme.streaming_vl <half>
+  return %svl_h : index
+}
+
+// -----
+
+// CHECK-LABEL: @arm_sme_streaming_vl_words
+// CHECK: "arm_sme.intr.cntsw"() : () -> i64
+func.func @arm_sme_streaming_vl_words() -> index {
+  %svl_w = arm_sme.streaming_vl <word>
+  return %svl_w : index
+}
+
+// -----
+
+// CHECK-LABEL: @arm_sme_streaming_vl_double_words
+// CHECK: "arm_sme.intr.cntsd"() : () -> i64
+func.func @arm_sme_streaming_vl_double_words() -> index {
+  %svl_d = arm_sme.streaming_vl <double>
+  return %svl_d : index
+}

mlir/test/Dialect/ArmSME/roundtrip.mlir

@@ -1095,3 +1095,39 @@ func.func @arm_sme_outerproduct_with_everything(%vecA: vector<[16]xi8>, %vecB: v
   %result = arm_sme.outerproduct %vecA, %vecB kind<sub> acc(%acc) masks(%maskA, %maskB) : vector<[16]xi8>, vector<[16]xi8>
   return %result : vector<[16]x[16]xi8>
 }
+
+//===----------------------------------------------------------------------===//
+// arm_sme.streaming_vl
+//===----------------------------------------------------------------------===//
+
+// -----
+
+func.func @arm_sme_streaming_vl_bytes() -> index {
+  // CHECK: arm_sme.streaming_vl <byte>
+  %svl_b = arm_sme.streaming_vl <byte>
+  return %svl_b : index
+}
+
+// -----
+
+func.func @arm_sme_streaming_vl_half_words() -> index {
+  // CHECK: arm_sme.streaming_vl <half>
+  %svl_h = arm_sme.streaming_vl <half>
+  return %svl_h : index
+}
+
+// -----
+
+func.func @arm_sme_streaming_vl_words() -> index {
+  // CHECK: arm_sme.streaming_vl <word>
+  %svl_w = arm_sme.streaming_vl <word>
+  return %svl_w : index
+}
+
+// -----
+
+func.func @arm_sme_streaming_vl_double_words() -> index {
+  // CHECK: arm_sme.streaming_vl <double>
+  %svl_d = arm_sme.streaming_vl <double>
+  return %svl_d : index
+}