[LLVM] Add llvm.experimental.vector.compress intrinsic (#92289)

This PR adds a new vector intrinsic `@llvm.experimental.vector.compress`
to "compress" data within a vector based on a selection mask, i.e., it
moves all selected values (i.e., where `mask[i] == 1`) to consecutive
lanes in the result vector. A `passthru` vector can be provided, from
which remaining lanes are filled.

The main reason for this is that the existing
`@llvm.masked.compressstore` has very strong constraints in that it can
only write values that were selected, resulting in guard branches for
all targets except AVX-512 (and even there the AMD implementation is
_very_ slow). More instruction sets support "compress" logic, but only
within registers. So to store the values, an additional store is needed.
But this combination is likely significantly faster on many target as it
avoids branches.

In follow up PRs, my plan is to add target-specific lowerings for x86,
SVE, and possibly RISCV. I also want to combine this with a store
instruction, as this is probably a common case and we can avoid some
memory writes in that case.

See [discussion in
forum](https://discourse.llvm.org/t/new-intrinsic-for-masked-vector-compress-without-store/78663)
for initial discussion on the design.

Author: lawben

llvm/docs/GlobalISel/GenericOpcode.rst

@@ -726,6 +726,13 @@ The type of the operand must be equal to or larger than the vector element
 type. If the operand is larger than the vector element type, the scalar is
 implicitly truncated to the vector element type.
 
+G_VECTOR_COMPRESS
+^^^^^^^^^^^^^^^^^
+
+Given an input vector, a mask vector, and a passthru vector, continuously place
+all selected (i.e., where mask[i] = true) input lanes in an output vector. All
+remaining lanes in the output are taken from passthru, which may be undef.
+
 Vector Reduction Operations
 ---------------------------
 

llvm/docs/LangRef.rst

@@ -19525,6 +19525,93 @@ the follow sequence of operations:
 
 The ``mask`` operand will apply to at least the gather and scatter operations.
 
+
+.. _int_vector_compress:
+
+'``llvm.experimental.vector.compress.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+LLVM provides an intrinsic for compressing data within a vector based on a selection mask.
+Semantically, this is similar to :ref:`llvm.masked.compressstore <int_compressstore>` but with weaker assumptions
+and without storing the results to memory, i.e., the data remains in the vector.
+
+Syntax:
+"""""""
+This is an overloaded intrinsic. A number of scalar values of integer, floating point or pointer data type are collected
+from an input vector and placed adjacently within the result vector. A mask defines which elements to collect from the vector.
+The remaining lanes are filled with values from ``passthru``.
+
+:: code-block:: llvm
+
+      declare <8 x i32> @llvm.experimental.vector.compress.v8i32(<8 x i32> <value>, <8 x i1> <mask>, <8 x i32> <passthru>)
+      declare <16 x float> @llvm.experimental.vector.compress.v16f32(<16 x float> <value>, <16 x i1> <mask>, <16 x float> undef)
+
+Overview:
+"""""""""
+
+Selects elements from input vector ``value`` according to the ``mask``.
+All selected elements are written into adjacent lanes in the result vector,
+from lower to higher.
+The mask holds an entry for each vector lane, and is used to select elements
+to be kept.
+If a ``passthru`` vector is given, all remaining lanes are filled with the
+corresponding lane's value from ``passthru``.
+The main difference to :ref:`llvm.masked.compressstore <int_compressstore>` is
+that the we do not need to guard against memory access for unselected lanes.
+This allows for branchless code and better optimization for all targets that
+do not support or have inefficient
+instructions of the explicit semantics of
+:ref:`llvm.masked.compressstore <int_compressstore>` but still have some form
+of compress operations.
+The result vector can be written with a similar effect, as all the selected
+values are at the lower positions of the vector, but without requiring
+branches to avoid writes where the mask is ``false``.
+
+Arguments:
+""""""""""
+
+The first operand is the input vector, from which elements are selected.
+The second operand is the mask, a vector of boolean values.
+The third operand is the passthru vector, from which elements are filled
+into remaining lanes.
+The mask and the input vector must have the same number of vector elements.
+The input and passthru vectors must have the same type.
+
+Semantics:
+""""""""""
+
+The ``llvm.experimental.vector.compress`` intrinsic compresses data within a vector.
+It collects elements from possibly non-adjacent lanes of a vector and places
+them contiguously in the result vector based on a selection mask, filling the
+remaining lanes with values from ``passthru``.
+This intrinsic performs the logic of the following C++ example.
+All values in ``out`` after the last selected one are undefined if
+``passthru`` is undefined.
+If all entries in the ``mask`` are 0, the ``out`` vector is ``passthru``.
+If any element of the mask is poison, all elements of the result are poison.
+Otherwise, if any element of the mask is undef, all elements of the result are undef.
+If ``passthru`` is undefined, the number of valid lanes is equal to the number
+of ``true`` entries in the mask, i.e., all lanes >= number-of-selected-values
+are undefined.
+
+.. code-block:: cpp
+
+    // Consecutively place selected values in a vector.
+    using VecT __attribute__((vector_size(N))) = int;
+    VecT compress(VecT vec, VecT mask, VecT passthru) {
+      VecT out;
+      int idx = 0;
+      for (int i = 0; i < N / sizeof(int); ++i) {
+        out[idx] = vec[i];
+        idx += static_cast<bool>(mask[i]);
+      }
+      for (; idx < N / sizeof(int); ++idx) {
+        out[idx] = passthru[idx];
+      }
+      return out;
+    }
+
+
 Matrix Intrinsics
 -----------------
 

llvm/docs/ReleaseNotes.rst

@@ -79,6 +79,7 @@ Changes to the LLVM IR
   * ``llvm.instprof.mcdc.tvbitmap.update``: 3rd argument has been
     removed. The next argument has been changed from byte index to bit
     index.
+* Added ``llvm.experimental.vector.compress`` intrinsic.
 
 Changes to LLVM infrastructure
 ------------------------------

llvm/include/llvm/CodeGen/GlobalISel/LegalizerHelper.h

@@ -412,6 +412,7 @@ class LegalizerHelper {
   LegalizeResult lowerUnmergeValues(MachineInstr &MI);
   LegalizeResult lowerExtractInsertVectorElt(MachineInstr &MI);
   LegalizeResult lowerShuffleVector(MachineInstr &MI);
+  LegalizeResult lowerVECTOR_COMPRESS(MachineInstr &MI);
   Register getDynStackAllocTargetPtr(Register SPReg, Register AllocSize,
                                      Align Alignment, LLT PtrTy);
   LegalizeResult lowerDynStackAlloc(MachineInstr &MI);

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -659,6 +659,14 @@ enum NodeType {
   /// non-constant operands.
   STEP_VECTOR,
 
+  /// VECTOR_COMPRESS(Vec, Mask, Passthru)
+  /// consecutively place vector elements based on mask
+  /// e.g., vec = {A, B, C, D} and mask = {1, 0, 1, 0}
+  ///         --> {A, C, ?, ?} where ? is undefined
+  /// If passthru is defined, ?s are replaced with elements from passthru.
+  /// If passthru is undef, ?s remain undefined.
+  VECTOR_COMPRESS,
+
   /// MULHU/MULHS - Multiply high - Multiply two integers of type iN,
   /// producing an unsigned/signed value of type i[2*N], then return the top
   /// part.

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -5496,6 +5496,10 @@ class TargetLowering : public TargetLoweringBase {
   /// method accepts vectors as its arguments.
   SDValue expandVectorSplice(SDNode *Node, SelectionDAG &DAG) const;
 
+  /// Expand a vector VECTOR_COMPRESS into a sequence of extract element, store
+  /// temporarily, advance store position, before re-loading the final vector.
+  SDValue expandVECTOR_COMPRESS(SDNode *Node, SelectionDAG &DAG) const;
+
   /// Legalize a SETCC or VP_SETCC with given LHS and RHS and condition code CC
   /// on the current target. A VP_SETCC will additionally be given a Mask
   /// and/or EVL not equal to SDValue().

llvm/include/llvm/IR/Intrinsics.td

@@ -2398,6 +2398,11 @@ def int_masked_compressstore:
             [IntrWriteMem, IntrArgMemOnly, IntrWillReturn,
              NoCapture<ArgIndex<1>>]>;
 
+def int_experimental_vector_compress:
+    DefaultAttrsIntrinsic<[llvm_anyvector_ty],
+              [LLVMMatchType<0>, LLVMScalarOrSameVectorWidth<0, llvm_i1_ty>, LLVMMatchType<0>],
+              [IntrNoMem, IntrWillReturn]>;
+
 // Test whether a pointer is associated with a type metadata identifier.
 def int_type_test : DefaultAttrsIntrinsic<[llvm_i1_ty], [llvm_ptr_ty, llvm_metadata_ty],
                               [IntrNoMem, IntrWillReturn, IntrSpeculatable]>;

llvm/include/llvm/Support/TargetOpcodes.def

@@ -754,6 +754,9 @@ HANDLE_TARGET_OPCODE(G_SHUFFLE_VECTOR)
 /// Generic splatvector.
 HANDLE_TARGET_OPCODE(G_SPLAT_VECTOR)
 
+/// Generic masked compress.
+HANDLE_TARGET_OPCODE(G_VECTOR_COMPRESS)
+
 /// Generic count trailing zeroes.
 HANDLE_TARGET_OPCODE(G_CTTZ)
 

llvm/include/llvm/Target/GenericOpcodes.td

@@ -1548,6 +1548,13 @@ def G_SPLAT_VECTOR: GenericInstruction {
   let hasSideEffects = false;
 }
 
+// Generic masked compress.
+def G_VECTOR_COMPRESS: GenericInstruction {
+  let OutOperandList = (outs type0:$dst);
+  let InOperandList = (ins type0:$vec, type1:$mask, type0:$passthru);
+  let hasSideEffects = false;
+}
+
 //------------------------------------------------------------------------------
 // Vector reductions
 //------------------------------------------------------------------------------

llvm/include/llvm/Target/GlobalISel/SelectionDAGCompat.td

@@ -193,6 +193,7 @@ def : GINodeEquiv<G_VECREDUCE_UMAX, vecreduce_umax>;
 def : GINodeEquiv<G_VECREDUCE_SMIN, vecreduce_smin>;
 def : GINodeEquiv<G_VECREDUCE_SMAX, vecreduce_smax>;
 def : GINodeEquiv<G_VECREDUCE_ADD, vecreduce_add>;
+def : GINodeEquiv<G_VECTOR_COMPRESS, vector_compress>;
 
 def : GINodeEquiv<G_STRICT_FADD, strict_fadd>;
 def : GINodeEquiv<G_STRICT_FSUB, strict_fsub>;