[VP] Declaration and docs for vp.select intrinsic

llvm.vp.select extends the regular select instruction with an explicit
vector length (%evl).

All lanes with indexes at and above %evl are
undefined. Lanes below %evl are taken from the first input where the
mask is true and from the second input otherwise.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D105351

Author: Simon Moll

llvm/docs/LangRef.rst

@@ -17745,6 +17745,64 @@ The use of an effective %evl is discouraged for those targets.  The function
 ``TargetTransformInfo::hasActiveVectorLength()`` returns true when the target
 has native support for %evl.
 
+.. _int_vp_select:
+
+'``llvm.vp.select.*``' Intrinsics
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+This is an overloaded intrinsic.
+
+::
+
+      declare <16 x i32>  @llvm.vp.select.v16i32 (<16 x i1> <condition>, <16 x i32> <on_true>, <16 x i32> <on_false>, i32 <evl>)
+      declare <vscale x 4 x i64>  @llvm.vp.select.nxv4i64 (<vscale x 4 x i1> <condition>, <vscale x 4 x i32> <on_true>, <vscale x 4 x i32> <on_false>, i32 <evl>)
+
+Overview:
+"""""""""
+
+The '``llvm.vp.select``' intrinsic is used to choose one value based on a
+condition vector, without IR-level branching.
+
+Arguments:
+""""""""""
+
+The first operand is a vector of ``i1`` and indicates the condition.  The
+second operand is the value that is selected where the condition vector is
+true.  The third operand is the value that is selected where the condition
+vector is false.  The vectors must be of the same size.  The fourth operand is
+the explicit vector length.
+
+#. The optional ``fast-math flags`` marker indicates that the select has one or
+   more :ref:`fast-math flags <fastmath>`. These are optimization hints to
+   enable otherwise unsafe floating-point optimizations. Fast-math flags are
+   only valid for selects that return a floating-point scalar or vector type,
+   or an array (nested to any depth) of floating-point scalar or vector types.
+
+Semantics:
+""""""""""
+
+The intrinsic selects lanes from the second and third operand depending on a
+condition vector.
+
+All result lanes at positions greater or equal than ``%evl`` are undefined.
+For all lanes below ``%evl`` where the condition vector is true the lane is
+taken from the second operand.  Otherwise, the lane is taken from the third
+operand.
+
+Example:
+""""""""
+
+.. code-block:: llvm
+
+      %r = call <4 x i32> @llvm.vp.select.v4i32(<4 x i1> %cond, <4 x i32> %on_true, <4 x i32> %on_false, i32 %evl)
+
+      ;;; Expansion.
+      ;; Any result is legal on lanes at and above %evl.
+      %also.r = select <4 x i1> %cond, <4 x i32> %on_true, <4 x i32> %on_false
+
+
 
 .. _int_vp_add:
 

llvm/include/llvm/IR/Intrinsics.td

@@ -1507,6 +1507,12 @@ let IntrProperties =
                                 LLVMScalarOrSameVectorWidth<0, llvm_i1_ty>,
                                 llvm_i32_ty]>;
 }
+// Shuffles.
+def int_vp_select : DefaultAttrsIntrinsic<[ llvm_anyvector_ty ],
+                              [ LLVMScalarOrSameVectorWidth<0, llvm_i1_ty>,
+                                LLVMMatchType<0>,
+                                LLVMMatchType<0>,
+                                llvm_i32_ty]>;
 
 // Reductions
 let IntrProperties = [IntrSpeculatable, IntrNoMem, IntrNoSync, IntrWillReturn] in {

llvm/include/llvm/IR/VPIntrinsics.def

@@ -333,6 +333,16 @@ HELPER_REGISTER_REDUCTION_SEQ_VP(vp_reduce_fmul, VP_REDUCE_FMUL,
 
 ///// } Reduction
 
+///// Shuffles {
+
+// llvm.vp.select(mask,on_true,on_false,vlen)
+BEGIN_REGISTER_VP_INTRINSIC(vp_select, 0, 3)
+// BEGIN_REGISTER_VP_SDNODE(VP_SELECT, -1, vp_select, 0, 4)
+// END_REGISTER_CASES(vp_select, VP_SELECT)
+END_REGISTER_VP_INTRINSIC(vp_select)
+
+///// } Shuffles
+
 #undef BEGIN_REGISTER_VP
 #undef BEGIN_REGISTER_VP_INTRINSIC
 #undef BEGIN_REGISTER_VP_SDNODE

llvm/lib/IR/IntrinsicInst.cpp

@@ -482,6 +482,9 @@ Function *VPIntrinsic::getDeclarationForParams(Module *M, Intrinsic::ID VPID,
     VPFunc = Intrinsic::getDeclaration(M, VPID, OverloadTy);
     break;
   }
+  case Intrinsic::vp_select:
+    VPFunc = Intrinsic::getDeclaration(M, VPID, {Params[1]->getType()});
+    break;
   case Intrinsic::vp_load:
     VPFunc = Intrinsic::getDeclaration(
         M, VPID,

llvm/unittests/IR/VPIntrinsicTest.cpp

@@ -68,6 +68,8 @@ class VPIntrinsicTest : public testing::Test {
       Str << " declare float @llvm.vp.reduce." << ReductionOpcode
           << ".v8f32(float, <8 x float>, <8 x i1>, i32) ";
 
+    Str << " declare <8 x i32> @llvm.vp.select.v8i32(<8 x i1>, <8 x i32>, <8 x "
+           "i32>, i32)";
     return parseAssemblyString(Str.str(), Err, C);
   }
 };