[RFC] Dynamic Vector Support (#4200)

Introduce a new concept in the IR that allows a producer to create
dynamic vector types. An IR function can now contain global value(s)
that represent a dynamic scaling factor, for a given fixed-width
vector type. A dynamic type is then created by 'multiplying' the
corresponding global value with a fixed-width type. These new types
can be used just like the existing types and the type system has a
set of hard-coded dynamic types, such as I32X4XN, which the user
defined types map onto. The dynamic types are also used explicitly
to create dynamic stack slots, which have no set size like their
existing counterparts. New IR instructions are added to access these
new stack entities.

Currently, during codegen, the dynamic scaling factor has to be
lowered to a constant so the dynamic slots do eventually have a
compile-time known size, as do spill slots.

The current lowering for aarch64 just targets Neon, using a dynamic
scale of 1.

Copyright (c) 2022, Arm Limited.

cranelift/codegen/src/ir/dfg.rs

@@ -3,12 +3,13 @@
 use crate::entity::{self, PrimaryMap, SecondaryMap};
 use crate::ir;
 use crate::ir::builder::ReplaceBuilder;
+use crate::ir::dynamic_type::{DynamicTypeData, DynamicTypes};
 use crate::ir::extfunc::ExtFuncData;
 use crate::ir::instructions::{BranchInfo, CallInfo, InstructionData};
 use crate::ir::{types, ConstantData, ConstantPool, Immediate};
 use crate::ir::{
-    Block, FuncRef, Inst, SigRef, Signature, SourceLoc, Type, Value, ValueLabelAssignments,
-    ValueList, ValueListPool,
+    Block, DynamicType, FuncRef, Inst, SigRef, Signature, SourceLoc, Type, Value,
+    ValueLabelAssignments, ValueList, ValueListPool,
 };
 use crate::packed_option::ReservedValue;
 use crate::write::write_operands;
@@ -50,6 +51,9 @@ pub struct DataFlowGraph {
     /// instructions contained in each block.
     blocks: PrimaryMap<Block, BlockData>,
 
+    /// Dynamic types created.
+    pub dynamic_types: DynamicTypes,
+
     /// Memory pool of value lists.
     ///
     /// The `ValueList` references into this pool appear in many places:
@@ -89,6 +93,7 @@ impl DataFlowGraph {
             insts: PrimaryMap::new(),
             results: SecondaryMap::new(),
             blocks: PrimaryMap::new(),
+            dynamic_types: DynamicTypes::new(),
             value_lists: ValueListPool::new(),
             values: PrimaryMap::new(),
             signatures: PrimaryMap::new(),
@@ -105,6 +110,7 @@ impl DataFlowGraph {
         self.insts.clear();
         self.results.clear();
         self.blocks.clear();
+        self.dynamic_types.clear();
         self.value_lists.clear();
         self.values.clear();
         self.signatures.clear();
@@ -557,6 +563,11 @@ impl DataFlowGraph {
         self.insts.push(data)
     }
 
+    /// Declares a dynamic vector type
+    pub fn make_dynamic_ty(&mut self, data: DynamicTypeData) -> DynamicType {
+        self.dynamic_types.push(data)
+    }
+
     /// Returns an object that displays `inst`.
     pub fn display_inst<'a>(&'a self, inst: Inst) -> DisplayInst<'a> {
         DisplayInst(self, inst)
@@ -1104,6 +1115,20 @@ impl DataFlowGraph {
         self.values[v].set_type(t);
     }
 
+    /// Check that the given concrete `Type` has been defined in the function.
+    pub fn check_dynamic_type(&mut self, ty: Type) -> Option<Type> {
+        debug_assert!(ty.is_dynamic_vector());
+        if self
+            .dynamic_types
+            .values()
+            .any(|dyn_ty_data| dyn_ty_data.concrete().unwrap() == ty)
+        {
+            Some(ty)
+        } else {
+            None
+        }
+    }
+
     /// Create result values for `inst`, reusing the provided detached values.
     /// This is similar to `make_inst_results_reusing` except it's only for use
     /// in the parser, which needs to reuse previously invalid values.
@@ -1130,6 +1155,10 @@ impl DataFlowGraph {
             let constraints = self.insts[inst].opcode().constraints();
             for res_idx in 0..constraints.num_fixed_results() {
                 let ty = constraints.result_type(res_idx, ctrl_typevar);
+                if ty.is_dynamic_vector() {
+                    self.check_dynamic_type(ty)
+                        .unwrap_or_else(|| panic!("Use of undeclared dynamic type: {}", ty));
+                }
                 if let Some(v) = reuse.get(res_idx) {
                     self.set_value_type_for_parser(*v, ty);
                 }