[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/isa/s390x/mod.rs

@@ -1,7 +1,7 @@
 //! IBM Z 64-bit Instruction Set Architecture.
 
 use crate::ir::condcodes::IntCC;
-use crate::ir::Function;
+use crate::ir::{Function, Type};
 use crate::isa::s390x::settings as s390x_settings;
 #[cfg(feature = "unwind")]
 use crate::isa::unwind::systemv::RegisterMappingError;
@@ -58,7 +58,7 @@ impl S390xBackend {
         flags: shared_settings::Flags,
     ) -> CodegenResult<(VCode<inst::Inst>, regalloc2::Output)> {
         let emit_info = EmitInfo::new(flags.clone(), self.isa_flags.clone());
-        let abi = Box::new(abi::S390xABICallee::new(func, flags, self.isa_flags())?);
+        let abi = Box::new(abi::S390xABICallee::new(func, self)?);
         compile::compile::<S390xBackend>(func, self, abi, &self.machine_env, emit_info)
     }
 }
@@ -77,7 +77,8 @@ impl TargetIsa for S390xBackend {
         let frame_size = emit_result.frame_size;
         let value_labels_ranges = emit_result.value_labels_ranges;
         let buffer = emit_result.buffer.finish();
-        let stackslot_offsets = emit_result.stackslot_offsets;
+        let sized_stackslot_offsets = emit_result.sized_stackslot_offsets;
+        let dynamic_stackslot_offsets = emit_result.dynamic_stackslot_offsets;
 
         if let Some(disasm) = emit_result.disasm.as_ref() {
             log::debug!("disassembly:\n{}", disasm);
@@ -88,7 +89,8 @@ impl TargetIsa for S390xBackend {
             frame_size,
             disasm: emit_result.disasm,
             value_labels_ranges,
-            stackslot_offsets,
+            sized_stackslot_offsets,
+            dynamic_stackslot_offsets,
             bb_starts: emit_result.bb_offsets,
             bb_edges: emit_result.bb_edges,
         })
@@ -110,6 +112,10 @@ impl TargetIsa for S390xBackend {
         self.isa_flags.iter().collect()
     }
 
+    fn dynamic_vector_bytes(&self, _dyn_ty: Type) -> u32 {
+        16
+    }
+
     fn unsigned_add_overflow_condition(&self) -> IntCC {
         // The ADD LOGICAL family of instructions set the condition code
         // differently from normal comparisons, in a way that cannot be