ARM64 backend, part 1 / 11: misc changes to existing code.

- Add a `simple_legalize()` function that invokes a predetermined set of
  legalizations, without depending on the details of the current
  backend design. This will be used by the new backend pipeline.

- Separate out `has_side_effect()` from the DCE pass. This will be used
  by the new backends' lowering code.

- Add documentation for the `Arm64Call` relocation type.

cranelift/codegen/meta/src/isa/arm64/mod.rs

@@ -54,7 +54,9 @@ pub(crate) fn define(shared_defs: &mut SharedDefinitions) -> TargetIsa {
     let mut a64 = CpuMode::new("A64");
 
     // TODO refine these.
+    let expand_flags = shared_defs.transform_groups.by_name("expand_flags");
     let narrow_flags = shared_defs.transform_groups.by_name("narrow_flags");
+    a64.legalize_monomorphic(expand_flags);
     a64.legalize_default(narrow_flags);
 
     let cpu_modes = vec![a64];

cranelift/codegen/src/binemit/mod.rs

@@ -54,7 +54,9 @@ pub enum Reloc {
     X86GOTPCRel4,
     /// Arm32 call target
     Arm32Call,
-    /// Arm64 call target
+    /// Arm64 call target. Encoded as bottom 26 bits of instruction. This
+    /// value is sign-extended, multiplied by 4, and added to the PC of
+    /// the call instruction to form the destination address.
     Arm64Call,
     /// RISC-V call target
     RiscvCall,

cranelift/codegen/src/dce.rs

@@ -40,6 +40,14 @@ fn is_load_with_defined_trapping(opcode: Opcode, data: &InstructionData) -> bool
     }
 }
 
+/// Does the given instruction have any side-effect that would preclude it from being removed when
+/// its value is unused?
+pub fn has_side_effect(func: &Function, inst: Inst) -> bool {
+    let data = &func.dfg[inst];
+    let opcode = data.opcode();
+    trivially_unsafe_for_dce(opcode) || is_load_with_defined_trapping(opcode, data)
+}
+
 /// Perform DCE on `func`.
 pub fn do_dce(func: &mut Function, domtree: &mut DominatorTree) {
     let _tt = timing::dce();
@@ -50,10 +58,7 @@ pub fn do_dce(func: &mut Function, domtree: &mut DominatorTree) {
         let mut pos = FuncCursor::new(func).at_bottom(block);
         while let Some(inst) = pos.prev_inst() {
             {
-                let data = &pos.func.dfg[inst];
+                if has_side_effect(pos.func, inst)
-                let opcode = data.opcode();
-                if trivially_unsafe_for_dce(opcode)
-                    || is_load_with_defined_trapping(opcode, &data)
                     || any_inst_results_used(inst, &live, &pos.func.dfg)
                 {
                     for arg in pos.func.dfg.inst_args(inst) {

cranelift/codegen/src/ir/immediates.rs

@@ -57,6 +57,11 @@ impl Imm64 {
     pub fn wrapping_neg(self) -> Self {
         Self(self.0.wrapping_neg())
     }
+
+    /// Return bits of this immediate.
+    pub fn bits(&self) -> i64 {
+        self.0
+    }
 }
 
 impl Into<i64> for Imm64 {

cranelift/codegen/src/legalizer/mod.rs

@@ -196,6 +196,55 @@ pub fn legalize_function(func: &mut ir::Function, cfg: &mut ControlFlowGraph, is
     }
 }
 
+/// Perform a simple legalization by expansion of the function, without
+/// platform-specific transforms.
+pub fn simple_legalize(func: &mut ir::Function, cfg: &mut ControlFlowGraph, isa: &dyn TargetIsa) {
+    let mut pos = FuncCursor::new(func);
+    let func_begin = pos.position();
+    pos.set_position(func_begin);
+    while let Some(_block) = pos.next_block() {
+        let mut prev_pos = pos.position();
+        while let Some(inst) = pos.next_inst() {
+            let expanded = match pos.func.dfg[inst].opcode() {
+                ir::Opcode::BrIcmp
+                | ir::Opcode::GlobalValue
+                | ir::Opcode::HeapAddr
+                | ir::Opcode::StackLoad
+                | ir::Opcode::StackStore
+                | ir::Opcode::TableAddr
+                | ir::Opcode::Trapnz
+                | ir::Opcode::Trapz
+                | ir::Opcode::BandImm
+                | ir::Opcode::BorImm
+                | ir::Opcode::BxorImm
+                | ir::Opcode::IaddImm
+                | ir::Opcode::IfcmpImm
+                | ir::Opcode::ImulImm
+                | ir::Opcode::IrsubImm
+                | ir::Opcode::IshlImm
+                | ir::Opcode::RotlImm
+                | ir::Opcode::RotrImm
+                | ir::Opcode::SdivImm
+                | ir::Opcode::SremImm
+                | ir::Opcode::SshrImm
+                | ir::Opcode::UdivImm
+                | ir::Opcode::UremImm
+                | ir::Opcode::UshrImm
+                | ir::Opcode::IcmpImm => expand(inst, &mut pos.func, cfg, isa),
+                _ => false,
+            };
+
+            if expanded {
+                // Legalization implementations require fixpoint loop
+                // here. TODO: fix this.
+                pos.set_position(prev_pos);
+            } else {
+                prev_pos = pos.position();
+            }
+        }
+    }
+}
+
 // Include legalization patterns that were generated by `gen_legalizer.rs` from the
 // `TransformGroup` in `cranelift-codegen/meta/shared/legalize.rs`.
 //