Detailed debug-info (DWARF) support in new backends (initially x64).

This PR propagates "value labels" all the way from CLIF to DWARF
metadata on the emitted machine code. The key idea is as follows:

- Translate value-label metadata on the input into "value_label"
  pseudo-instructions when lowering into VCode. These
  pseudo-instructions take a register as input, denote a value label,
  and semantically are like a "move into value label" -- i.e., they
  update the current value (as seen by debugging tools) of the given
  local. These pseudo-instructions emit no machine code.

- Perform a dataflow analysis *at the machine-code level*, tracking
  value-labels that propagate into registers and into [SP+constant]
  stack storage. This is a forward dataflow fixpoint analysis where each
  storage location can contain a *set* of value labels, and each value
  label can reside in a *set* of storage locations. (Meet function is
  pairwise intersection by storage location.)

  This analysis traces value labels symbolically through loads and
  stores and reg-to-reg moves, so it will naturally handle spills and
  reloads without knowing anything special about them.

- When this analysis converges, we have, at each machine-code offset, a
  mapping from value labels to some number of storage locations; for
  each offset for each label, we choose the best location (prefer
  registers). Note that we can choose any location, as the symbolic
  dataflow analysis is sound and guarantees that the value at the
  value_label instruction propagates to all of the named locations.

- Then we can convert this mapping into a format that the DWARF
  generation code (wasmtime's debug crate) can use.

This PR also adds the new-backend variant to the gdb tests on CI.

cranelift/codegen/src/isa/x64/mod.rs

@@ -4,13 +4,14 @@ use self::inst::EmitInfo;
 
 use super::TargetIsa;
 use crate::ir::{condcodes::IntCC, Function};
+use crate::isa::unwind::systemv::RegisterMappingError;
 use crate::isa::x64::{inst::regs::create_reg_universe_systemv, settings as x64_settings};
 use crate::isa::Builder as IsaBuilder;
 use crate::machinst::{compile, MachBackend, MachCompileResult, TargetIsaAdapter, VCode};
 use crate::result::CodegenResult;
 use crate::settings::{self as shared_settings, Flags};
 use alloc::boxed::Box;
-use regalloc::{PrettyPrint, RealRegUniverse};
+use regalloc::{PrettyPrint, RealRegUniverse, Reg};
 use target_lexicon::Triple;
 
 mod abi;
@@ -60,6 +61,7 @@ impl MachBackend for X64Backend {
         let buffer = buffer.finish();
         let frame_size = vcode.frame_size();
         let unwind_info = vcode.unwind_info()?;
+        let value_labels_ranges = vcode.value_labels_ranges()?;
 
         let disasm = if want_disasm {
             Some(vcode.show_rru(Some(&create_reg_universe_systemv(flags))))
@@ -72,6 +74,7 @@ impl MachBackend for X64Backend {
             frame_size,
             disasm,
             unwind_info,
+            value_labels_ranges,
         })
     }
 
@@ -127,6 +130,11 @@ impl MachBackend for X64Backend {
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
         Some(inst::unwind::systemv::create_cie())
     }
+
+    #[cfg(feature = "unwind")]
+    fn map_reg_to_dwarf(&self, reg: Reg) -> Result<u16, RegisterMappingError> {
+        inst::unwind::systemv::map_reg(reg).map(|reg| reg.0)
+    }
 }
 
 /// Create a new `isa::Builder`.