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/value_label.rs

@@ -1,13 +1,16 @@
 use crate::ir::{Function, SourceLoc, Value, ValueLabel, ValueLabelAssignments, ValueLoc};
 use crate::isa::TargetIsa;
+use crate::machinst::MachCompileResult;
 use crate::regalloc::{Context, RegDiversions};
 use crate::HashMap;
 use alloc::collections::BTreeMap;
 use alloc::vec::Vec;
 use core::cmp::Ordering;
+use core::convert::From;
 use core::iter::Iterator;
 use core::ops::Bound::*;
 use core::ops::Deref;
+use regalloc::Reg;
 
 #[cfg(feature = "enable-serde")]
 use serde::{Deserialize, Serialize};
@@ -17,13 +20,31 @@ use serde::{Deserialize, Serialize};
 #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
 pub struct ValueLocRange {
     /// The ValueLoc containing a ValueLabel during this range.
-    pub loc: ValueLoc,
+    pub loc: LabelValueLoc,
     /// The start of the range. It is an offset in the generated code.
     pub start: u32,
     /// The end of the range. It is an offset in the generated code.
     pub end: u32,
 }
 
+/// The particular location for a value.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
+pub enum LabelValueLoc {
+    /// Old-backend location: RegUnit, StackSlot, or Unassigned.
+    ValueLoc(ValueLoc),
+    /// New-backend Reg.
+    Reg(Reg),
+    /// New-backend offset from stack pointer.
+    SPOffset(i64),
+}
+
+impl From<ValueLoc> for LabelValueLoc {
+    fn from(v: ValueLoc) -> Self {
+        LabelValueLoc::ValueLoc(v)
+    }
+}
+
 /// Resulting map of Value labels and their ranges/locations.
 pub type ValueLabelsRanges = HashMap<ValueLabel, Vec<ValueLocRange>>;
 
@@ -86,14 +107,18 @@ where
 pub fn build_value_labels_ranges<T>(
     func: &Function,
     regalloc: &Context,
+    mach_compile_result: Option<&MachCompileResult>,
     isa: &dyn TargetIsa,
 ) -> ValueLabelsRanges
 where
     T: From<SourceLoc> + Deref<Target = SourceLoc> + Ord + Copy,
 {
-    // FIXME(#1523): New-style backend does not yet have debug info.
-    if isa.get_mach_backend().is_some() {
-        return HashMap::new();
+    if mach_compile_result.is_some() && mach_compile_result.unwrap().value_labels_ranges.is_some() {
+        return mach_compile_result
+            .unwrap()
+            .value_labels_ranges
+            .clone()
+            .unwrap();
     }
 
     let values_labels = build_value_labels_index::<T>(func);
@@ -113,7 +138,7 @@ where
             .entry(label)
             .or_insert_with(Vec::new)
             .push(ValueLocRange {
-                loc,
+                loc: loc.into(),
                 start: range.0,
                 end: range.1,
             });