Reftypes part two: add support for stackmaps.

This commit adds support for generating stackmaps at safepoints to the
new backend framework and to the AArch64 backend in particular. It has
been tested to work with SpiderMonkey.

cranelift/codegen/src/machinst/buffer.rs

@@ -140,7 +140,7 @@
 //! Given these invariants, we argue why each optimization preserves execution
 //! semantics below (grep for "Preserves execution semantics").
 
-use crate::binemit::{Addend, CodeOffset, CodeSink, Reloc};
+use crate::binemit::{Addend, CodeOffset, CodeSink, Reloc, Stackmap};
 use crate::ir::{ExternalName, Opcode, SourceLoc, TrapCode};
 use crate::machinst::{BlockIndex, MachInstLabelUse, VCodeInst};
 
@@ -168,6 +168,8 @@ pub struct MachBuffer<I: VCodeInst> {
     call_sites: SmallVec<[MachCallSite; 16]>,
     /// Any source location mappings referring to this code.
     srclocs: SmallVec<[MachSrcLoc; 64]>,
+    /// Any stackmaps referring to this code.
+    stackmaps: SmallVec<[MachStackMap; 8]>,
     /// The current source location in progress (after `start_srcloc()` and
     /// before `end_srcloc()`).  This is a (start_offset, src_loc) tuple.
     cur_srcloc: Option<(CodeOffset, SourceLoc)>,
@@ -228,6 +230,8 @@ pub struct MachBufferFinalized {
     call_sites: SmallVec<[MachCallSite; 16]>,
     /// Any source location mappings referring to this code.
     srclocs: SmallVec<[MachSrcLoc; 64]>,
+    /// Any stackmaps referring to this code.
+    stackmaps: SmallVec<[MachStackMap; 8]>,
 }
 
 static UNKNOWN_LABEL_OFFSET: CodeOffset = 0xffff_ffff;
@@ -262,6 +266,7 @@ impl<I: VCodeInst> MachBuffer<I> {
             traps: SmallVec::new(),
             call_sites: SmallVec::new(),
             srclocs: SmallVec::new(),
+            stackmaps: SmallVec::new(),
             cur_srcloc: None,
             label_offsets: SmallVec::new(),
             label_aliases: SmallVec::new(),
@@ -1090,6 +1095,7 @@ impl<I: VCodeInst> MachBuffer<I> {
             traps: self.traps,
             call_sites: self.call_sites,
             srclocs: self.srclocs,
+            stackmaps: self.stackmaps,
         }
     }
 
@@ -1149,6 +1155,22 @@ impl<I: VCodeInst> MachBuffer<I> {
             self.srclocs.push(MachSrcLoc { start, end, loc });
         }
     }
+
+    /// Add stackmap metadata for this program point: a set of stack offsets
+    /// (from SP upward) that contain live references.
+    ///
+    /// The `offset_to_fp` value is the offset from the nominal SP (at which the
+    /// `stack_offsets` are based) and the FP value. By subtracting
+    /// `offset_to_fp` from each `stack_offsets` element, one can obtain
+    /// live-reference offsets from FP instead.
+    pub fn add_stackmap(&mut self, insn_len: CodeOffset, stackmap: Stackmap) {
+        let offset = self.cur_offset();
+        self.stackmaps.push(MachStackMap {
+            offset,
+            offset_end: offset + insn_len,
+            stackmap,
+        });
+    }
 }
 
 impl MachBufferFinalized {
@@ -1207,6 +1229,11 @@ impl MachBufferFinalized {
         sink.begin_rodata();
         sink.end_codegen();
     }
+
+    /// Get the stackmap metadata for this code.
+    pub fn stackmaps(&self) -> &[MachStackMap] {
+        &self.stackmaps[..]
+    }
 }
 
 /// A constant that is deferred to the next constant-pool opportunity.
@@ -1286,6 +1313,18 @@ pub struct MachSrcLoc {
     pub loc: SourceLoc,
 }
 
+/// Record of stackmap metadata: stack offsets containing references.
+#[derive(Clone, Debug)]
+pub struct MachStackMap {
+    /// The code offset at which this stackmap applies.
+    pub offset: CodeOffset,
+    /// The code offset at the *end* of the instruction at which this stackmap
+    /// applies.
+    pub offset_end: CodeOffset,
+    /// The Stackmap itself.
+    pub stackmap: Stackmap,
+}
+
 /// Record of branch instruction in the buffer, to facilitate editing.
 #[derive(Clone, Debug)]
 struct MachBranch {