Fix and enable GC on new x64 backend.

One critical bit of plumbing was missing: the `StackMapSink` passed to
`compile_and_emit` was not actually receiving stackmaps. This seemingly
very basic issue was not caught because the other major user of reftype
support, SpiderMonkey, extracts stackmaps with a lower-level API. The
SM integration was built this way to avoid an awkward API quirk when
passing stackmaps through a `CodeSink` that proxies them to a
`StackMapSink`: the `CodeSink` wants `Value`s for each reference slot,
while the actual `StackMapSink` does not require these. This PR tweaks
the plumbing in a slightly different way to make `wasmtime` GC tests,
and presumably other consumers of stack-map info from the top-level
Cranelift interface, happy.

cranelift/codegen/src/context.rs

@@ -22,7 +22,7 @@ use crate::legalize_function;
 use crate::legalizer::simple_legalize;
 use crate::licm::do_licm;
 use crate::loop_analysis::LoopAnalysis;
-use crate::machinst::MachCompileResult;
+use crate::machinst::{MachCompileResult, MachStackMap};
 use crate::nan_canonicalization::do_nan_canonicalization;
 use crate::postopt::do_postopt;
 use crate::redundant_reload_remover::RedundantReloadRemover;
@@ -239,10 +239,23 @@ impl Context {
         let mut sink = MemoryCodeSink::new(mem, relocs, traps, stack_maps);
         if let Some(ref result) = &self.mach_compile_result {
             result.buffer.emit(&mut sink);
+            let info = sink.info;
+            // New backends do not emit StackMaps through the `CodeSink` because its interface
+            // requires `Value`s; instead, the `StackMap` objects are directly accessible via
+            // `result.buffer.stack_maps()`.
+            for &MachStackMap {
+                offset_end,
+                ref stack_map,
+                ..
+            } in result.buffer.stack_maps()
+            {
+                stack_maps.add_stack_map(offset_end, stack_map.clone());
+            }
+            info
         } else {
             isa.emit_function_to_memory(&self.func, &mut sink);
+            sink.info
         }
-        sink.info
     }
 
     /// Creates unwind information for the function.

tests/all/gc.rs

@@ -24,7 +24,6 @@ impl Drop for GcOnDrop {
 }
 
 #[test]
-#[cfg_attr(feature = "experimental_x64", ignore)] // TODO #2079 investigate.
 fn smoke_test_gc() -> anyhow::Result<()> {
     let (store, module) = ref_types_module(
         r#"
@@ -121,7 +120,6 @@ fn wasm_dropping_refs() -> anyhow::Result<()> {
 }
 
 #[test]
-#[cfg_attr(feature = "experimental_x64", ignore)] // TODO #2079 investigate.
 fn many_live_refs() -> anyhow::Result<()> {
     let mut wat = r#"
         (module
@@ -375,7 +373,6 @@ fn gc_during_gc_when_passing_refs_into_wasm() -> anyhow::Result<()> {
 }
 
 #[test]
-#[cfg_attr(feature = "experimental_x64", ignore)] // TODO #2079 investigate.
 fn gc_during_gc_from_many_table_gets() -> anyhow::Result<()> {
     let (store, module) = ref_types_module(
         r#"