Replace the global-exports mechanism with a caller-vmctx mechanism. (#789)

* Replace the global-exports mechanism with a caller-vmctx mechanism.

This eliminates the global exports mechanism, and instead adds a
caller-vmctx argument to wasm functions so that WASI can obtain the
memory and other things from the caller rather than looking them up in a
global registry.

This replaces #390.

* Fixup some merge conflicts

* Rustfmt

* Ensure VMContext is aligned to 16 bytes

With the removal of `global_exports` it "just so happens" that this
isn't happening naturally any more.

* Fixup some bugs with double vmctx in wasmtime crate

* Trampoline stub needed adjusting
* Use pointer type instead of always using I64 for caller vmctx
* Don't store `ir::Signature` in `Func` since we don't know the pointer
  size at creation time.
* Skip the first 2 arguments in IR signatures since that's the two vmctx
  parameters.

* Update cranelift to 0.56.0

* Handle more merge conflicts

* Rustfmt

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/environ/Cargo.toml

@@ -13,9 +13,9 @@ edition = "2018"
 
 [dependencies]
 anyhow = "1.0"
-cranelift-codegen = { version = "0.55", features = ["enable-serde"] }
-cranelift-entity = { version = "0.55", features = ["enable-serde"] }
-cranelift-wasm = { version = "0.55", features = ["enable-serde"] }
+cranelift-codegen = { version = "0.56", features = ["enable-serde"] }
+cranelift-entity = { version = "0.56", features = ["enable-serde"] }
+cranelift-wasm = { version = "0.56", features = ["enable-serde"] }
 wasmparser = "0.47.0"
 lightbeam = { path = "../lightbeam", optional = true, version = "0.9.0" }
 indexmap = "1.0.2"
@@ -46,7 +46,7 @@ tempfile = "3"
 target-lexicon = { version = "0.10.0", default-features = false }
 pretty_env_logger = "0.3.0"
 rand = { version = "0.7.0", default-features = false, features = ["small_rng"] }
-cranelift-codegen = { version = "0.55", features = ["enable-serde", "all-arch"] }
+cranelift-codegen = { version = "0.56", features = ["enable-serde", "all-arch"] }
 filetime = "0.2.7"
 
 [badges]

crates/environ/src/func_environ.rs

@@ -362,8 +362,10 @@ impl<'module_environment> TargetEnvironment for FuncEnvironment<'module_environm
 }
 
 impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'module_environment> {
-    fn is_wasm_parameter(&self, signature: &ir::Signature, index: usize) -> bool {
-        signature.params[index].purpose == ir::ArgumentPurpose::Normal
+    fn is_wasm_parameter(&self, _signature: &ir::Signature, index: usize) -> bool {
+        // The first two parameters are the vmctx and caller vmctx. The rest are
+        // the wasm parameters.
+        index >= 2
     }
 
     fn make_table(&mut self, func: &mut ir::Function, index: TableIndex) -> WasmResult<ir::Table> {
@@ -615,7 +617,8 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
             }
         }
 
-        let mut real_call_args = Vec::with_capacity(call_args.len() + 1);
+        let mut real_call_args = Vec::with_capacity(call_args.len() + 2);
+        let caller_vmctx = pos.func.special_param(ArgumentPurpose::VMContext).unwrap();
 
         // First append the callee vmctx address.
         let vmctx = pos.ins().load(
@@ -625,6 +628,7 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
             i32::from(self.offsets.vmcaller_checked_anyfunc_vmctx()),
         );
         real_call_args.push(vmctx);
+        real_call_args.push(caller_vmctx);
 
         // Then append the regular call arguments.
         real_call_args.extend_from_slice(call_args);
@@ -639,12 +643,17 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
         callee: ir::FuncRef,
         call_args: &[ir::Value],
     ) -> WasmResult<ir::Inst> {
-        let mut real_call_args = Vec::with_capacity(call_args.len() + 1);
+        let mut real_call_args = Vec::with_capacity(call_args.len() + 2);
+        let caller_vmctx = pos.func.special_param(ArgumentPurpose::VMContext).unwrap();
 
         // Handle direct calls to locally-defined functions.
         if !self.module.is_imported_function(callee_index) {
-            // First append the callee vmctx address.
-            real_call_args.push(pos.func.special_param(ArgumentPurpose::VMContext).unwrap());
+            // First append the callee vmctx address, which is the same as the caller vmctx in
+            // this case.
+            real_call_args.push(caller_vmctx);
+
+            // Then append the caller vmctx address.
+            real_call_args.push(caller_vmctx);
 
             // Then append the regular call arguments.
             real_call_args.extend_from_slice(call_args);
@@ -671,6 +680,7 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
             i32::try_from(self.offsets.vmctx_vmfunction_import_vmctx(callee_index)).unwrap();
         let vmctx = pos.ins().load(pointer_type, mem_flags, base, vmctx_offset);
         real_call_args.push(vmctx);
+        real_call_args.push(caller_vmctx);
 
         // Then append the regular call arguments.
         real_call_args.extend_from_slice(call_args);

crates/environ/src/module_environ.rs

@@ -383,6 +383,8 @@ pub fn translate_signature(mut sig: ir::Signature, pointer_type: ir::Type) -> ir
         0,
         AbiParam::special(pointer_type, ArgumentPurpose::VMContext),
     );
+    // Prepend the caller vmctx argument.
+    sig.params.insert(1, AbiParam::new(pointer_type));
     sig
 }