From 7eea5d8d438bf0d65710410c0acc58e5b27018d7 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Fri, 10 Apr 2020 12:52:06 -0500
Subject: [PATCH] Optimize codegen in `Func::wrap` (#1491)

This commit optimizes the codegen of `Func::wrap` such that if you do
something like `Func::wrap(&store, || {})` then the shim generated
contains zero code (as expected). In general this means that the extra
tidbits generated by wasmtime are all eligible to be entirely optimized
away so long as you don't actually rely on something.
---
 crates/api/src/func.rs          | 8 ++++++--
 crates/runtime/src/instance.rs  | 2 ++
 crates/runtime/src/vmcontext.rs | 2 ++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/crates/api/src/func.rs b/crates/api/src/func.rs
index 24ff6f9c43..d70195946e 100644
--- a/crates/api/src/func.rs
+++ b/crates/api/src/func.rs
@@ -1030,8 +1030,12 @@ macro_rules! impl_into_func {
                     R: WasmRet,
                 {
                     let ret = {
-                        let instance = InstanceHandle::from_vmctx(vmctx);
-                        let (func, store) = instance.host_state().downcast_ref::<(F, Store)>().expect("state");
+                        let state = (*vmctx).host_state();
+                        // Double-check ourselves in debug mode, but we control
+                        // the `Any` here so an unsafe downcast should also
+                        // work.
+                        debug_assert!(state.is::<(F, Store)>());
+                        let (func, store) = &*(state as *const _ as *const (F, Store));
                         panic::catch_unwind(AssertUnwindSafe(|| {
                             func(
                                 Caller { store, caller_vmctx },
diff --git a/crates/runtime/src/instance.rs b/crates/runtime/src/instance.rs
index 1c5e6540f4..caa56b4eff 100644
--- a/crates/runtime/src/instance.rs
+++ b/crates/runtime/src/instance.rs
@@ -376,6 +376,7 @@ impl Instance {
     }
 
     /// Return a reference to the custom state attached to this instance.
+    #[inline]
     pub fn host_state(&self) -> &dyn Any {
         &*self.host_state
     }
@@ -416,6 +417,7 @@ impl Instance {
     }
 
     /// Return the offset from the vmctx pointer to its containing Instance.
+    #[inline]
     pub(crate) fn vmctx_offset() -> isize {
         offset_of!(Self, vmctx) as isize
     }
diff --git a/crates/runtime/src/vmcontext.rs b/crates/runtime/src/vmcontext.rs
index 23e6384bfc..c986c21718 100644
--- a/crates/runtime/src/vmcontext.rs
+++ b/crates/runtime/src/vmcontext.rs
@@ -632,6 +632,7 @@ impl VMContext {
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
     #[allow(clippy::cast_ptr_alignment)]
+    #[inline]
     pub(crate) unsafe fn instance(&self) -> &Instance {
         &*((self as *const Self as *mut u8).offset(-Instance::vmctx_offset()) as *const Instance)
     }
@@ -641,6 +642,7 @@ impl VMContext {
     /// # Safety
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
+    #[inline]
     pub unsafe fn host_state(&self) -> &dyn Any {
         self.instance().host_state()
     }