From cff25e4147d46e4adbe5c032a36f197f6eaf38ea Mon Sep 17 00:00:00 2001
From: Stiopa Koltsov <nga@meta.com>
Date: Wed, 6 Sep 2023 13:19:49 -0700
Subject: [PATCH] Fix the issue with TypeId alignment on new Rust Apple Silicon

Summary:
* `TypeId` is `u128` [in Rust 1.72](https://github.com/rust-lang/rust/pull/109953/)
* `u128` is 16 bytes aligned on Apple Silicon
* we require 8 byte alignment for `StarlarkValue`

Maybe we should fix heap allocations: insert padding to allow any alignment.

But practically we don't need it and should not do it: generated machine code is identical for aligned and 8-byte aligned `u128`: [compiler explorer](https://rust.godbolt.org/z/96KsnjW77) (note it is identical if we load it from memory not pass by value, but this is what we do).

So for now just force 8 byte alignment on `TypeId`.

Fixes issue https://github.com/facebook/buck2/issues/408

Reviewed By: ndmitchell, shayne-fletcher

Differential Revision: D49019964

fbshipit-source-id: 62c8d7db83e9e4bdbe58fe7a62d5daae41996e61
---
 .../starlark/src/values/starlark_type_id.rs   | 30 +++++++++++++++++++
 .../values/typing/type_compiled/matchers.rs   |  7 +++--
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/starlark-rust/starlark/src/values/starlark_type_id.rs b/starlark-rust/starlark/src/values/starlark_type_id.rs
index abc441057426..4ba5c7c170f6 100644
--- a/starlark-rust/starlark/src/values/starlark_type_id.rs
+++ b/starlark-rust/starlark/src/values/starlark_type_id.rs
@@ -47,3 +47,33 @@ impl StarlarkTypeId {
         StarlarkTypeId(ConstTypeId::of::<T::StaticType>())
     }
 }
+
+/// We require alignment 8 for `StarlarkValue`.
+/// `TypeId` is 16 bytes aligned on Rust 1.72 on Apple Silicon.
+/// Use this struct to put `ConstTypeId` in a `StarlarkValue`.
+// TODO(nga): remove alignment requirement from `Heap`.
+#[repr(packed(8))]
+#[derive(Allocative, Eq, Clone, Copy, Dupe, Debug)]
+#[allocative(skip)] // There are no heap allocations in this struct.
+pub(crate) struct StarlarkTypeIdAligned {
+    starlark_type_id: StarlarkTypeId,
+}
+
+impl PartialEq for StarlarkTypeIdAligned {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.get() == other.get()
+    }
+}
+
+impl StarlarkTypeIdAligned {
+    #[inline]
+    pub(crate) const fn new(starlark_type_id: StarlarkTypeId) -> StarlarkTypeIdAligned {
+        StarlarkTypeIdAligned { starlark_type_id }
+    }
+
+    #[inline]
+    pub(crate) const fn get(&self) -> StarlarkTypeId {
+        self.starlark_type_id
+    }
+}
diff --git a/starlark-rust/starlark/src/values/typing/type_compiled/matchers.rs b/starlark-rust/starlark/src/values/typing/type_compiled/matchers.rs
index 7288ef490216..7a8ea5712208 100644
--- a/starlark-rust/starlark/src/values/typing/type_compiled/matchers.rs
+++ b/starlark-rust/starlark/src/values/typing/type_compiled/matchers.rs
@@ -27,6 +27,7 @@ use crate::values::dict::DictRef;
 use crate::values::list::value::FrozenList;
 use crate::values::list::ListRef;
 use crate::values::starlark_type_id::StarlarkTypeId;
+use crate::values::starlark_type_id::StarlarkTypeIdAligned;
 use crate::values::tuple::value::Tuple;
 use crate::values::types::int_or_big::StarlarkIntRef;
 use crate::values::typing::type_compiled::matcher::TypeMatcher;
@@ -245,20 +246,20 @@ impl TypeMatcher for IsNone {
 
 #[derive(Allocative, Debug, Clone)]
 pub(crate) struct StarlarkTypeIdMatcher {
-    starlark_type_id: StarlarkTypeId,
+    starlark_type_id: StarlarkTypeIdAligned,
 }
 
 impl StarlarkTypeIdMatcher {
     pub(crate) fn new(ty: TyStarlarkValue) -> StarlarkTypeIdMatcher {
         StarlarkTypeIdMatcher {
-            starlark_type_id: ty.starlark_type_id(),
+            starlark_type_id: StarlarkTypeIdAligned::new(ty.starlark_type_id()),
         }
     }
 }
 
 impl TypeMatcher for StarlarkTypeIdMatcher {
     fn matches(&self, value: Value) -> bool {
-        value.starlark_type_id() == self.starlark_type_id
+        value.starlark_type_id() == self.starlark_type_id.get()
     }
 }