Rollup merge of #141507 - RalfJung:atomic-intrinsics, r=bjorn3

atomic_load intrinsic: use const generic parameter for ordering

We have a gazillion intrinsics for the atomics because we encode the ordering into the intrinsic name rather than making it a parameter. This is particularly bad for those operations that take two orderings. Let's fix that!

This PR only converts `load`, to see if there's any feedback that would fundamentally change the strategy we pursue for the const generic intrinsics.

The first two commits are preparation and could be a separate PR if you prefer.

`@BoxyUwU` -- I hope this is a use of const generics that is unlikely to explode? All we need is a const generic of enum type. We could funnel it through an integer if we had to but an enum is obviously nicer...

`@bjorn3` it seems like the cranelift backend entirely ignores the ordering?

2 files changed, 35 insertions, 3 deletions

diff --git a/compiler/rustc_middle/src/ty/consts/int.rs b/compiler/rustc_middle/src/ty/consts/int.rs
index 9c9cd695339..b087ae25486 100644
--- a/compiler/rustc_middle/src/ty/consts/int.rs
+++ b/compiler/rustc_middle/src/ty/consts/int.rs
@@ -26,6 +26,19 @@ impl ConstInt {
     }
 }
 
+/// An enum to represent the compiler-side view of `intrinsics::AtomicOrdering`.
+/// This lives here because there's a method in this file that needs it and it is entirely unclear
+/// where else to put this...
+#[derive(Debug, Copy, Clone)]
+pub enum AtomicOrdering {
+    // These values must match `intrinsics::AtomicOrdering`!
+    Relaxed = 0,
+    Release = 1,
+    Acquire = 2,
+    AcqRel = 3,
+    SeqCst = 4,
+}
+
 impl std::fmt::Debug for ConstInt {
     fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let Self { int, signed, is_ptr_sized_integral } = *self;
@@ -318,6 +331,25 @@ impl ScalarInt {
         self.to_uint(tcx.data_layout.pointer_size).try_into().unwrap()
     }
 
+    #[inline]
+    pub fn to_atomic_ordering(self) -> AtomicOrdering {
+        use AtomicOrdering::*;
+        let val = self.to_u32();
+        if val == Relaxed as u32 {
+            Relaxed
+        } else if val == Release as u32 {
+            Release
+        } else if val == Acquire as u32 {
+            Acquire
+        } else if val == AcqRel as u32 {
+            AcqRel
+        } else if val == SeqCst as u32 {
+            SeqCst
+        } else {
+            panic!("not a valid atomic ordering")
+        }
+    }
+
     /// Converts the `ScalarInt` to `bool`.
     /// Panics if the `size` of the `ScalarInt` is not equal to 1 byte.
     /// Errors if it is not a valid `bool`.
@@ -488,7 +520,7 @@ from_scalar_int_for_x_signed!(i8, i16, i32, i64, i128);
 impl From<std::cmp::Ordering> for ScalarInt {
     #[inline]
     fn from(c: std::cmp::Ordering) -> Self {
-        // Here we rely on `Ordering` having the same values in host and target!
+        // Here we rely on `cmp::Ordering` having the same values in host and target!
         ScalarInt::from(c as i8)
     }
 }
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 78c0812b08f..af31f7ed33b 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -74,8 +74,8 @@ pub use self::closure::{
     place_to_string_for_capture,
 };
 pub use self::consts::{
-    AnonConstKind, Const, ConstInt, ConstKind, Expr, ExprKind, ScalarInt, UnevaluatedConst,
-    ValTree, ValTreeKind, Value,
+    AnonConstKind, AtomicOrdering, Const, ConstInt, ConstKind, Expr, ExprKind, ScalarInt,
+    UnevaluatedConst, ValTree, ValTreeKind, Value,
 };
 pub use self::context::{
     CtxtInterners, CurrentGcx, DeducedParamAttrs, Feed, FreeRegionInfo, GlobalCtxt, Lift, TyCtxt,