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?

Author: matthiaskrgr

src/intrinsics/atomic.rs

@@ -1,4 +1,5 @@
 use rustc_middle::mir::BinOp;
+use rustc_middle::ty::AtomicOrdering;
 use rustc_middle::{mir, ty};
 
 use self::helpers::check_intrinsic_arg_count;
@@ -19,6 +20,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
     fn emulate_atomic_intrinsic(
         &mut self,
         intrinsic_name: &str,
+        generic_args: ty::GenericArgsRef<'tcx>,
         args: &[OpTy<'tcx>],
         dest: &MPlaceTy<'tcx>,
     ) -> InterpResult<'tcx, EmulateItemResult> {
@@ -35,6 +37,15 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
         }
 
+        fn read_ord_const_generic(o: AtomicOrdering) -> AtomicReadOrd {
+            match o {
+                AtomicOrdering::SeqCst => AtomicReadOrd::SeqCst,
+                AtomicOrdering::Acquire => AtomicReadOrd::Acquire,
+                AtomicOrdering::Relaxed => AtomicReadOrd::Relaxed,
+                _ => panic!("invalid read ordering `{o:?}`"),
+            }
+        }
+
         fn write_ord(ord: &str) -> AtomicWriteOrd {
             match ord {
                 "seqcst" => AtomicWriteOrd::SeqCst,
@@ -66,7 +77,15 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         }
 
         match &*intrinsic_structure {
-            ["load", ord] => this.atomic_load(args, dest, read_ord(ord))?,
+            // New-style intrinsics that use const generics
+            ["load"] => {
+                let ordering = generic_args.const_at(1).to_value();
+                let ordering =
+                    ordering.valtree.unwrap_branch()[0].unwrap_leaf().to_atomic_ordering();
+                this.atomic_load(args, dest, read_ord_const_generic(ordering))?;
+            }
+
+            // Old-style intrinsics that have the ordering in the intrinsic name
             ["store", ord] => this.atomic_store(args, write_ord(ord))?,
 
             ["fence", ord] => this.atomic_fence_intrinsic(args, fence_ord(ord))?,

src/intrinsics/mod.rs

@@ -97,7 +97,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         let this = self.eval_context_mut();
 
         if let Some(name) = intrinsic_name.strip_prefix("atomic_") {
-            return this.emulate_atomic_intrinsic(name, args, dest);
+            return this.emulate_atomic_intrinsic(name, generic_args, args, dest);
         }
         if let Some(name) = intrinsic_name.strip_prefix("simd_") {
             return this.emulate_simd_intrinsic(name, generic_args, args, dest);

tests/fail/unaligned_pointers/atomic_unaligned.rs

@@ -1,13 +1,14 @@
 //@compile-flags: -Zmiri-symbolic-alignment-check -Cdebug-assertions=no
 #![feature(core_intrinsics)]
+use std::intrinsics;
 
 fn main() {
     // Do a 4-aligned u64 atomic access. That should be UB on all platforms,
     // even if u64 only has alignment 4.
     let z = [0u32; 2];
     let zptr = &z as *const _ as *const u64;
     unsafe {
-        ::std::intrinsics::atomic_load_seqcst(zptr);
+        intrinsics::atomic_load::<_, { intrinsics::AtomicOrdering::SeqCst }>(zptr);
         //~^ERROR: accessing memory with alignment 4, but alignment 8 is required
     }
 }

tests/fail/unaligned_pointers/atomic_unaligned.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
   --> tests/fail/unaligned_pointers/atomic_unaligned.rs:LL:CC
    |
-LL |         ::std::intrinsics::atomic_load_seqcst(zptr);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+LL |         intrinsics::atomic_load::<_, { intrinsics::AtomicOrdering::SeqCst }>(zptr);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
    |
    = help: this usually indicates that your program performed an invalid operation and caused Undefined Behavior
    = help: but due to `-Zmiri-symbolic-alignment-check`, alignment errors can also be false positives