Simplify align_of_val::<[T]>(…) → align_of::<T>()

Author: scottmcm

compiler/rustc_mir_transform/src/instsimplify.rs

@@ -55,6 +55,7 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
 
             let terminator = block.terminator.as_mut().unwrap();
             ctx.simplify_primitive_clone(terminator, &mut block.statements);
+            ctx.simplify_align_of_slice_val(terminator, &mut block.statements);
             ctx.simplify_intrinsic_assert(terminator);
             ctx.simplify_nounwind_call(terminator);
             simplify_duplicate_switch_targets(terminator);
@@ -252,6 +253,36 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         terminator.kind = TerminatorKind::Goto { target: *destination_block };
     }
 
+    // Convert `align_of_val::<[T]>(ptr)` to `align_of::<T>()`, since the
+    // alignment of a slice doesn't actually depend on metadata at all
+    // and the element type is always `Sized`.
+    //
+    // This is here so it can run after inlining, where it's more useful.
+    // (LowerIntrinsics is done in cleanup, before the optimization passes.)
+    fn simplify_align_of_slice_val(
+        &self,
+        terminator: &mut Terminator<'tcx>,
+        statements: &mut Vec<Statement<'tcx>>,
+    ) {
+        if let TerminatorKind::Call {
+            func, args, destination, target: Some(destination_block), ..
+        } = &terminator.kind
+            && args.len() == 1
+            && let Some((fn_def_id, generics)) = func.const_fn_def()
+            && self.tcx.is_intrinsic(fn_def_id, sym::align_of_val)
+            && let ty::Slice(elem_ty) = *generics.type_at(0).kind()
+        {
+            statements.push(Statement::new(
+                terminator.source_info,
+                StatementKind::Assign(Box::new((
+                    *destination,
+                    Rvalue::NullaryOp(NullOp::AlignOf, elem_ty),
+                ))),
+            ));
+            terminator.kind = TerminatorKind::Goto { target: *destination_block };
+        }
+    }
+
     fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
         let TerminatorKind::Call { ref func, ref mut unwind, .. } = terminator.kind else {
             return;

tests/mir-opt/instsimplify/align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff

@@ -9,7 +9,9 @@
       bb0: {
           StorageLive(_2);
           _2 = &raw const (*_1);
-          _0 = std::intrinsics::align_of_val::<[T]>(move _2) -> [return: bb1, unwind unreachable];
+-         _0 = std::intrinsics::align_of_val::<[T]>(move _2) -> [return: bb1, unwind unreachable];
++         _0 = AlignOf(T);
++         goto -> bb1;
       }
 
       bb1: {

tests/mir-opt/instsimplify/align_of_slice.rs

@@ -7,7 +7,6 @@
 // EMIT_MIR align_of_slice.of_val_slice.InstSimplify-after-simplifycfg.diff
 pub fn of_val_slice<T>(slice: &[T]) -> usize {
     // CHECK-LABEL: fn of_val_slice(_1: &[T])
-    // CHECK: _2 = &raw const (*_1);
-    // CHECK: _0 = std::intrinsics::align_of_val::<[T]>(move _2)
+    // CHECK: _0 = AlignOf(T);
     unsafe { core::intrinsics::align_of_val(slice) }
 }