Auto merge of #152689 - scottmcm:also-simplify-of-sized-val-alt, r=<try>

bors · bors · commit 85c13679ed60 · 2026-02-16T02:10:29.000Z
Simplify `size/align_of_val&lt;T: Sized&gt;` to `size/align_of&lt;T&gt;` instead
diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs
@@ -56,7 +56,7 @@ 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_size_or_align_of_val(terminator, &mut block.statements);
             ctx.simplify_intrinsic_assert(terminator);
             ctx.simplify_nounwind_call(terminator);
             simplify_duplicate_switch_targets(terminator);
@@ -246,13 +246,18 @@ 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(
+    /// Simplify `size_of_val` and `align_of_val` if we don't actually need
+    /// to look at the value in order to calculate the result:
+    /// - For `Sized` types we can always do this for both,
+    /// - For `align_of_val::<[T]>` we can return `align_of::<T>()`, since it
+    ///   doesn't depend on the slice's length and the elements are sized.
+    ///
+    /// This is here so it can run after inlining, where it's more useful.
+    /// (LowerIntrinsics is done in cleanup, before the optimization passes.)
+    ///
+    /// Note that we intentionally just produce the lang item constants so this
+    /// works on generic types and avoids any risk of layout calculation cycles.
+    fn simplify_size_or_align_of_val(
         &self,
         terminator: &mut Terminator<'tcx>,
         statements: &mut Vec<Statement<'tcx>>,
@@ -263,19 +268,35 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
         } = &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()
         {
-            let align_def_id = self.tcx.require_lang_item(LangItem::AlignOf, source_info.span);
-            let align_const = Operand::unevaluated_constant(
+            let lang_item = if self.tcx.is_intrinsic(fn_def_id, sym::size_of_val) {
+                LangItem::SizeOf
+            } else if self.tcx.is_intrinsic(fn_def_id, sym::align_of_val) {
+                LangItem::AlignOf
+            } else {
+                return;
+            };
+            let generic_ty = generics.type_at(0);
+            let ty = if generic_ty.is_sized(self.tcx, self.typing_env) {
+                generic_ty
+            } else if let LangItem::AlignOf = lang_item
+                && let ty::Slice(elem_ty) = *generic_ty.kind()
+            {
+                elem_ty
+            } else {
+                return;
+            };
+
+            let const_def_id = self.tcx.require_lang_item(lang_item, source_info.span);
+            let const_op = Operand::unevaluated_constant(
                 self.tcx,
-                align_def_id,
-                &[elem_ty.into()],
+                const_def_id,
+                &[ty.into()],
                 source_info.span,
             );
             statements.push(Statement::new(
                 source_info,
-                StatementKind::Assign(Box::new((*destination, Rvalue::Use(align_const)))),
+                StatementKind::Assign(Box::new((*destination, Rvalue::Use(const_op)))),
             ));
             terminator.kind = TerminatorKind::Goto { target: *destination_block };
         }
diff --git a/tests/mir-opt/instsimplify/align_or_size_of_sized_val.align_of_val_sized.InstSimplify-after-simplifycfg.diff b/tests/mir-opt/instsimplify/align_or_size_of_sized_val.align_of_val_sized.InstSimplify-after-simplifycfg.diff
@@ -0,0 +1,22 @@
+- // MIR for `align_of_val_sized` before InstSimplify-after-simplifycfg
++ // MIR for `align_of_val_sized` after InstSimplify-after-simplifycfg
+  
+  fn align_of_val_sized(_1: &T) -> usize {
+      debug val => _1;
+      let mut _0: usize;
+      let mut _2: *const T;
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = &raw const (*_1);
+-         _0 = std::intrinsics::align_of_val::<T>(move _2) -> [return: bb1, unwind unreachable];
++         _0 = const <T as std::mem::SizedTypeProperties>::ALIGN;
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/instsimplify/align_or_size_of_sized_val.rs b/tests/mir-opt/instsimplify/align_or_size_of_sized_val.rs
@@ -0,0 +1,19 @@
+//@ test-mir-pass: InstSimplify-after-simplifycfg
+//@ needs-unwind
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+// EMIT_MIR align_or_size_of_sized_val.align_of_val_sized.InstSimplify-after-simplifycfg.diff
+pub fn align_of_val_sized<T>(val: &T) -> usize {
+    // CHECK-LABEL: fn align_of_val_sized
+    // CHECK: _0 = const <T as std::mem::SizedTypeProperties>::ALIGN;
+    unsafe { core::intrinsics::align_of_val(val) }
+}
+
+// EMIT_MIR align_or_size_of_sized_val.size_of_val_sized.InstSimplify-after-simplifycfg.diff
+pub fn size_of_val_sized<T>(val: &T) -> usize {
+    // CHECK-LABEL: fn size_of_val_sized
+    // CHECK: _0 = const <T as std::mem::SizedTypeProperties>::SIZE;
+    unsafe { core::intrinsics::size_of_val(val) }
+}
diff --git a/tests/mir-opt/instsimplify/align_or_size_of_sized_val.size_of_val_sized.InstSimplify-after-simplifycfg.diff b/tests/mir-opt/instsimplify/align_or_size_of_sized_val.size_of_val_sized.InstSimplify-after-simplifycfg.diff
@@ -0,0 +1,22 @@
+- // MIR for `size_of_val_sized` before InstSimplify-after-simplifycfg
++ // MIR for `size_of_val_sized` after InstSimplify-after-simplifycfg
+  
+  fn size_of_val_sized(_1: &T) -> usize {
+      debug val => _1;
+      let mut _0: usize;
+      let mut _2: *const T;
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = &raw const (*_1);
+-         _0 = std::intrinsics::size_of_val::<T>(move _2) -> [return: bb1, unwind unreachable];
++         _0 = const <T as std::mem::SizedTypeProperties>::SIZE;
++         goto -> bb1;
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_bytes.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_bytes.PreCodegen.after.panic-abort.mir
@@ -0,0 +1,81 @@
+// MIR for `drop_bytes` after PreCodegen
+
+fn drop_bytes(_1: *mut Box<[u8; 1024]>) -> () {
+    debug x => _1;
+    let mut _0: ();
+    scope 1 (inlined drop_in_place::<Box<[u8; 1024]>> - shim(Some(Box<[u8; 1024]>))) {
+        scope 2 (inlined <Box<[u8; 1024]> as Drop>::drop) {
+            let _2: std::ptr::NonNull<[u8; 1024]>;
+            let _4: ();
+            scope 3 {
+                scope 4 {
+                    scope 17 (inlined Layout::size) {
+                    }
+                    scope 18 (inlined std::ptr::Unique::<[u8; 1024]>::cast::<u8>) {
+                        scope 19 (inlined NonNull::<[u8; 1024]>::cast::<u8>) {
+                            scope 20 (inlined NonNull::<[u8; 1024]>::as_ptr) {
+                            }
+                        }
+                    }
+                    scope 21 (inlined <NonNull<u8> as From<std::ptr::Unique<u8>>>::from) {
+                        scope 22 (inlined std::ptr::Unique::<u8>::as_non_null_ptr) {
+                        }
+                    }
+                    scope 23 (inlined <std::alloc::Global as Allocator>::deallocate) {
+                        scope 24 (inlined std::alloc::Global::deallocate_impl) {
+                            scope 25 (inlined std::alloc::Global::deallocate_impl_runtime) {
+                                let mut _3: *mut u8;
+                                scope 26 (inlined Layout::size) {
+                                }
+                                scope 27 (inlined NonNull::<u8>::as_ptr) {
+                                }
+                                scope 28 (inlined std::alloc::dealloc) {
+                                    scope 29 (inlined Layout::size) {
+                                    }
+                                    scope 30 (inlined Layout::alignment) {
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+                scope 5 (inlined std::ptr::Unique::<[u8; 1024]>::as_ptr) {
+                    scope 6 (inlined NonNull::<[u8; 1024]>::as_ptr) {
+                    }
+                }
+                scope 7 (inlined Layout::for_value_raw::<[u8; 1024]>) {
+                    scope 8 {
+                        scope 16 (inlined #[track_caller] Layout::from_size_alignment_unchecked) {
+                        }
+                    }
+                    scope 9 (inlined size_of_val_raw::<[u8; 1024]>) {
+                    }
+                    scope 10 (inlined std::ptr::Alignment::of_val_raw::<[u8; 1024]>) {
+                        scope 11 {
+                            scope 13 (inlined #[track_caller] std::ptr::Alignment::new_unchecked) {
+                                scope 14 (inlined core::ub_checks::check_language_ub) {
+                                    scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
+                                    }
+                                }
+                            }
+                        }
+                        scope 12 (inlined align_of_val_raw::<[u8; 1024]>) {
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    bb0: {
+        _2 = copy (((*_1).0: std::ptr::Unique<[u8; 1024]>).0: std::ptr::NonNull<[u8; 1024]>);
+        StorageLive(_3);
+        _3 = copy _2 as *mut u8 (Transmute);
+        _4 = alloc::alloc::__rust_dealloc(move _3, const 1024_usize, const std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }}) -> [return: bb1, unwind unreachable];
+    }
+
+    bb1: {
+        StorageDead(_3);
+        return;
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_bytes.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_bytes.PreCodegen.after.panic-unwind.mir
@@ -0,0 +1,81 @@
+// MIR for `drop_bytes` after PreCodegen
+
+fn drop_bytes(_1: *mut Box<[u8; 1024]>) -> () {
+    debug x => _1;
+    let mut _0: ();
+    scope 1 (inlined drop_in_place::<Box<[u8; 1024]>> - shim(Some(Box<[u8; 1024]>))) {
+        scope 2 (inlined <Box<[u8; 1024]> as Drop>::drop) {
+            let _2: std::ptr::NonNull<[u8; 1024]>;
+            let _4: ();
+            scope 3 {
+                scope 4 {
+                    scope 17 (inlined Layout::size) {
+                    }
+                    scope 18 (inlined std::ptr::Unique::<[u8; 1024]>::cast::<u8>) {
+                        scope 19 (inlined NonNull::<[u8; 1024]>::cast::<u8>) {
+                            scope 20 (inlined NonNull::<[u8; 1024]>::as_ptr) {
+                            }
+                        }
+                    }
+                    scope 21 (inlined <NonNull<u8> as From<std::ptr::Unique<u8>>>::from) {
+                        scope 22 (inlined std::ptr::Unique::<u8>::as_non_null_ptr) {
+                        }
+                    }
+                    scope 23 (inlined <std::alloc::Global as Allocator>::deallocate) {
+                        scope 24 (inlined std::alloc::Global::deallocate_impl) {
+                            scope 25 (inlined std::alloc::Global::deallocate_impl_runtime) {
+                                let mut _3: *mut u8;
+                                scope 26 (inlined Layout::size) {
+                                }
+                                scope 27 (inlined NonNull::<u8>::as_ptr) {
+                                }
+                                scope 28 (inlined std::alloc::dealloc) {
+                                    scope 29 (inlined Layout::size) {
+                                    }
+                                    scope 30 (inlined Layout::alignment) {
+                                    }
+                                }
+                            }
+                        }
+                    }
+                }
+                scope 5 (inlined std::ptr::Unique::<[u8; 1024]>::as_ptr) {
+                    scope 6 (inlined NonNull::<[u8; 1024]>::as_ptr) {
+                    }
+                }
+                scope 7 (inlined Layout::for_value_raw::<[u8; 1024]>) {
+                    scope 8 {
+                        scope 16 (inlined #[track_caller] Layout::from_size_alignment_unchecked) {
+                        }
+                    }
+                    scope 9 (inlined size_of_val_raw::<[u8; 1024]>) {
+                    }
+                    scope 10 (inlined std::ptr::Alignment::of_val_raw::<[u8; 1024]>) {
+                        scope 11 {
+                            scope 13 (inlined #[track_caller] std::ptr::Alignment::new_unchecked) {
+                                scope 14 (inlined core::ub_checks::check_language_ub) {
+                                    scope 15 (inlined core::ub_checks::check_language_ub::runtime) {
+                                    }
+                                }
+                            }
+                        }
+                        scope 12 (inlined align_of_val_raw::<[u8; 1024]>) {
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    bb0: {
+        _2 = copy (((*_1).0: std::ptr::Unique<[u8; 1024]>).0: std::ptr::NonNull<[u8; 1024]>);
+        StorageLive(_3);
+        _3 = copy _2 as *mut u8 (Transmute);
+        _4 = alloc::alloc::__rust_dealloc(move _3, const 1024_usize, const std::ptr::Alignment {{ _inner_repr_trick: std::ptr::alignment::AlignmentEnum::_Align1Shl0 }}) -> [return: bb1, unwind unreachable];
+    }
+
+    bb1: {
+        StorageDead(_3);
+        return;
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_generic.PreCodegen.after.panic-abort.mir b/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_generic.PreCodegen.after.panic-abort.mir
diff --git a/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_generic.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/pre-codegen/drop_box_of_sized.drop_generic.PreCodegen.after.panic-unwind.mir
diff --git a/tests/mir-opt/pre-codegen/drop_box_of_sized.rs b/tests/mir-opt/pre-codegen/drop_box_of_sized.rs
