compiler: BackendRepr::inherent_{size,align} -> scalar_{size,align}

This pair of fn was introduced to perform invariant checks for scalars.
Their current behavior doesn't mesh as well with checking SIMD types,
so change the name of the fn to reflect their actual use-case and
refactor the corresponding checks.

Also simplify the returns from Option<AbiAndPrefAlign> to Option<Align>,
because every site was mapping away the "preferred" alignment anyways.

Author: workingjubilee

compiler/rustc_abi/src/layout.rs

@@ -308,10 +308,10 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         let mut align = if repr.pack.is_some() { dl.i8_align } else { dl.aggregate_align };
         let mut max_repr_align = repr.align;
 
-        // If all the non-ZST fields have the same ABI and union ABI optimizations aren't
-        // disabled, we can use that common ABI for the union as a whole.
+        // If all the non-ZST fields have the same repr and union repr optimizations aren't
+        // disabled, we can use that common repr for the union as a whole.
         struct AbiMismatch;
-        let mut common_non_zst_abi_and_align = if repr.inhibits_union_abi_opt() {
+        let mut common_non_zst_repr_and_align = if repr.inhibits_union_abi_opt() {
             // Can't optimize
             Err(AbiMismatch)
         } else {
@@ -335,14 +335,14 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 continue;
             }
 
-            if let Ok(common) = common_non_zst_abi_and_align {
+            if let Ok(common) = common_non_zst_repr_and_align {
                 // Discard valid range information and allow undef
                 let field_abi = field.backend_repr.to_union();
 
                 if let Some((common_abi, common_align)) = common {
                     if common_abi != field_abi {
                         // Different fields have different ABI: disable opt
-                        common_non_zst_abi_and_align = Err(AbiMismatch);
+                        common_non_zst_repr_and_align = Err(AbiMismatch);
                     } else {
                         // Fields with the same non-Aggregate ABI should also
                         // have the same alignment
@@ -355,7 +355,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                     }
                 } else {
                     // First non-ZST field: record its ABI and alignment
-                    common_non_zst_abi_and_align = Ok(Some((field_abi, field.align.abi)));
+                    common_non_zst_repr_and_align = Ok(Some((field_abi, field.align.abi)));
                 }
             }
         }
@@ -374,16 +374,26 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
 
         // If all non-ZST fields have the same ABI, we may forward that ABI
         // for the union as a whole, unless otherwise inhibited.
-        let abi = match common_non_zst_abi_and_align {
+        let backend_repr = match common_non_zst_repr_and_align {
             Err(AbiMismatch) | Ok(None) => BackendRepr::Memory { sized: true },
-            Ok(Some((abi, _))) => {
-                if abi.inherent_align(dl).map(|a| a.abi) != Some(align.abi) {
-                    // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
+            Ok(Some((repr, _))) => match repr {
+                // Mismatched alignment (e.g. union is #[repr(packed)]): disable opt
+                BackendRepr::Scalar(_) | BackendRepr::ScalarPair(_, _)
+                    if repr.scalar_align(dl).unwrap() != align.abi =>
+                {
                     BackendRepr::Memory { sized: true }
-                } else {
-                    abi
                 }
-            }
+                // Vectors require at least element alignment, else disable the opt
+                BackendRepr::Vector { element, count: _ } if element.align(dl).abi > align.abi => {
+                    BackendRepr::Memory { sized: true }
+                }
+                // the alignment tests passed and we can use this
+                BackendRepr::Scalar(..)
+                | BackendRepr::ScalarPair(..)
+                | BackendRepr::Vector { .. }
+                | BackendRepr::Memory { .. } => repr,
+                BackendRepr::Uninhabited { .. } => BackendRepr::Memory { sized: true },
+            },
         };
 
         let Some(union_field_count) = NonZeroUsize::new(only_variant.len()) else {
@@ -398,7 +408,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         Ok(LayoutData {
             variants: Variants::Single { index: only_variant_idx },
             fields: FieldsShape::Union(union_field_count),
-            backend_repr: abi,
+            backend_repr,
             largest_niche: None,
             align,
             size: size.align_to(align.abi),

compiler/rustc_abi/src/lib.rs

@@ -1462,37 +1462,42 @@ impl BackendRepr {
         matches!(*self, BackendRepr::Scalar(s) if s.is_bool())
     }
 
-    /// Returns the fixed alignment of this ABI, if any is mandated.
-    pub fn inherent_align<C: HasDataLayout>(&self, cx: &C) -> Option<AbiAndPrefAlign> {
-        Some(match *self {
-            BackendRepr::Scalar(s) => s.align(cx),
-            BackendRepr::ScalarPair(s1, s2) => s1.align(cx).max(s2.align(cx)),
-            BackendRepr::Vector { element, count } => {
-                cx.data_layout().vector_align(element.size(cx) * count)
-            }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
-        })
+    /// The psABI alignment for a `Scalar` or `ScalarPair`
+    ///
+    /// `None` for other variants.
+    pub fn scalar_align<C: HasDataLayout>(&self, cx: &C) -> Option<Align> {
+        match *self {
+            BackendRepr::Scalar(s) => Some(s.align(cx).abi),
+            BackendRepr::ScalarPair(s1, s2) => Some(s1.align(cx).max(s2.align(cx)).abi),
+            // The align of a Vector can vary in surprising ways
+            BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => None,
+            // FIXME: rebased away soon
+            BackendRepr::Uninhabited => None,
+        }
     }
 
-    /// Returns the fixed size of this ABI, if any is mandated.
-    pub fn inherent_size<C: HasDataLayout>(&self, cx: &C) -> Option<Size> {
-        Some(match *self {
-            BackendRepr::Scalar(s) => {
-                // No padding in scalars.
-                s.size(cx)
-            }
+    /// The psABI size for a `Scalar` or `ScalarPair`
+    ///
+    /// `None` for other variants
+    pub fn scalar_size<C: HasDataLayout>(&self, cx: &C) -> Option<Size> {
+        match *self {
+            // No padding in scalars.
+            BackendRepr::Scalar(s) => Some(s.size(cx)),
+            // May have some padding between the pair.
             BackendRepr::ScalarPair(s1, s2) => {
-                // May have some padding between the pair.
                 let field2_offset = s1.size(cx).align_to(s2.align(cx).abi);
-                (field2_offset + s2.size(cx)).align_to(self.inherent_align(cx)?.abi)
-            }
-            BackendRepr::Vector { element, count } => {
-                // No padding in vectors, except possibly for trailing padding
-                // to make the size a multiple of align (e.g. for vectors of size 3).
-                (element.size(cx) * count).align_to(self.inherent_align(cx)?.abi)
+                let size = (field2_offset + s2.size(cx)).align_to(
+                    self.scalar_align(cx)
+                        // We absolutely must have an answer here or everything is FUBAR.
+                        .unwrap(),
+                );
+                Some(size)
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
-        })
+            // The size of a Vector can vary in surprising ways
+            BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => None,
+            // FIXME: rebased away soon
+            BackendRepr::Uninhabited => None,
+        }
     }
 
     /// Discard validity range information and allow undef.

compiler/rustc_ty_utils/src/layout/invariant.rs

@@ -65,31 +65,30 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
     }
 
     fn check_layout_abi<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayout<'tcx>) {
-        // Verify the ABI mandated alignment and size.
-        let align = layout.backend_repr.inherent_align(cx).map(|align| align.abi);
-        let size = layout.backend_repr.inherent_size(cx);
-        let Some((align, size)) = align.zip(size) else {
-            assert_matches!(
-                layout.layout.backend_repr(),
-                BackendRepr::Uninhabited | BackendRepr::Memory { .. },
-                "ABI unexpectedly missing alignment and/or size in {layout:#?}"
+        // Verify the ABI-mandated alignment and size for scalars.
+        let align = layout.backend_repr.scalar_align(cx);
+        let size = layout.backend_repr.scalar_size(cx);
+        if let Some(align) = align {
+            assert_eq!(
+                layout.layout.align().abi,
+                align,
+                "alignment mismatch between ABI and layout in {layout:#?}"
             );
-            return;
-        };
-        assert_eq!(
-            layout.layout.align().abi,
-            align,
-            "alignment mismatch between ABI and layout in {layout:#?}"
-        );
-        assert_eq!(
-            layout.layout.size(),
-            size,
-            "size mismatch between ABI and layout in {layout:#?}"
-        );
+        }
+        if let Some(size) = size {
+            assert_eq!(
+                layout.layout.size(),
+                size,
+                "size mismatch between ABI and layout in {layout:#?}"
+            );
+        }
 
         // Verify per-ABI invariants
         match layout.layout.backend_repr() {
             BackendRepr::Scalar(_) => {
+                // we already asserted this
+                let align = align.unwrap();
+                let size = size.unwrap();
                 // Check that this matches the underlying field.
                 let inner = skip_newtypes(cx, layout);
                 assert!(
@@ -231,9 +230,15 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     "`ScalarPair` second field with bad ABI in {inner:#?}",
                 );
             }
-            BackendRepr::Vector { element, .. } => {
-                assert!(align >= element.align(cx).abi); // just sanity-checking `vector_align`.
-                // FIXME: Do some kind of check of the inner type, like for Scalar and ScalarPair.
+            BackendRepr::Vector { element, count } => {
+                let align = layout.align.abi;
+                let size = layout.size;
+                let element_align = element.align(cx).abi;
+                let element_size = element.size(cx);
+                // Currently, vectors must always be aligned to at least their elements:
+                assert!(align >= element_align);
+                // And the size has to be element * count, of course
+                assert!(size >= (element_size * count).align_to(align));
             }
             BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {} // Nothing to check.
         }