Auto merge of #87254 - rusticstuff:rustc_codegen_llvm_dont_emit_zero_sized_padding, r=eddyb

LLVM codegen: Don't emit zero-sized padding for fields

Currently padding is emitted before fields of a struct and at the end of the struct regardless of the ABI. Even if no padding is required zero-sized padding fields are emitted. This is not useful and - more importantly - it make it impossible to generate the exact vector types that LLVM expects for certain ARM SIMD intrinsics. This change should unblock the implementation of many ARM intrinsics using the `unadjusted` ABI, see https://github.com/rust-lang/stdarch/issues/1143#issuecomment-827404092.

This is a proof of concept only because the field lookup now takes O(number of fields) time compared to O(1) before since it recalculates the mapping at every lookup. I would like to find out how big the performance impact actually is before implementing caching or restricting this behavior to the `unadjusted` ABI.

cc `@SparrowLii` `@bjorn3`

([Discussion on internals](https://internals.rust-lang.org/t/feature-request-add-a-way-in-rustc-for-generating-struct-type-llvm-ir-without-paddings/15007))

compiler/rustc_codegen_llvm/src/context.rs

@@ -24,6 +24,7 @@ use rustc_span::source_map::{Span, DUMMY_SP};
 use rustc_span::symbol::Symbol;
 use rustc_target::abi::{HasDataLayout, LayoutOf, PointeeInfo, Size, TargetDataLayout, VariantIdx};
 use rustc_target::spec::{HasTargetSpec, RelocModel, Target, TlsModel};
+use smallvec::SmallVec;
 
 use std::cell::{Cell, RefCell};
 use std::ffi::CStr;
@@ -74,8 +75,12 @@ pub struct CodegenCx<'ll, 'tcx> {
     /// See <https://llvm.org/docs/LangRef.html#the-llvm-used-global-variable> for details
     pub used_statics: RefCell<Vec<&'ll Value>>,
 
-    pub lltypes: RefCell<FxHashMap<(Ty<'tcx>, Option<VariantIdx>), &'ll Type>>,
+    /// Mapping of non-scalar types to llvm types and field remapping if needed.
+    pub type_lowering: RefCell<FxHashMap<(Ty<'tcx>, Option<VariantIdx>), TypeLowering<'ll>>>,
+
+    /// Mapping of scalar types to llvm types.
     pub scalar_lltypes: RefCell<FxHashMap<Ty<'tcx>, &'ll Type>>,
+
     pub pointee_infos: RefCell<FxHashMap<(Ty<'tcx>, Size), Option<PointeeInfo>>>,
     pub isize_ty: &'ll Type,
 
@@ -92,6 +97,15 @@ pub struct CodegenCx<'ll, 'tcx> {
     local_gen_sym_counter: Cell<usize>,
 }
 
+pub struct TypeLowering<'ll> {
+    /// Associated LLVM type
+    pub lltype: &'ll Type,
+
+    /// If padding is used the slice maps fields from source order
+    /// to llvm order.
+    pub field_remapping: Option<SmallVec<[u32; 4]>>,
+}
+
 fn to_llvm_tls_model(tls_model: TlsModel) -> llvm::ThreadLocalMode {
     match tls_model {
         TlsModel::GeneralDynamic => llvm::ThreadLocalMode::GeneralDynamic,
@@ -304,7 +318,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
             const_globals: Default::default(),
             statics_to_rauw: RefCell::new(Vec::new()),
             used_statics: RefCell::new(Vec::new()),
-            lltypes: Default::default(),
+            type_lowering: Default::default(),
             scalar_lltypes: Default::default(),
             pointee_infos: Default::default(),
             isize_ty,

compiler/rustc_codegen_llvm/src/type_.rs

@@ -266,7 +266,7 @@ impl LayoutTypeMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         layout.is_llvm_scalar_pair()
     }
     fn backend_field_index(&self, layout: TyAndLayout<'tcx>, index: usize) -> u64 {
-        layout.llvm_field_index(index)
+        layout.llvm_field_index(self, index)
     }
     fn scalar_pair_element_backend_type(
         &self,

compiler/rustc_codegen_llvm/src/type_of.rs

@@ -1,5 +1,6 @@
 use crate::abi::FnAbi;
 use crate::common::*;
+use crate::context::TypeLowering;
 use crate::type_::Type;
 use rustc_codegen_ssa::traits::*;
 use rustc_middle::bug;
@@ -9,6 +10,7 @@ use rustc_middle::ty::{self, Ty, TypeFoldable};
 use rustc_target::abi::{Abi, AddressSpace, Align, FieldsShape};
 use rustc_target::abi::{Int, Pointer, F32, F64};
 use rustc_target::abi::{LayoutOf, PointeeInfo, Scalar, Size, TyAndLayoutMethods, Variants};
+use smallvec::{smallvec, SmallVec};
 use tracing::debug;
 
 use std::fmt::Write;
@@ -17,6 +19,7 @@ fn uncached_llvm_type<'a, 'tcx>(
     cx: &CodegenCx<'a, 'tcx>,
     layout: TyAndLayout<'tcx>,
     defer: &mut Option<(&'a Type, TyAndLayout<'tcx>)>,
+    field_remapping: &mut Option<SmallVec<[u32; 4]>>,
 ) -> &'a Type {
     match layout.abi {
         Abi::Scalar(_) => bug!("handled elsewhere"),
@@ -75,7 +78,8 @@ fn uncached_llvm_type<'a, 'tcx>(
         FieldsShape::Array { count, .. } => cx.type_array(layout.field(cx, 0).llvm_type(cx), count),
         FieldsShape::Arbitrary { .. } => match name {
             None => {
-                let (llfields, packed) = struct_llfields(cx, layout);
+                let (llfields, packed, new_field_remapping) = struct_llfields(cx, layout);
+                *field_remapping = new_field_remapping;
                 cx.type_struct(&llfields, packed)
             }
             Some(ref name) => {
@@ -90,7 +94,7 @@ fn uncached_llvm_type<'a, 'tcx>(
 fn struct_llfields<'a, 'tcx>(
     cx: &CodegenCx<'a, 'tcx>,
     layout: TyAndLayout<'tcx>,
-) -> (Vec<&'a Type>, bool) {
+) -> (Vec<&'a Type>, bool, Option<SmallVec<[u32; 4]>>) {
     debug!("struct_llfields: {:#?}", layout);
     let field_count = layout.fields.count();
 
@@ -98,6 +102,7 @@ fn struct_llfields<'a, 'tcx>(
     let mut offset = Size::ZERO;
     let mut prev_effective_align = layout.align.abi;
     let mut result: Vec<_> = Vec::with_capacity(1 + field_count * 2);
+    let mut field_remapping = smallvec![0; field_count];
     for i in layout.fields.index_by_increasing_offset() {
         let target_offset = layout.fields.offset(i as usize);
         let field = layout.field(cx, i);
@@ -116,33 +121,37 @@ fn struct_llfields<'a, 'tcx>(
         );
         assert!(target_offset >= offset);
         let padding = target_offset - offset;
-        let padding_align = prev_effective_align.min(effective_field_align);
-        assert_eq!(offset.align_to(padding_align) + padding, target_offset);
-        result.push(cx.type_padding_filler(padding, padding_align));
-        debug!("    padding before: {:?}", padding);
-
+        if padding != Size::ZERO {
+            let padding_align = prev_effective_align.min(effective_field_align);
+            assert_eq!(offset.align_to(padding_align) + padding, target_offset);
+            result.push(cx.type_padding_filler(padding, padding_align));
+            debug!("    padding before: {:?}", padding);
+        }
+        field_remapping[i] = result.len() as u32;
         result.push(field.llvm_type(cx));
         offset = target_offset + field.size;
         prev_effective_align = effective_field_align;
     }
+    let padding_used = result.len() > field_count;
     if !layout.is_unsized() && field_count > 0 {
         if offset > layout.size {
             bug!("layout: {:#?} stride: {:?} offset: {:?}", layout, layout.size, offset);
         }
         let padding = layout.size - offset;
-        let padding_align = prev_effective_align;
-        assert_eq!(offset.align_to(padding_align) + padding, layout.size);
-        debug!(
-            "struct_llfields: pad_bytes: {:?} offset: {:?} stride: {:?}",
-            padding, offset, layout.size
-        );
-        result.push(cx.type_padding_filler(padding, padding_align));
-        assert_eq!(result.len(), 1 + field_count * 2);
+        if padding != Size::ZERO {
+            let padding_align = prev_effective_align;
+            assert_eq!(offset.align_to(padding_align) + padding, layout.size);
+            debug!(
+                "struct_llfields: pad_bytes: {:?} offset: {:?} stride: {:?}",
+                padding, offset, layout.size
+            );
+            result.push(cx.type_padding_filler(padding, padding_align));
+        }
     } else {
         debug!("struct_llfields: offset: {:?} stride: {:?}", offset, layout.size);
     }
-
-    (result, packed)
+    let field_remapping = if padding_used { Some(field_remapping) } else { None };
+    (result, packed, field_remapping)
 }
 
 impl<'a, 'tcx> CodegenCx<'a, 'tcx> {
@@ -177,7 +186,7 @@ pub trait LayoutLlvmExt<'tcx> {
         index: usize,
         immediate: bool,
     ) -> &'a Type;
-    fn llvm_field_index(&self, index: usize) -> u64;
+    fn llvm_field_index<'a>(&self, cx: &CodegenCx<'a, 'tcx>, index: usize) -> u64;
     fn pointee_info_at<'a>(&self, cx: &CodegenCx<'a, 'tcx>, offset: Size) -> Option<PointeeInfo>;
 }
 
@@ -234,8 +243,8 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
             Variants::Single { index } => Some(index),
             _ => None,
         };
-        if let Some(&llty) = cx.lltypes.borrow().get(&(self.ty, variant_index)) {
-            return llty;
+        if let Some(ref llty) = cx.type_lowering.borrow().get(&(self.ty, variant_index)) {
+            return llty.lltype;
         }
 
         debug!("llvm_type({:#?})", self);
@@ -247,6 +256,7 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
         let normal_ty = cx.tcx.erase_regions(self.ty);
 
         let mut defer = None;
+        let mut field_remapping = None;
         let llty = if self.ty != normal_ty {
             let mut layout = cx.layout_of(normal_ty);
             if let Some(v) = variant_index {
@@ -254,17 +264,24 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
             }
             layout.llvm_type(cx)
         } else {
-            uncached_llvm_type(cx, *self, &mut defer)
+            uncached_llvm_type(cx, *self, &mut defer, &mut field_remapping)
         };
         debug!("--> mapped {:#?} to llty={:?}", self, llty);
 
-        cx.lltypes.borrow_mut().insert((self.ty, variant_index), llty);
+        cx.type_lowering.borrow_mut().insert(
+            (self.ty, variant_index),
+            TypeLowering { lltype: llty, field_remapping: field_remapping },
+        );
 
         if let Some((llty, layout)) = defer {
-            let (llfields, packed) = struct_llfields(cx, layout);
-            cx.set_struct_body(llty, &llfields, packed)
+            let (llfields, packed, new_field_remapping) = struct_llfields(cx, layout);
+            cx.set_struct_body(llty, &llfields, packed);
+            cx.type_lowering
+                .borrow_mut()
+                .get_mut(&(self.ty, variant_index))
+                .unwrap()
+                .field_remapping = new_field_remapping;
         }
-
         llty
     }
 
@@ -340,7 +357,7 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
         self.scalar_llvm_type_at(cx, scalar, offset)
     }
 
-    fn llvm_field_index(&self, index: usize) -> u64 {
+    fn llvm_field_index<'a>(&self, cx: &CodegenCx<'a, 'tcx>, index: usize) -> u64 {
         match self.abi {
             Abi::Scalar(_) | Abi::ScalarPair(..) => {
                 bug!("TyAndLayout::llvm_field_index({:?}): not applicable", self)
@@ -354,7 +371,25 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
 
             FieldsShape::Array { .. } => index as u64,
 
-            FieldsShape::Arbitrary { .. } => 1 + (self.fields.memory_index(index) as u64) * 2,
+            FieldsShape::Arbitrary { .. } => {
+                let variant_index = match self.variants {
+                    Variants::Single { index } => Some(index),
+                    _ => None,
+                };
+
+                // Look up llvm field if indexes do not match memory order due to padding. If
+                // `field_remapping` is `None` no padding was used and the llvm field index
+                // matches the memory index.
+                match cx.type_lowering.borrow().get(&(self.ty, variant_index)) {
+                    Some(TypeLowering { field_remapping: Some(ref remap), .. }) => {
+                        remap[index] as u64
+                    }
+                    Some(_) => self.fields.memory_index(index) as u64,
+                    None => {
+                        bug!("TyAndLayout::llvm_field_index({:?}): type info not found", self)
+                    }
+                }
+            }
         }
     }
 

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -98,7 +98,8 @@ fn emit_aapcs_va_arg(
     // Implementation of the AAPCS64 calling convention for va_args see
     // https://github.com/ARM-software/abi-aa/blob/master/aapcs64/aapcs64.rst
     let va_list_addr = list.immediate();
-    let va_list_ty = list.deref(bx.cx).layout.llvm_type(bx);
+    let va_list_layout = list.deref(bx.cx).layout;
+    let va_list_ty = va_list_layout.llvm_type(bx);
     let layout = bx.cx.layout_of(target_ty);
 
     let mut maybe_reg = bx.build_sibling_block("va_arg.maybe_reg");
@@ -110,13 +111,15 @@ fn emit_aapcs_va_arg(
 
     let gr_type = target_ty.is_any_ptr() || target_ty.is_integral();
     let (reg_off, reg_top_index, slot_size) = if gr_type {
-        let gr_offs = bx.struct_gep(va_list_ty, va_list_addr, 7);
+        let gr_offs =
+            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 3));
         let nreg = (layout.size.bytes() + 7) / 8;
-        (gr_offs, 3, nreg * 8)
+        (gr_offs, va_list_layout.llvm_field_index(bx.cx, 1), nreg * 8)
     } else {
-        let vr_off = bx.struct_gep(va_list_ty, va_list_addr, 9);
+        let vr_off =
+            bx.struct_gep(va_list_ty, va_list_addr, va_list_layout.llvm_field_index(bx.cx, 4));
         let nreg = (layout.size.bytes() + 15) / 16;
-        (vr_off, 5, nreg * 16)
+        (vr_off, va_list_layout.llvm_field_index(bx.cx, 2), nreg * 16)
     };
 
     // if the offset >= 0 then the value will be on the stack

src/test/codegen/align-enum.rs

@@ -8,7 +8,7 @@ pub enum Align64 {
     A(u32),
     B(u32),
 }
-// CHECK: %Align64 = type { [0 x i32], i32, [15 x i32] }
+// CHECK: %Align64 = type { i32, [15 x i32] }
 
 pub struct Nested64 {
     a: u8,

src/test/codegen/align-struct.rs

@@ -5,7 +5,7 @@
 
 #[repr(align(64))]
 pub struct Align64(i32);
-// CHECK: %Align64 = type { [0 x i32], i32, [15 x i32] }
+// CHECK: %Align64 = type { i32, [15 x i32] }
 
 pub struct Nested64 {
     a: Align64,
@@ -13,20 +13,20 @@ pub struct Nested64 {
     c: i32,
     d: i8,
 }
-// CHECK: %Nested64 = type { [0 x i64], %Align64, [0 x i32], i32, [0 x i32], i32, [0 x i8], i8, [55 x i8] }
+// CHECK: %Nested64 = type { %Align64, i32, i32, i8, [55 x i8] }
 
 pub enum Enum4 {
     A(i32),
     B(i32),
 }
-// CHECK: %"Enum4::A" = type { [1 x i32], i32, [0 x i32] }
+// CHECK: %"Enum4::A" = type { [1 x i32], i32 }
 
 pub enum Enum64 {
     A(Align64),
     B(i32),
 }
-// CHECK: %Enum64 = type { [0 x i32], i32, [31 x i32] }
-// CHECK: %"Enum64::A" = type { [8 x i64], %Align64, [0 x i64] }
+// CHECK: %Enum64 = type { i32, [31 x i32] }
+// CHECK: %"Enum64::A" = type { [8 x i64], %Align64 }
 
 // CHECK-LABEL: @align64
 #[no_mangle]

src/test/codegen/unpadded-simd.rs

@@ -0,0 +1,14 @@
+// Make sure that no 0-sized padding is inserted in structs and that
+// structs are represented as expected by Neon intrinsics in LLVM.
+// See #87254.
+
+#![crate_type = "lib"]
+#![feature(repr_simd)]
+
+#[derive(Copy, Clone, Debug)]
+#[repr(simd)]
+pub struct int16x4_t(pub i16, pub i16, pub i16, pub i16);
+
+#[derive(Copy, Clone, Debug)]
+pub struct int16x4x2_t(pub int16x4_t, pub int16x4_t);
+// CHECK: %int16x4x2_t = type { <4 x i16>, <4 x i16> }