Use `load`-`store` instead of `memcpy` for short integer arrays

2 files changed, 36 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/type_.rs b/compiler/rustc_codegen_llvm/src/type_.rs
index d3fad5699c8..4ffa2b9c6a3 100644
--- a/compiler/rustc_codegen_llvm/src/type_.rs
+++ b/compiler/rustc_codegen_llvm/src/type_.rs
@@ -288,6 +288,9 @@ impl<'ll, 'tcx> LayoutTypeMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     fn reg_backend_type(&self, ty: &Reg) -> &'ll Type {
         ty.llvm_type(self)
     }
+    fn scalar_copy_backend_type(&self, layout: TyAndLayout<'tcx>) -> Option<Self::Type> {
+        layout.scalar_copy_llvm_type(self)
+    }
 }
 
 impl<'ll, 'tcx> TypeMembershipMethods<'tcx> for CodegenCx<'ll, 'tcx> {
diff --git a/compiler/rustc_codegen_llvm/src/type_of.rs b/compiler/rustc_codegen_llvm/src/type_of.rs
index a493c9c0548..3339e4e07ed 100644
--- a/compiler/rustc_codegen_llvm/src/type_of.rs
+++ b/compiler/rustc_codegen_llvm/src/type_of.rs
@@ -6,6 +6,7 @@ use rustc_middle::bug;
 use rustc_middle::ty::layout::{FnAbiOf, LayoutOf, TyAndLayout};
 use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
+use rustc_target::abi::HasDataLayout;
 use rustc_target::abi::{Abi, Align, FieldsShape};
 use rustc_target::abi::{Int, Pointer, F32, F64};
 use rustc_target::abi::{PointeeInfo, Scalar, Size, TyAbiInterface, Variants};
@@ -192,6 +193,7 @@ pub trait LayoutLlvmExt<'tcx> {
     ) -> &'a Type;
     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>;
+    fn scalar_copy_llvm_type<'a>(&self, cx: &CodegenCx<'a, 'tcx>) -> Option<&'a Type>;
 }
 
 impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
@@ -414,4 +416,35 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
         cx.pointee_infos.borrow_mut().insert((self.ty, offset), result);
         result
     }
+
+    fn scalar_copy_llvm_type<'a>(&self, cx: &CodegenCx<'a, 'tcx>) -> Option<&'a Type> {
+        debug_assert!(self.is_sized());
+
+        // FIXME: this is a fairly arbitrary choice, but 128 bits on WASM
+        // (matching the 128-bit SIMD types proposal) and 256 bits on x64
+        // (like AVX2 registers) seems at least like a tolerable starting point.
+        let threshold = cx.data_layout().pointer_size * 4;
+        if self.layout.size() > threshold {
+            return None;
+        }
+
+        // Vectors, even for non-power-of-two sizes, have the same layout as
+        // arrays but don't count as aggregate types
+        if let FieldsShape::Array { count, .. } = self.layout.fields()
+            && let element = self.field(cx, 0)
+            && element.ty.is_integral()
+        {
+            // `cx.type_ix(bits)` is tempting here, but while that works great
+            // for things that *stay* as memory-to-memory copies, it also ends
+            // up suppressing vectorization as it introduces shifts when it
+            // extracts all the individual values.
+
+            let ety = element.llvm_type(cx);
+            return Some(cx.type_vector(ety, *count));
+        }
+
+        // FIXME: The above only handled integer arrays; surely more things
+        // would also be possible. Be careful about provenance, though!
+        None
+    }
 }