[libc][NFC] refactor Cortex memcpy code (#148204)

gchatelet · web-flow · commit 7c69c3b0baa5 · 2025-07-16T10:06:18.000+02:00
This patch is in preparation for the Cortex `memset` implementation.
It improves the codegen by generating a prefetch for large sizes.
diff --git a/libc/src/string/memory_utils/CMakeLists.txt b/libc/src/string/memory_utils/CMakeLists.txt
@@ -7,6 +7,7 @@ add_header_library(
     aarch64/inline_memcpy.h
     aarch64/inline_memmove.h
     aarch64/inline_memset.h
+    arm/common.h
     arm/inline_memcpy.h
     generic/aligned_access.h
     generic/byte_per_byte.h
diff --git a/libc/src/string/memory_utils/arm/common.h b/libc/src/string/memory_utils/arm/common.h
@@ -0,0 +1,52 @@
+//===-- Common constants and defines for arm --------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_COMMON_H
+#define LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_COMMON_H
+
+#include "src/__support/macros/attributes.h" // LIBC_INLINE_VAR
+#include "src/string/memory_utils/utils.h"   // CPtr, Ptr, distance_to_align
+
+#include <stddef.h> // size_t
+
+// https://libc.llvm.org/compiler_support.html
+// Support for [[likely]] / [[unlikely]]
+//  [X] GCC 12.2
+//  [X] Clang 12
+//  [ ] Clang 11
+#define LIBC_ATTR_LIKELY [[likely]]
+#define LIBC_ATTR_UNLIKELY [[unlikely]]
+
+#if defined(LIBC_COMPILER_IS_CLANG)
+#if LIBC_COMPILER_CLANG_VER < 1200
+#undef LIBC_ATTR_LIKELY
+#undef LIBC_ATTR_UNLIKELY
+#define LIBC_ATTR_LIKELY
+#define LIBC_ATTR_UNLIKELY
+#endif
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+LIBC_INLINE_VAR constexpr size_t kWordSize = sizeof(uint32_t);
+
+enum class AssumeAccess { kUnknown, kAligned };
+enum class BlockOp { kFull, kByWord };
+
+LIBC_INLINE auto misaligned(CPtr ptr) {
+  return distance_to_align_down<kWordSize>(ptr);
+}
+
+LIBC_INLINE CPtr bitwise_or(CPtr a, CPtr b) {
+  return cpp::bit_cast<CPtr>(cpp::bit_cast<uintptr_t>(a) |
+                             cpp::bit_cast<uintptr_t>(b));
+}
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_COMMON_H
diff --git a/libc/src/string/memory_utils/arm/inline_memcpy.h b/libc/src/string/memory_utils/arm/inline_memcpy.h
@@ -5,63 +5,56 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+// The functions defined in this file give approximate code size. These sizes
+// assume the following configuration options:
+// - LIBC_CONF_KEEP_FRAME_POINTER = false
+// - LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR = false
+// - LIBC_ADD_NULL_CHECKS = false
 #ifndef LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
 #define LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
 
 #include "src/__support/macros/attributes.h"   // LIBC_INLINE
 #include "src/__support/macros/optimization.h" // LIBC_LOOP_NOUNROLL
+#include "src/string/memory_utils/arm/common.h" // LIBC_ATTR_LIKELY, LIBC_ATTR_UNLIKELY
 #include "src/string/memory_utils/utils.h" // memcpy_inline, distance_to_align
 
 #include <stddef.h> // size_t
 
-// https://libc.llvm.org/compiler_support.html
-// Support for [[likely]] / [[unlikely]]
-//  [X] GCC 12.2
-//  [X] Clang 12
-//  [ ] Clang 11
-#define LIBC_ATTR_LIKELY [[likely]]
-#define LIBC_ATTR_UNLIKELY [[unlikely]]
-
-#if defined(LIBC_COMPILER_IS_CLANG)
-#if LIBC_COMPILER_CLANG_VER < 1200
-#undef LIBC_ATTR_LIKELY
-#undef LIBC_ATTR_UNLIKELY
-#define LIBC_ATTR_LIKELY
-#define LIBC_ATTR_UNLIKELY
-#endif
-#endif
-
 namespace LIBC_NAMESPACE_DECL {
 
 namespace {
 
-LIBC_INLINE_VAR constexpr size_t kWordSize = sizeof(uint32_t);
-
-enum Strategy {
-  ForceWordLdStChain,
-  AssumeWordAligned,
-  AssumeUnaligned,
-};
+// Performs a copy of `bytes` byte from `src` to `dst`. This function has the
+// semantics of `memcpy` where `src` and `dst` are `__restrict`. The compiler is
+// free to use whatever instruction is best for the size and assumed access.
+template <size_t bytes, AssumeAccess access>
+LIBC_INLINE void copy(void *dst, const void *src) {
+  if constexpr (access == AssumeAccess::kAligned) {
+    constexpr size_t alignment = bytes > kWordSize ? kWordSize : bytes;
+    memcpy_inline<bytes>(assume_aligned<alignment>(dst),
+                         assume_aligned<alignment>(src));
+  } else if constexpr (access == AssumeAccess::kUnknown) {
+    memcpy_inline<bytes>(dst, src);
+  } else {
+    static_assert(false);
+  }
+}
 
-template <size_t bytes, Strategy strategy = AssumeUnaligned>
-LIBC_INLINE void copy_and_bump_pointers(Ptr &dst, CPtr &src) {
-  if constexpr (strategy == AssumeUnaligned) {
-    memcpy_inline<bytes>(assume_aligned<1>(dst), assume_aligned<1>(src));
-  } else if constexpr (strategy == AssumeWordAligned) {
-    static_assert(bytes >= kWordSize);
-    memcpy_inline<bytes>(assume_aligned<kWordSize>(dst),
-                         assume_aligned<kWordSize>(src));
-  } else if constexpr (strategy == ForceWordLdStChain) {
+template <size_t bytes, BlockOp block_op = BlockOp::kFull,
+          AssumeAccess access = AssumeAccess::kUnknown>
+LIBC_INLINE void copy_block_and_bump_pointers(Ptr &dst, CPtr &src) {
+  if constexpr (block_op == BlockOp::kFull) {
+    copy<bytes, access>(dst, src);
+  } else if constexpr (block_op == BlockOp::kByWord) {
     // We restrict loads/stores to 4 byte to prevent the use of load/store
-    // multiple (LDM, STM) and load/store double (LDRD, STRD). First, they may
-    // fault (see notes below) and second, they use more registers which in turn
-    // adds push/pop instructions in the hot path.
+    // multiple (LDM, STM) and load/store double (LDRD, STRD).
     static_assert((bytes % kWordSize == 0) && (bytes >= kWordSize));
     LIBC_LOOP_UNROLL
-    for (size_t i = 0; i < bytes / kWordSize; ++i) {
-      const size_t offset = i * kWordSize;
-      memcpy_inline<kWordSize>(dst + offset, src + offset);
+    for (size_t offset = 0; offset < bytes; offset += kWordSize) {
+      copy<kWordSize, access>(dst + offset, src + offset);
     }
+  } else {
+    static_assert(false, "Invalid BlockOp");
   }
   // In the 1, 2, 4 byte copy case, the compiler can fold pointer offsetting
   // into the load/store instructions.
@@ -72,39 +65,27 @@ LIBC_INLINE void copy_and_bump_pointers(Ptr &dst, CPtr &src) {
   src += bytes;
 }
 
-LIBC_INLINE void copy_bytes_and_bump_pointers(Ptr &dst, CPtr &src,
-                                              const size_t size) {
+template <size_t bytes, BlockOp block_op, AssumeAccess access>
+LIBC_INLINE void consume_by_block(Ptr &dst, CPtr &src, size_t &size) {
   LIBC_LOOP_NOUNROLL
-  for (size_t i = 0; i < size; ++i)
-    *dst++ = *src++;
+  for (size_t i = 0; i < size / bytes; ++i)
+    copy_block_and_bump_pointers<bytes, block_op, access>(dst, src);
+  size %= bytes;
 }
 
-template <size_t block_size, Strategy strategy>
-LIBC_INLINE void copy_blocks_and_update_args(Ptr &dst, CPtr &src,
-                                             size_t &size) {
+[[maybe_unused]] LIBC_INLINE void
+copy_bytes_and_bump_pointers(Ptr &dst, CPtr &src, size_t size) {
   LIBC_LOOP_NOUNROLL
-  for (size_t i = 0; i < size / block_size; ++i)
-    copy_and_bump_pointers<block_size, strategy>(dst, src);
-  // Update `size` once at the end instead of once per iteration.
-  size %= block_size;
-}
-
-LIBC_INLINE CPtr bitwise_or(CPtr a, CPtr b) {
-  return cpp::bit_cast<CPtr>(cpp::bit_cast<uintptr_t>(a) |
-                             cpp::bit_cast<uintptr_t>(b));
-}
-
-LIBC_INLINE auto misaligned(CPtr a) {
-  return distance_to_align_down<kWordSize>(a);
+  for (size_t i = 0; i < size; ++i)
+    *dst++ = *src++;
 }
 
 } // namespace
 
-// Implementation for Cortex-M0, M0+, M1.
-// Notes:
-// - It compiles down to 196 bytes, but 220 bytes when used through `memcpy`
-//   that also needs to return the `dst` ptr.
-// - These cores do not allow for unaligned loads/stores.
+// Implementation for Cortex-M0, M0+, M1 cores that do not allow for unaligned
+// loads/stores. It compiles down to 208 bytes when used through `memcpy` that
+// also needs to return the `dst` ptr.
+// Note:
 // - When `src` and `dst` are coaligned, we start by aligning them and perform
 //   bulk copies. We let the compiler know the pointers are aligned so it can
 //   use load/store multiple (LDM, STM). This significantly increase throughput
@@ -125,9 +106,18 @@ LIBC_INLINE auto misaligned(CPtr a) {
     if (src_alignment == 0)
       LIBC_ATTR_LIKELY {
         // Both `src` and `dst` are now word-aligned.
-        copy_blocks_and_update_args<64, AssumeWordAligned>(dst, src, size);
-        copy_blocks_and_update_args<16, AssumeWordAligned>(dst, src, size);
-        copy_blocks_and_update_args<4, AssumeWordAligned>(dst, src, size);
+        // We first copy by blocks of 64 bytes, the compiler will use 4
+        // load/store multiple (LDM, STM), each of 4 words. This requires more
+        // registers so additional push/pop are needed but the speedup is worth
+        // it.
+        consume_by_block<64, BlockOp::kFull, AssumeAccess::kAligned>(dst, src,
+                                                                     size);
+        // Then we use blocks of 4 word load/store.
+        consume_by_block<16, BlockOp::kByWord, AssumeAccess::kAligned>(dst, src,
+                                                                       size);
+        // Then we use word by word copy.
+        consume_by_block<4, BlockOp::kByWord, AssumeAccess::kAligned>(dst, src,
+                                                                      size);
       }
     else {
       // `dst` is aligned but `src` is not.
@@ -138,7 +128,7 @@ LIBC_INLINE auto misaligned(CPtr a) {
             src_alignment == 2
                 ? load_aligned<uint32_t, uint16_t, uint16_t>(src)
                 : load_aligned<uint32_t, uint8_t, uint16_t, uint8_t>(src);
-        memcpy_inline<kWordSize>(assume_aligned<kWordSize>(dst), &value);
+        copy<kWordSize, AssumeAccess::kAligned>(dst, &value);
         dst += kWordSize;
         src += kWordSize;
         size -= kWordSize;
@@ -151,56 +141,69 @@ LIBC_INLINE auto misaligned(CPtr a) {
 }
 
 // Implementation for Cortex-M3, M4, M7, M23, M33, M35P, M52 with hardware
-// support for unaligned loads and stores.
-// Notes:
-// - It compiles down to 266 bytes.
-// - `dst` and `src` are not `__restrict` to prevent the compiler from
-//   reordering loads/stores.
-// - We keep state variables to a strict minimum to keep everything in the free
-//   registers and prevent costly push / pop.
-// - If unaligned single loads/stores to normal memory are supported, unaligned
-//   accesses for load/store multiple (LDM, STM) and load/store double (LDRD,
-//   STRD) instructions are generally not supported and will still fault so we
-//   make sure to restrict unrolling to word loads/stores.
+// support for unaligned loads and stores. It compiles down to 272 bytes when
+// used through `memcpy` that also needs to return the `dst` ptr.
 [[maybe_unused]] LIBC_INLINE void inline_memcpy_arm_mid_end(Ptr dst, CPtr src,
                                                             size_t size) {
   if (misaligned(bitwise_or(src, dst)))
     LIBC_ATTR_UNLIKELY {
       if (size < 8)
         LIBC_ATTR_UNLIKELY {
           if (size & 1)
-            copy_and_bump_pointers<1>(dst, src);
+            copy_block_and_bump_pointers<1>(dst, src);
           if (size & 2)
-            copy_and_bump_pointers<2>(dst, src);
+            copy_block_and_bump_pointers<2>(dst, src);
           if (size & 4)
-            copy_and_bump_pointers<4>(dst, src);
+            copy_block_and_bump_pointers<4>(dst, src);
           return;
         }
       if (misaligned(src))
         LIBC_ATTR_UNLIKELY {
           const size_t offset = distance_to_align_up<kWordSize>(dst);
           if (offset & 1)
-            copy_and_bump_pointers<1>(dst, src);
+            copy_block_and_bump_pointers<1>(dst, src);
           if (offset & 2)
-            copy_and_bump_pointers<2>(dst, src);
+            copy_block_and_bump_pointers<2>(dst, src);
           size -= offset;
         }
     }
-  copy_blocks_and_update_args<64, ForceWordLdStChain>(dst, src, size);
-  copy_blocks_and_update_args<16, ForceWordLdStChain>(dst, src, size);
-  copy_blocks_and_update_args<4, AssumeUnaligned>(dst, src, size);
+  // `dst` and `src` are not necessarily both aligned at that point but this
+  // implementation assumes hardware support for unaligned loads and stores so
+  // it is still fast to perform unrolled word by word copy. Note that wider
+  // accesses through the use of load/store multiple (LDM, STM) and load/store
+  // double (LDRD, STRD) instructions are generally not supported and can fault.
+  // By forcing decomposition of 64 bytes copy into word by word copy, the
+  // compiler can use the first load to prefetch memory:
+  //   ldr  r3, [r1, #64]!  <- prefetch next cache line
+  //   str  r3, [r0]
+  //   ldr  r3, [r1, #0x4]
+  //   str  r3, [r0, #0x4]
+  //   ...
+  //   ldr  r3, [r1, #0x3c]
+  //   str  r3, [r0, #0x3c]
+  // This is a bit detrimental for sizes between 64 and 256 (less than 10%
+  // penalty) but the prefetch yields better throughput for larger copies.
+  consume_by_block<64, BlockOp::kByWord, AssumeAccess::kUnknown>(dst, src,
+                                                                 size);
+  consume_by_block<16, BlockOp::kByWord, AssumeAccess::kUnknown>(dst, src,
+                                                                 size);
+  consume_by_block<4, BlockOp::kByWord, AssumeAccess::kUnknown>(dst, src, size);
   if (size & 1)
-    copy_and_bump_pointers<1>(dst, src);
+    copy_block_and_bump_pointers<1>(dst, src);
   if (size & 2)
-    LIBC_ATTR_UNLIKELY
-  copy_and_bump_pointers<2>(dst, src);
+    copy_block_and_bump_pointers<2>(dst, src);
 }
 
-[[maybe_unused]] LIBC_INLINE void inline_memcpy_arm(void *__restrict dst_,
-                                                    const void *__restrict src_,
+[[maybe_unused]] LIBC_INLINE void inline_memcpy_arm(Ptr dst, CPtr src,
                                                     size_t size) {
-  Ptr dst = cpp::bit_cast<Ptr>(dst_);
-  CPtr src = cpp::bit_cast<CPtr>(src_);
+  // The compiler performs alias analysis and is able to prove that `dst` and
+  // `src` do not alias by propagating the `__restrict` keyword from the
+  // `memcpy` prototype. This allows the compiler to merge consecutive
+  // load/store (LDR, STR) instructions generated in
+  // `copy_block_and_bump_pointers` with `BlockOp::kByWord` into load/store
+  // double (LDRD, STRD) instructions, this is is undesirable so we prevent the
+  // compiler from inferring `__restrict` with the following line.
+  asm volatile("" : "+r"(dst), "+r"(src));
 #ifdef __ARM_FEATURE_UNALIGNED
   return inline_memcpy_arm_mid_end(dst, src, size);
 #else
@@ -210,8 +213,4 @@ LIBC_INLINE auto misaligned(CPtr a) {
 
 } // namespace LIBC_NAMESPACE_DECL
 
-// Cleanup local macros
-#undef LIBC_ATTR_LIKELY
-#undef LIBC_ATTR_UNLIKELY
-
 #endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_ARM_INLINE_MEMCPY_H
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -4448,6 +4448,7 @@ libc_support_library(
         "src/string/memory_utils/aarch64/inline_memcpy.h",
         "src/string/memory_utils/aarch64/inline_memmove.h",
         "src/string/memory_utils/aarch64/inline_memset.h",
+        "src/string/memory_utils/arm/common.h",
         "src/string/memory_utils/arm/inline_memcpy.h",
         "src/string/memory_utils/generic/aligned_access.h",
         "src/string/memory_utils/generic/byte_per_byte.h",
