[libc] Rely on __builtin_memcpy_inline for memcpy implementation

[gchatelet]

Summary

This patch removes usage of -mllvm -combiner-global-alias-analysis
and relies on compiler builtin to implement memcpy.

Note that -mllvm -combiner-global-alias-analysis is actually only useful for
functions where buffers can alias (namely memcpy and memmove). The other
memory functions where not benefiting from the flag anyways.

The upside is that the memory functions can now be compiled from source with
thinlto (thinlto would not be able to carry on the flag when doing inlining).

The downside is that for compilers other than clang (i.e. not providing
__builtin_memcpy_inline) the codegen may be worse.

[courbet]

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

833

Let's so that in a separate patch.

[gchatelet]

• Address comments

libc/src/string/CMakeLists.txt

function(add_implementation name impl_name)
   cmake_parse_arguments(
     "ADD_IMPL"
     "" # Optional arguments
     "" # Single value arguments
     "REQUIRE;SRCS;HDRS;DEPENDS;COMPILE_OPTIONS;MLLVM_COMPILE_OPTIONS" # Multi value arguments
     ${ARGN})
 
   if("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
-    list(APPEND ADD_IMPL_MLLVM_COMPILE_OPTIONS "-combiner-global-alias-analysis")
     # Note that '-mllvm' needs to be prefixed with 'SHELL:' to prevent CMake flag deduplication.
     foreach(opt IN LISTS ADD_IMPL_MLLVM_COMPILE_OPTIONS)
       list(APPEND ADD_IMPL_COMPILE_OPTIONS "SHELL:-mllvm ${opt}")
     endforeach()
   endif()
 
   add_entrypoint_object(${impl_name}
     NAME ${name}

libc/src/string/memory_utils/memcpy_implementations.h

 //   implementation parameters.
 // - As compilers and processors get better, the generated code is improved
 //   with little change on the code side.
 
 namespace __llvm_libc {
 
 static inline void inline_memcpy(char *__restrict dst,
                                  const char *__restrict src, size_t count) {
+  using namespace __llvm_libc::builtin;
 #if defined(LLVM_LIBC_ARCH_X86)
   /////////////////////////////////////////////////////////////////////////////
   // LLVM_LIBC_ARCH_X86
   /////////////////////////////////////////////////////////////////////////////
-  using namespace __llvm_libc::x86;
 
   // Whether to use only rep;movsb.
   constexpr bool USE_ONLY_REP_MOVSB =
       LLVM_LIBC_IS_DEFINED(LLVM_LIBC_MEMCPY_X86_USE_ONLY_REPMOVSB);
 
   // kRepMovsBSize == -1 : Only CopyAligned is used.
   // kRepMovsBSize ==  0 : Only RepMovsb is used.
   // else CopyAligned is used up to kRepMovsBSize and then RepMovsb.

 
 #if defined(__AVX__)
   using LoopBlockSize = _64;
 #else
   using LoopBlockSize = _32;
 #endif
 
   if (USE_ONLY_REP_MOVSB)
-    return copy<Accelerator>(dst, src, count);
+    return copy<x86::Accelerator>(dst, src, count);
 
   if (count == 0)
     return;
   if (count == 1)
     return copy<_1>(dst, src);
   if (count == 2)
     return copy<_2>(dst, src);
   if (count == 3)
if (count < 64)
     return copy<HeadTail<_32>>(dst, src, count);
   if (count < 128)
     return copy<HeadTail<_64>>(dst, src, count);
   if (HAS_AVX && count < 256)
     return copy<HeadTail<_128>>(dst, src, count);
   if (count <= REP_MOVS_B_SIZE)
     return copy<Align<_32, Arg::Dst>::Then<Loop<LoopBlockSize>>>(dst, src,
                                                                  count);
-  return copy<Accelerator>(dst, src, count);
+  return copy<x86::Accelerator>(dst, src, count);
 #elif defined(LLVM_LIBC_ARCH_AARCH64)
   /////////////////////////////////////////////////////////////////////////////
   // LLVM_LIBC_ARCH_AARCH64
   /////////////////////////////////////////////////////////////////////////////
-  using namespace __llvm_libc::scalar;
   if (count == 0)
     return;
   if (count == 1)
     return copy<_1>(dst, src);
   if (count == 2)
     return copy<_2>(dst, src);
   if (count == 3)
     return copy<_3>(dst, src);
if (count < 64)
     return copy<HeadTail<_32>>(dst, src, count);
   if (count < 128)
     return copy<HeadTail<_64>>(dst, src, count);
   return copy<Align<_16, Arg::Src>::Then<Loop<_64>>>(dst, src, count);
 #else
   /////////////////////////////////////////////////////////////////////////////
   // Default
   /////////////////////////////////////////////////////////////////////////////
-  using namespace __llvm_libc::scalar;
   if (count == 0)
     return;
   if (count == 1)
     return copy<_1>(dst, src);
   if (count == 2)
     return copy<_2>(dst, src);
   if (count == 3)
     return copy<_3>(dst, src);

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

 
 libc_function(
     name = "memcpy",
     srcs = ["src/string/memcpy.cpp"],
     hdrs = ["src/string/memcpy.h"],
     copts = [
         "-fno-builtin-memcpy",
         "-fno-builtin-memmove",
-        "-mllvm -combiner-global-alias-analysis",
         "-mllvm --tail-merge-threshold=0",
     ],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memset",
     srcs = ["src/string/memset.cpp"],
     hdrs = ["src/string/memset.h"],
     copts = [
         "-fno-builtin-memset",
-        "-mllvm -combiner-global-alias-analysis",
     ],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memmove",
     srcs = ["src/string/memmove.cpp"],
     hdrs = ["src/string/memmove.h"],
     copts = [
         "-fno-builtin-memmove",
-        "-mllvm -combiner-global-alias-analysis",
     ],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":__support_integer_operations",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memcmp",
     srcs = ["src/string/memcmp.cpp"],
     hdrs = ["src/string/memcmp.h"],
     copts = [
         "-fno-builtin-memcmp",
-        "-mllvm -combiner-global-alias-analysis",
     ],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":__support_integer_operations",
         ":string_memory_utils",
     ],
 )

 
 libc_function(
     name = "bzero",
     srcs = ["src/string/bzero.cpp"],
     hdrs = ["src/string/bzero.h"],
     copts = [
         "-fno-builtin-bzero",
         "-fno-builtin-memset",
-        "-mllvm -combiner-global-alias-analysis",
     ],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )