Benchmark cmake: Added SWIFT_BENCHMARK_USE_OS_LIBRARIES

Cleanup and document the configuration of the library path and rpath.

With SWIFT_BENCHMARK_USE_OS_LIBRARIES, it's now possible to directly
build benchmarks for a target device and run those benchmarks on the
device without building or installing Swift.

It's also possible now to specify an absolute SWIFT_LIBRARY_PATH to be
used as an rpath so installation can be skipped.

benchmark/cmake/modules/AddSwiftBenchmarkSuite.cmake

@@ -24,6 +24,9 @@ macro(configure_build)
     if(${c_compiler} STREQUAL "clang")
       set(CLANG_EXEC ${CMAKE_C_COMPILER})
     else()
+      if(NOT SWIFT_DARWIN_XCRUN_TOOLCHAIN)
+        set(SWIFT_DARWIN_XCRUN_TOOLCHAIN "XcodeDefault")
+      endif()
       runcmd(COMMAND "xcrun" "-toolchain" "${SWIFT_DARWIN_XCRUN_TOOLCHAIN}" "-f" "clang"
              VARIABLE CLANG_EXEC
              ERROR "Unable to find Clang driver")
@@ -57,12 +60,47 @@ macro(configure_build)
     endif()
   endif()
 
-  # We always infer the SWIFT_LIBRARY_PATH from SWIFT_EXEC unless
-  # SWIFT_LIBRARY_PATH is specified explicitly.
-  if(NOT SWIFT_LIBRARY_PATH)
-    get_filename_component(tmp_dir "${SWIFT_EXEC}" DIRECTORY)
-    get_filename_component(tmp_dir "${tmp_dir}" DIRECTORY)
-    set(SWIFT_LIBRARY_PATH "${tmp_dir}/lib/swift")
+  # Set LIBRARY_PATH and either RPATH or RPATH_BASE. To build and run
+  # for multiple platforms, RPATH_BASE must be set instead of RPATH. It
+  # is the platform-independent runtime library directory. The platform
+  # subdirectory name will be appended to form a different RPATH for
+  # on platform.
+
+  # If requested, use Swift-in-the-OS. This way, the benchmarks may be built
+  # standalone on the host, and the binaries can run directly from a temp dir
+  # on any target machine. Of course, this factors out performance changes in
+  # stdlib or overlays.
+  if(SWIFT_BENCHMARK_USE_OS_LIBRARIES)
+    set(SWIFT_RPATH "/usr/lib/swift")
+  endif()
+
+  # When SWIFT_LIBRARY_PATH is specified explicitly for a standalone
+  # build, use it as an absolute RPATH_BASE. This only works when
+  # running benchmarks on the host machine. Otherwise, RPATH is set
+  # assuming that libraries will be installed later (manually)
+  # relative to the benchmark binaries.
+  #
+  # When not building standalone, SWIFT_LIBRARY_PATH is set by LLVM
+  # cmake to the build directory for Swift dylibs. Otherwise, assume
+  # that the dylibs are built relative to SWIFT_EXEC.
+  if(SWIFT_LIBRARY_PATH AND SWIFT_BENCHMARK_BUILT_STANDALONE)
+    if (NOT SWIFT_RPATH)
+      set(SWIFT_RPATH_BASE ${SWIFT_LIBRARY_PATH})
+    endif()
+  else()
+    if (NOT SWIFT_LIBRARY_PATH)
+      get_filename_component(tmp_dir "${SWIFT_EXEC}" DIRECTORY)
+      get_filename_component(tmp_dir "${tmp_dir}" DIRECTORY)
+      set(SWIFT_LIBRARY_PATH "${tmp_dir}/lib/swift")
+    endif()
+    if (NOT SWIFT_RPATH)
+      # If the benchmarks are built against a local swift build, assume that
+      # either the benchmarks will be installed in the swift build dir,
+      # or the swift libraries will be installed in the benchmark location in
+      # a platform specific subdirectory.
+      # This way, performance always factors in changes to the libraries.
+      set(SWIFT_RPATH_BASE "@executable_path/../lib/swift")
+    endif()
   endif()
 endmacro()
 
@@ -571,6 +609,11 @@ function (swift_benchmark_compile_archopts)
   # both do exactly the same thing with both sets of arguments. It also lets us
   # avoid issues around code-signing.
   if (is_darwin)
+    if (SWIFT_RPATH)
+      set(SWIFT_LINK_RPATH "${SWIFT_RPATH}")
+    else()
+      set(SWIFT_LINK_RPATH "${SWIFT_RPATH_BASE}/${BENCH_COMPILE_ARCHOPTS_PLATFORM}")
+    endif()
     add_custom_command(
         OUTPUT "${OUTPUT_EXEC}"
         DEPENDS
@@ -593,7 +636,7 @@ function (swift_benchmark_compile_archopts)
           "-lobjc"
           "-L${SWIFT_LIBRARY_PATH}/${BENCH_COMPILE_ARCHOPTS_PLATFORM}"
           "-Xlinker" "-rpath"
-          "-Xlinker" "@executable_path/../lib/swift/${BENCH_COMPILE_ARCHOPTS_PLATFORM}"
+          "-Xlinker" "${SWIFT_LINK_RPATH}"
           ${bench_library_objects}
           ${bench_driver_objects}
           ${SWIFT_BENCH_OBJFILES}
@@ -612,7 +655,7 @@ function (swift_benchmark_compile_archopts)
           "${SWIFT_EXEC}"
           "-O"
           "-target" "${target}"
-          "-L${SWIFT_LIBRARY_PATH}/${BENCH_COMPILE_ARCHOPTS_PLATFORM}"
+          "-L${SWIFT_LIBRARY_PATH}"
           ${bench_library_objects}
           ${bench_driver_objects}
           ${SWIFT_BENCH_OBJFILES}