[BOLT] Make BOLT_ENABLE_RUNTIME user-configurable

[urnathan]

Summary

This makes BOLT_ENABLE_RUNTIME a user-configurable option. It defaults (almost) the same as the current behaviour, but allows one to explicitly disable the runtime.

• It adds a check for Linux or Darwin, AFAICT those are the only two hosts the runtime supports. So don't default on if not on those hosts.

• If the runtime is enabled, it adds a check that /proc/self/map_files is readable. The RHEL7 VM dev machine I have is configured such that it is not, and consequently the runtime fails. The failure mode is that one can open() the dir, but getdents fails with EPERM. This is apparently not too uncommon.

[rafauler]

LGTM

bolt/CMakeLists.txt

 include(ExternalProject)
 
 set(BOLT_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(BOLT_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 set(CMAKE_CXX_STANDARD 17)
 
-set(BOLT_ENABLE_RUNTIME OFF)
+set(BOLT_ENABLE_RUNTIME_default OFF)
 if (CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64"
+    AND (CMAKE_SYSTEM_NAME STREQUAL "Linux"
+      OR CMAKE_SYSTEM_NAME STREQUAL "Darwin")
     AND "X86" IN_LIST LLVM_TARGETS_TO_BUILD)
-  set(BOLT_ENABLE_RUNTIME ON)
+  set(BOLT_ENABLE_RUNTIME_default ON)
+endif()
+option(BOLT_ENABLE_RUNTIME "Enable BOLT runtime" ${BOLT_ENABLE_RUNTIME_default})
+if (BOLT_ENABLE_RUNTIME)
+  # Some systems prevent reading /proc/self/map_files
+  execute_process(COMMAND ls /proc/self/map_files
+    RESULT_VARIABLE LS OUTPUT_QUIET ERROR_QUIET)
+  if (LS)
+    set(BOLT_ENABLE_RUNTIME OFF)
+    message(WARNING
+      "BOLT runtime is disabled as /proc/self/map_files is unreadable.")
+  endif()
 endif()
 
 set(BOLT_CLANG_EXE "" CACHE FILEPATH "Path to clang executable for the target \
 architecture for use in BOLT tests")
 set(BOLT_LLD_EXE "" CACHE FILEPATH "Path to lld executable for the target \
 architecture for use in BOLT tests")
 
 set(BOLT_INCLUDE_TESTS OFF)