[Offload] Detect native ELF machine from preprocessor (#91282)

Summary:
This gets the target's corresponding ELF value from the preprocessor.
We use this to detect if a given ELF is compatible with the CPU
offloading impolementation for OpenMP. Previously we used defitions from
CMake, but this is easier for people to understand as there may be new
users of this in the future.

Author: jhuber6

offload/plugins-nextgen/common/include/Utils/ELF.h

@@ -24,6 +24,9 @@ namespace elf {
 /// Returns true or false if the \p Buffer is an ELF file.
 bool isELF(llvm::StringRef Buffer);
 
+/// Returns the ELF e_machine value of the current compilation target.
+uint16_t getTargetMachine();
+
 /// Checks if the given \p Object is a valid ELF matching the e_machine value.
 llvm::Expected<bool> checkMachine(llvm::StringRef Object, uint16_t EMachine);
 

offload/plugins-nextgen/common/src/Utils/ELF.cpp

@@ -36,6 +36,21 @@ bool utils::elf::isELF(StringRef Buffer) {
   }
 }
 
+uint16_t utils::elf::getTargetMachine() {
+#if defined(__x86_64__)
+  return EM_X86_64;
+#elif defined(__s390x__)
+  return EM_S390;
+#elif defined(__aarch64__)
+  return EM_AARCH64;
+#elif defined(__powerpc64__)
+  return EM_PPC64;
+#else
+#warning "Unknown ELF compilation target architecture"
+  return EM_NONE;
+#endif
+}
+
 template <class ELFT>
 static Expected<bool>
 checkMachineImpl(const object::ELFObjectFile<ELFT> &ELFObj, uint16_t EMachine) {

offload/plugins-nextgen/host/CMakeLists.txt

@@ -52,27 +52,22 @@ endif()
 
 # Define the target specific triples and ELF machine values.
 if(CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64le$")
-  target_compile_definitions(omptarget.rtl.host PRIVATE TARGET_ELF_ID=EM_PPC64)
   list(APPEND LIBOMPTARGET_SYSTEM_TARGETS 
        "powerpc64le-ibm-linux-gnu" "powerpc64le-ibm-linux-gnu-LTO")
   set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}" PARENT_SCOPE)
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "ppc64$")
-  target_compile_definitions(omptarget.rtl.host PRIVATE TARGET_ELF_ID=EM_PPC64)
   list(APPEND LIBOMPTARGET_SYSTEM_TARGETS 
        "powerpc64-ibm-linux-gnu" "powerpc64-ibm-linux-gnu-LTO")
   set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}" PARENT_SCOPE)
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64$")
-  target_compile_definitions(omptarget.rtl.host PRIVATE TARGET_ELF_ID=EM_X86_64)
   list(APPEND LIBOMPTARGET_SYSTEM_TARGETS 
        "x86_64-pc-linux-gnu" "x86_64-pc-linux-gnu-LTO")
   set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}" PARENT_SCOPE)
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64$")
-  target_compile_definitions(omptarget.rtl.host PRIVATE TARGET_ELF_ID=EM_AARCH64)
   list(APPEND LIBOMPTARGET_SYSTEM_TARGETS 
        "aarch64-unknown-linux-gnu" "aarch64-unknown-linux-gnu-LTO")
   set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}" PARENT_SCOPE)
 elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "s390x$")
-  target_compile_definitions(omptarget.rtl.host PRIVATE TARGET_ELF_ID=EM_S390)
   list(APPEND LIBOMPTARGET_SYSTEM_TARGETS 
        "s390x-ibm-linux-gnu" "s390x-ibm-linux-gnu-LTO")
   set(LIBOMPTARGET_SYSTEM_TARGETS "${LIBOMPTARGET_SYSTEM_TARGETS}" PARENT_SCOPE)

offload/plugins-nextgen/host/src/rtl.cpp

@@ -18,6 +18,7 @@
 
 #include "Shared/Debug.h"
 #include "Shared/Environment.h"
+#include "Utils/ELF.h"
 
 #include "GlobalHandler.h"
 #include "OpenMP/OMPT/Callback.h"
@@ -44,11 +45,6 @@
 // The number of devices in this plugin.
 #define NUM_DEVICES 4
 
-// The ELF ID should be defined at compile-time by the build system.
-#ifndef TARGET_ELF_ID
-#define TARGET_ELF_ID EM_NONE
-#endif
-
 namespace llvm {
 namespace omp {
 namespace target {
@@ -416,7 +412,9 @@ struct GenELF64PluginTy final : public GenericPluginTy {
   }
 
   /// Get the ELF code to recognize the compatible binary images.
-  uint16_t getMagicElfBits() const override { return ELF::TARGET_ELF_ID; }
+  uint16_t getMagicElfBits() const override {
+    return utils::elf::getTargetMachine();
+  }
 
   /// This plugin does not support exchanging data between two devices.
   bool isDataExchangable(int32_t SrcDeviceId, int32_t DstDeviceId) override {