Link Time Optimization (LTO)

The LTO information generated by IBM Open XL C/C++ for AIX 17.1.0 is incompatible with the Interprocedural Analysis (IPA) information of IBM XL C/C++ for AIX 16.1.0 in the following aspects:

• The IPA information in object files that are created by IBM XL C/C++ for AIX 16.1.0 or earlier releases is silently ignored by IBM Open XL C/C++ for AIX 17.1.0. 
• Object files that are created by IBM XL C/C++ for AIX 16.1.0 or earlier releases using the -qipa=noobject option contain only IPA information, so they are unusable in IBM Open XL C/C++ for AIX 17.1.0. 
• Object files that are created by IBM Open XL C/C++ for AIX 17.1.0 using the -flto option are not readable by IBM XL C/C++ for AIX 16.1.0 or earlier releases.
  Note: In IBM Open XL C/C++ for AIX 17.1.0, the -flto=thin option is not supported. LTO mode can be set only to the full mode.

To compile a program with LTO, specify the -flto option on both the compile and link steps:

• When -flto is specified on the compile step, the compiler generates LLVM IR instead of object code, in preparation for LTO on the link step.
• When -flto is specified on the link step, the compiler adds extra linker options such as -bplugin:<path-to-libLTO.so> when invoking the linker. The linker can handle a mixture of native XCOFF objects and LLVM IR objects, and then invokes the LTO optimizations on the LLVM IR objects via the libLTO.so plugin.