Subversion Repositories QNX 8.QNX8 linker driver

QNX 8.0 'gcc' linker driver drop-in replacement by Pierre-Marie Baty

RATIONALE

=========

Until a proper QNXToolchain class is implemented in the Clang driver, Clang

relies on GNU GCC to drive the system linker when it hasn't enough knowledge

of the underlying platform. So, if we are to provide a LLVM/Clang-based native

toolchain for QNX, we *HAVE* to provide a 'gcc' executable for Clang to be

able to link QNX files. This is an either/or situation and it was decided

that reimplementing an external linker driver would be faster than doing it

in the LLVM codebase (mostly because of the very long iteration times that

rebuilding LLVM causes, even when using a compiler cache.)

Why does Clang chain-call gcc with the intent of linking ? Because GNU GCC is

itself a 'driver' which can also invoke either a compiler or a linker ('gcc'

invokes 'collect2' which in turn invokes 'ld', the UNIX linker). This means

our drop-in GCC replacement will need to either chain-call 'clang' when it is

called with the intent of *compiling* and chain-call ld' when it is called

with the intent of *linking*. This can be decided by looking at its command-

line arguments.

How will our drop-in gcc replacement know how to call 'ld' correctly ? Because

when proper support is added to it, GNU GCC contains a 'linker specification'

for the system (a string of symbols describing a sort of decision tree) which

it uses to construct the linker command line. The 'ld' linker specifications

for QNX 8.0 can be extracted from 'gcc -dumpspecs' from the cross-compilers

(Win64->QNX and Linux->QNX) provided by Blackberry and have been reimplemented

in this file.

The GNU specs language is documented here:

https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html

The following environment variables control the behaviour of the driver:

   DEBUG_LINKER_DRIVER - when defined to any value (including an empty

                         string), the constructed ld command line is printed

                         to the standard error before calling the linker.

Build with:

	qnxsdp_env

	ntox86_64-gcc linkerdriver.c -o gcc

or just 'make' on Win32.

-- 

Pierre-Marie Baty <pm@pmbaty.com>