[swift-dev] [RFC] Moving to gold linker

[Joe Groff]

> On Jan 19, 2016, at 8:31 AM, William Dillon <william at housedillon.com> wrote:
> 
>>> 
>>> Second, Orlando believes that it’s possible to use C++ to generate the object files rather that assembler.  This would make these source files much more portable and easier to maintain.  There is a catch, however. In the assembler version there is no runtime impact at all; the C++ version requires a subtraction at load time.  The cost is pretty minimal, but it’s worth considering.  Changing from one implementation to another is very simple.
>> 
>> How different is the .s file across platforms? I'd expect it to just contain data directives.
>> 
> 
> There are very minor differences.  Here are the current files:
> 
> swift_begin.S:
> 
> #if defined(__arm__)
> .section .swift2_protocol_conformances, "aw",%progbits
> #else
> .section .swift2_protocol_conformances, "aw", at progbits
> #endif
> .global .swift2_protocol_conformances_start
> .swift2_protocol_conformances_start:
> 
> #if defined(__x86_64__)
> .quad (.swift2_protocol_conformances_end - .swift2_protocol_conformances_start) - 8
> #else
> .long (.swift2_protocol_conformances_end - .swift2_protocol_conformances_start) - 8
> .long 0
> #endif
> 
> 
> swift_end.S:
> 
> #if defined(__arm__)
> .section .swift2_protocol_conformances, "aw",%progbits
> #else
> .section .swift2_protocol_conformances, "aw", at progbits
> #endif
> .global .swift2_protocol_conformances_end
> .swift2_protocol_conformances_end:
> 
> Whereas the C++ version would be something like this (for begin):
> 
> __attribute__((section(".swift2_protocol_conformances")))
> extern char swift2_protocol_conformances_end[] __asm__(".swift2_protocol_conformances_end");
> 
> __attribute__((section(".swift2_protocol_conformances")))
> long long swift2_protocol_conformances_start __asm__(".swift2_protocol_conformances_start") =
> (long long)((char *)&swift2_protocol_conformances_end - (char *)&swift2_protocol_conformances_start - sizeof(swift2_protocol_conformances_start));
> 
> So, it’s not necessary to break-out the different sections, the generated initializer for swift2_protocol_conformances_start would have a subtraction.

A .ll file containing LLVM IR might be able to adequately abstract away the per-platform syntactical differences in .s files, while allowing enough control to avoid the extra relocation implied by the C++ code.

-Joe