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

[William Dillon]

> On Jan 20, 2016, at 12:53 PM, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> 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.
> 

Thanks for the idea, Joe!

We’ll look into it. :)

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