[llvm-dev] GC for defsym'd symbols in LLD

[Peter Smith via llvm-dev]

On Wed, 4 Dec 2019 at 07:05, Fāng-ruì Sòng <maskray at google.com> wrote:
>
> On Tue, Dec 3, 2019 at 7:02 PM Shoaib Meenai via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > LLD treats any symbol referenced from a linker script as a GC root, which makes sense. Unfortunately, it also processes --defsym as a linker script fragment internally, so all target symbols of a --defsym also get treated as GC roots (i.e., if you have something like --defsym SRC=TGT, TGT will become a GC root). I believe this to be unnecessary for defsym specifically, since you're just aliasing a symbol, and if the original or aliased symbols are referenced from anywhere, the symbol's section will get preserved anyway. (There's also cases where the defsym target can be an expression instead of just a symbol name, which I admittedly haven't thought about too hard, but I believe the same logic  should hold in terms of any needed sections getting preserved regardless.) I want to change defsym targets specifically to not be considered as GC roots, so that they can be dead code eliminated. Does anyone foresee any issues with this?
>
> % cat a.s
> .globl _start, foo, bar
> .text; _start: movabs $d, %rax
> .section .text_foo,"ax"; foo: ret
> .section .text_bar,"ax"; bar: nop
> % as a.s -o a.o
>
> % ld.bfd a.o --defsym d=foo --gc-sections -o a => .text_foo is retained
> % ld.bfd a.o --defsym d=bar --gc-sections -o a => .text_bar is retained
> % ld.bfd a.o --defsym d=1 --gc-sections -o a => Neither .text_foo nor
> .text_bar is retained
> % ld.bfd a.o --defsym c=foo --defsym d=1 --gc-sections -o a => Neither
> .text_foo nor .text_bar is retained; lld will retain .text_foo.
>
> For --defsym from=an_expression_with_to, GNU ld appears to add a
> reference from 'from' to 'to'. lld's behavior
> (https://reviews.llvm.org/D34195) is more conservative.
>
> If we stop treating script->referencedSymbols as GC roots,
> instructions like `movabs $d, %rax` will no longer be able to access
> the intended section. We can tweak our behavior to be like GNU ld, but
> the additional complexity may not be worthwhile.

I think it would be a step too far for defsym symbol=expression to
have no effect on GC. I'd expect that something like defsym foo=bar is
used because some live code refers to foo, but does not refer to bar,
so ideally we'd like defsym foo=bar to keep bar live. I've seen this
idiom used in embedded systems in the presence of binary only
libraries. It is true that the programmer can always go the extra mile
to force bar to be marked live, however I think the expectation would
be defsym foo=bar would do it.

I think the GNU ld behaviour is reasonable. If nothing refers to
either foo or bar then there is no reason to mark them live. On the
implementation cost-benefit trade off I guess we won't know until
there is a prototype, and some idea of what implementing it will save
on a real example.

Peter