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

[Shoaib Meenai via llvm-dev]

I completely agree that --defsym foo=bar should keep bar (or more precisely the section containing bar) alive if foo is referenced.

My mental model of how --defsym foo=bar behaves is that (assuming bar is a defined symbol) we create a symbol foo that points to the same location as bar (as in it has the same section + address within that section). Any reference to foo should therefore prevent that section from getting garbage collected. bar doesn't need to enter the picture directly (and we don't need to store any sort of explicit link between foo and bar); its section getting preserved just naturally falls out of foo getting preserved.

For example, in Fāng-ruì's movabs example, the symbol _start (which is the entry point and therefore a GC root) will have a relocation against d, so d will be kept alive too. With --defsym d=foo, the symbol d should point to the same section as foo, so that section will be preserved; it doesn't matter if the symbol foo itself is preserved (unless there are other non-dead references to it, of course, but then those references should cause foo to be marked alive as well).

I haven't actually studied how LLD models a defsym though, so my mental model might be way off. I apologize for not having done so before replying, but it'll be at least a few days before I have the chance to get to that. If my mental model is accurate, preserving the needed section for defsym should just fall out naturally from it (without needing to give the target of a defsym any special treatment), but if not, the whole thing might be much more complicated and not worth it.

On 12/4/19, 1:35 AM, "Peter Smith" <peter.smith at linaro.org> wrote:

    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://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D34195&d=DwIFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=MpiPCWMhZJFZg0s-e1lhHtcCr-BLzG6zbJ44d0isoMc&s=7j_hrwm8LBMCPNgU_IXbhye_YKPQFgGJlU3YMAtWGLE&e= ) 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