[PATCH] D149541: [JITLink] Process null symbols

[Job Noorman via Phabricator via llvm-commits]

jobnoorman added inline comments.


================
Comment at: llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h:552-553
+
+      auto &GSym = G->addAbsoluteSymbol(*Name, orc::ExecutorAddr(0), 0,
+                                        Linkage::Strong, Scope::Local, false);
+      setGraphSymbol(SymIndex, GSym);
----------------
lhames wrote:
> jobnoorman wrote:
> > lhames wrote:
> > > Let's name these `__jitlink_ELF_SYM_UND_<st_value>` -- it'll make them easy to spot when debugging, and allow us to keep the invariant that all absolute symbols are named (which means that the change to `JITLink.cpp` would no longer be needed).
> > > 
> > > Side note: We should probably change `Symbol::hasName` so that empty symbols still count as valid names, but that's outside the scope of this patch.
> > > Let's name these `__jitlink_ELF_SYM_UND_<st_value>` -- it'll make them easy to spot when debugging
> > 
> > Should we maybe go for `__jitlink_ELF_SYM_UND_<SymIndex>` to make sure the name is always unique? `st_value` will always be 0 in this case.
> > 
> > > and allow us to keep the invariant that all absolute symbols are named (which means that the change to `JITLink.cpp` would no longer be needed).
> > 
> > I might be missing something here but wouldn't this change still be needed? It handles printing edges with targets that are `!isDefined()` rather than `!hasName()`.
> > Should we maybe go for __jitlink_ELF_SYM_UND_<SymIndex> to make sure the name is always unique? st_value will always be 0 in this case.
> 
> Yep -- that sounds good to me.
> 
> > I might be missing something here but wouldn't this change still be needed? It handles printing edges with targets that are !isDefined() rather than !hasName().
> 
> If all absolute symbols are named then they're handled under the `if (TargetSym.hasName())` condition, and all symbols are either named or have an associated Block (the `else` case).
> > Should we maybe go for __jitlink_ELF_SYM_UND_<SymIndex> to make sure the name is always unique? st_value will always be 0 in this case.
> 
> Yep -- that sounds good to me.

Just noticed there's a practical issue here: `Symbol` stores its name as a `StringRef`  so I don't think there's a safe way to construct a name on the fly.

So I see two options:
- Call the symbol `__jitlink_ELF_SYM_NULL` (or something like that) and `assert` there is only one;
- Update `Symbol` to own its name in a `std::string`.

> > I might be missing something here but wouldn't this change still be needed? It handles printing edges with targets that are !isDefined() rather than !hasName().
> 
> If all absolute symbols are named then they're handled under the `if (TargetSym.hasName())` condition, and all symbols are either named or have an associated Block (the `else` case).

Ah, of course! Will remove these changes in the next update.




Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D149541/new/

https://reviews.llvm.org/D149541