[PATCH] D89891: [llvm-ar][Object] Fix detection of need for 64-bit archive symbol tables

[Andrew Ng via Phabricator via llvm-commits]

andrewng marked 2 inline comments as done.
andrewng added inline comments.


================
Comment at: llvm/lib/Object/ArchiveWriter.cpp:610
+    raw_svector_ostream SymtabHeader(SymtabHeaderBuf);
+    writeSymbolTableHeader(SymtabHeader, Kind, Deterministic, SymtabSize);
+
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grimar wrote:
> andrewng wrote:
> > grimar wrote:
> > > grimar wrote:
> > > > grimar wrote:
> > > > > andrewng wrote:
> > > > > > grimar wrote:
> > > > > > > I was a bit confused when saw `writeSymbolTableHeader` here.
> > > > > > > `Kind` can be changed below:
> > > > > > > 
> > > > > > > ```
> > > > > > >     if (LastOffset >= (1ULL << Sym64Threshold)) {
> > > > > > >       if (Kind == object::Archive::K_DARWIN)
> > > > > > >         Kind = object::Archive::K_DARWIN64;
> > > > > > >       else
> > > > > > >         Kind = object::Archive::K_GNU64;
> > > > > > >     }
> > > > > > > ```
> > > > > > > 
> > > > > > > I think would be more natural to call `writeSymbolTableHeader` after last possible modification of `Kind`, i.e. after this code block. Is it possible?
> > > > > > This call to `writeSymbolTableHeader` is purely used to determine the size of the header and is not a write to the output which happens later.
> > > > > This wasn't addressed?
> > > > > 
> > > > > aside: could you please mask addressed comments as "Done"? So that reviewers can see the status.
> > > > mask -> mark
> > > > This call to writeSymbolTableHeader is purely used to determine the size of the header and is not a write to the output which happens later.
> > > 
> > > `writeSymbolTableHeader` still uses `Kind`. It is just not clean to give it one `Kind` and change it to another later.
> > But this is by design. This call to `writeSymbolTableHeader` is used as part of the calculation of the 32-bit symbol table size in order to determine whether a 64-bit symbol table is required. If this is required, then the `Kind` is changed and the appropriate format is output. Does that make sense?
> I believe that the behavior of original code was:
> 1) Calculate the size of the symbol table.
> 2) Switch to 64-bit symbol table Kind if needed.
> 3) Write the header and the body of the symbol table in one move.
> 
> Now the behavior with this patch is:
> 1) Calculate the size of the symbol table.
> 2) Write the symbol table header using the `Kind` (Kind1) we have at this step.
> 3) Switch to 64-bit symbol table `Kind` (Kind2) if needed.
> 4) Write the body of the symbol table using the new Kind (Kind2)
> 
> `computeSymbolTableSize` uses `Kind` for `isBSDLike(Kind)` call.
> Perhaps instead of `Kind` it could take `bool IsBSDLike`? That way we know that
> changing of `Kind` from 32->64 does not affect anything. What do you think?
> 
Sorry if I haven't explained clearly, but the behaviour with this patch is:

1) Correctly calculate `LastOffset` including the correct calculation for the 32-bit symbol table size and also include the file signature.
2) The above calculation includes a call to `writeSymbolTableHeader` with the original `Kind` to a temporary buffer in order to determine the symbol table header size.
3) Switch to a 64-bit symbol table `Kind` if required.
4) Write archive which includes a call to `writeSymbolTableHeader` with the final `Kind` and the actual output buffer.

Hope that clarifies the situation.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D89891/new/

https://reviews.llvm.org/D89891