[LLVMdev] RFC: Merge MCSymbol with MCSymbolData, optimizing for object file emission

[Duncan P. N. Exon Smith]

+llvm-commits, bcc:llvmdev

> On 2015-May-13, at 14:48, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> Right now MC is optimized for emitting assembly, but as Rafael pointed
> out to me over IRC the optimized path should be emitting object files.
> 
> This WIP patch moves MCSymbolData into MCSymbol.h and makes it a
> field inside MCSymbol.  This eliminates a pointer from MCSymbolData back
> to MCSymbol, the DenseMap<const MCSymbol *, MCSymbolData *> in
> MCAssembler, and converts the iplist in MCAssembler into a std::vector
> (along with some churn to pass around MCSymbols instead of
> MCSymbolData).  As a result, during object emission we save ~6 pointers
> per MCSymbol, eliminate lookup indirection between MCSymbol and
> MCSymbolData, and avoid allocation overhead for MCSymbolData.
> 
> I've measured ~4% memory savings on `llc` with this patch (using the
> same -flto -g input as r236642 and r236629 before it), dropping memory
> usage from 1058 MB down to 1017 MB.
> 
> Before I clean this up and commit it (obviously in more incremental
> patches (and I'll do the same cleanup for MCSection)), I wanted to
> confirm the direction.
> 
> <symbol-data-wip.patch>

I went ahead and committed r237486 and r237487, which converted the
iplist in MCAssembler into a std::vector, and I've started to clean
up for commit.  (More to do still...)

The one patch (so far) I want pre-commit review on is 0002 -- i.e.,
is this direction okay? -- since the rest are just natural fallout.
I've attached them for reference anyway so you can see where I'm
headed (0001 is just moving code as prep for 0002).

I know Rafael's on board -- he nerd-sniped me into this -- but he's
on vacation ;).  Any chance of a go-ahead from someone else that's
spent some time in MC?


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