[PATCH] D62091: [CommandLine] Reduce size of Option class by moving more members into bit field

[Don Hinton via Phabricator via llvm-commits]

hintonda marked an inline comment as done.
hintonda added inline comments.


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Comment at: llvm/include/llvm/Support/CommandLine.h:288
       Categories;                    // The Categories this option belongs to
   SmallPtrSet<SubCommand *, 4> Subs; // The subcommands this option belongs to.
 
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beanz wrote:
> beanz wrote:
> > Side note: I wonder how many members actually get put into this.  The common-case for most options is surely 0.
> From eyeballing (not an exhaustive search), I don't see anywhere that more than one Sub command is set. You might be able to save more space by shrinking the inline size here. If I read the code right, SmallPtrSet is 32 bytes + (sizeof (T) * N). In this case that should be 64 bytes. Changing the size to 2, would save another 16 bytes per option.
Actually, it's empty about 99% or the time and defaults to TopLevelSubCommand at that point.  This is how all the options in passes are setup -- well over 1,000.  Only a few tools currently use SubCommands -- the most I've seen is 4 by llvm-pdbutil.

So, I was considering something a bit more radical.  Instead of each Option keeping track of SubCommand, and Category, membership, why not let the SubCommand's and Category's do it instead?




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https://reviews.llvm.org/D62091