[PATCH] ADT: Shave a pointer off of SmallVector<T, 1>

[Duncan Exon Smith]

I agree it's probably worth formalizing a smaller vector type, but nothing specifically in mind -- I just noticed when looking at the struct layout of MCRelaxableFragment with Pete that the SmallVector<MCFixup,1> was 56 bytes instead of the 48 bytes I thought it would be.

(BTW, it's crazy that every relaxable fragment carries around a full copy of the MCSubtargetInfo.  Until today I thought that was a reference!)

-- dpnes

> On Jun 30, 2015, at 8:52 PM, Sean Silva <chisophugis at gmail.com> wrote:
> 
> Is there a specific place where we're storing SmallVectors in a datastructure that you're trying to optimize with this? It might be worth investing in a more general small-sizeof vector than TinyPtrVector. The way the SmallVector currently works (such as actually containing a 3 pointer "vector header") imposes some fundamental limitations on its sizeof.
> 
> There are various standard ways to greatly reduce sizeof of vectors especially in 64-bit address spaces (and certain assumptions on the OS...) where often the high bits are redundant, allowing the vector header to be compressed to a single pointer in size across a very large size range. Even without the spacious high-bits of a 64-bit address space, we can often store at least up to size 4 or 8 in the low bits, which for many use cases is plenty in the common case.
> 
> (to be clear, I think this patch makes sense; just had a high-level question)
> 
> -- Sean Silva
> 
>> On Tue, Jun 30, 2015 at 5:34 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> I'd just commit this, but I want to check:
>> 
>>  1. Are my static asserts valid under MSVC?  Sanitizers?  If not, what
>>     *is* a portable way to check that we don't waste memory here?
>>  2. I like them in SmallVector.h (assuming they're portable).
>>     Anyone strongly prefer ADTTests?
>> 
>> Avoid an unused byte -- which for `sizeof(T) >= sizeof(void *)` costs
>> a pointer -- on `SmallVector<T, 1>`.  Shave the same byte off of
>> `SmallVector<T, 0>`.  Otherwise, no functionality change.
>> 
>> Note that `SmallVector<T, 0>` has the same memory footprint as
>> `SmallVector<T, 1>`, which seems like a bug to me.  We should probably
>> fix that too.
>> 
>> 
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