[cfe-dev] [llvm-dev] [RFC] Loading Bitfields with Smallest Needed Types

[John McCall via cfe-dev]

On 26 May 2020, at 20:31, Arthur O'Dwyer wrote:
> On Tue, May 26, 2020 at 7:32 PM John McCall via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> On 26 May 2020, at 18:28, Bill Wendling via llvm-dev wrote:
>>> [...] The store is a byte:
>>>
>>>     orb    $0x1,0x4a(%rbx)
>>>
>>> while the read is a word:
>>>
>>>     movzwl 0x4a(%r12),%r15d
>>>
>>> The problem is that between the store and the load the value hasn't
>>> been retired / placed in the cache. One would expect store-to-load
>>> forwarding to kick in, but on x86 that doesn't happen because x86
>>> requires the store to be of equal or greater size than the load. So
>>> instead the load takes the slow path, causing unacceptable 
>>> slowdowns.
>> [...]
>>
>> Clang used to generate narrower loads and stores for bit-fields, but 
>> a
>> long time ago it was intentionally changed to generate wider loads
>> and stores, IIRC by Chandler.  There are some cases where I think the
>> “new” code goes overboard, but in this case I don’t 
>> particularly have
>> an issue with the wider loads and stores.  I guess we could make a
>> best-effort attempt to stick to the storage-unit size when the
>> bit-fields break evenly on a boundary.  But mostly I think the 
>> frontend’s
>> responsibility ends with it generating same-size accesses in both
>> places, and if inconsistent access sizes trigger poor performance,
>> the backend should be more careful about intentionally changing 
>> access
>> sizes.
>>
>
> FWIW, when I was at Green Hills, I recall the rule being "Always use 
> the
> declared type of the bitfield to govern the size of the read or 
> write."
> (There was a similar rule for the meaning of `volatile`. I hope I'm 
> not
> just getting confused between the two. Actually, since of the 
> compilers on
> Godbolt, only MSVC follows this rule <https://godbolt.org/z/Aq_APH>, 
> I'm
> *probably* wrong.)  That is, if the bitfield is declared `int16_t`, 
> then
> use 16-bit loads and stores for it; if it's declared `int32_t`, then 
> use
> 32-bit loads and stores.

I’ve always liked MSVC’s bit-field rules as a coherent whole, but 
they are
quite different from the standard Unix rules.  On Windows, `T x : 3`
literally allocates an entire `T` in the structure, and successive
bit-fields get packed into that `T` only if their base type is of the
same size (and they haven’t exhausted the original `T`).  So of course
all accesses to that bit-field are basically of the full size of the 
`T`;
there’s no overlap to be concerned with.  On Unix, bit-fields will 
typically
get packed together regardless of the base type; the base type does have
some influence, but it’s target-specific and somewhat odd.

I’d prefer if we degraded to a Windows-like access behavior as much
as we can, but it’s not always possible because of that packing.

John.
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