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

[Bill Wendling via llvm-dev]

On Fri, May 29, 2020 at 4:00 PM Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Fri, 29 May 2020 at 11:06, John McCall via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> On 28 May 2020, at 18:42, Bill Wendling wrote:
>>
>> > On Tue, May 26, 2020 at 7:49 PM James Y Knight via llvm-dev
>> > <llvm-dev at lists.llvm.org> wrote:
>> >>
>> >> At least in this test-case, the "bitfield" part of this seems to be a
>> >> distraction. As Eli notes, Clang has lowered the function to LLVM IR
>> >> containing consistent i16 operations. Despite that being a different
>> >> choice from GCC, it should still be correct and consistent.
>> >>
>> > I suspect that this is more prevalent with bitfields as they're more
>> > likely to have the load / bitwise op / store operations done on them,
>> > resulting in an access type that can be shortened. But yes, it's not
>> > specific to just bitfields.
>> >
>> > I'm more interested in consistency, to be honest. If the loads and
>> > stores for the bitfields (or other such shorten-able objects) were the
>> > same, then we wouldn't run into the store-to-load forwarding issue on
>> > x86 (I don't know about other platforms, but suspect that consistency
>> > wouldn't hurt). I liked Arthur's idea of accessing the object using
>> > the type size the bitfield was defined with (i8, i16, i256). It would
>> > help with improving the heuristic. The downside is that it could lead
>> > to un-optimal code, but that's the situation we have now, so...
>>
>> Okay, but what concretely are you suggesting here?  Clang IRGen is
>> emitting accesses with consistent sizes, and LLVM is making them
>> inconsistent.  Are you just asking Clang to emit smaller accesses
>> in the hope that LLVM won’t mess them up?
>
>
> I don't think this has anything to do with bit-fields or Clang's lowering. This seems to "just" be an optimizer issue (one that happens to show up for bit-field access patterns, but also affects other cases). Much-reduced testcase:
>
> unsigned short n;
> void set() { n |= 1; }
>
> For this testcase, -O2 generates a 1-byte 'or' instruction, which will often be a pessimization when there are also full-width accesses. I don't think the frontend can or should be working around this.

We're a bit (heh) tied in what we can do. LLVM's IR doesn't convey
that we're working with a bitfield, but as many pointed out it's not
bitfield-specific (I didn't mean to imply that it was *only* bitfields
affected by this with my initial post, that was just where it showed
up). The front-end generates code that's perfectly valid for the
various accesses, and changing what it generates doesn't seem like it
would be worth the hassle. (E.g. using i1, i3, etc. for the bitfields
(note I'm *not* suggesting we do this, I'm just using as an example).)

So what we're left with is trying to ensure that we don't generate
sub-optimal code via the DAG combiner and other passes. Some ideas off
the top of my head:

1. Avoid generating byte-sized bitwise operations, because as Richard
pointed out they can lead to pessimizations.

2. Come up with a better heuristic to determine if we're going to
generate inconsistent load/store accesses to the same object.

3. Transform the machine instructions before register allocation to
ensure that all accesses to the same address are the same size, or at
least that the stores are of equal or greater size than the loads.

Option (1) may be the simplest and could potentially solve our immediate issue.

Option (2) is difficult because the DAG combiner operates on a single
BB at a time and doesn't have visibility into past DAGs it modified.

Option (3) allows us to retain the current behavior and gives us
visibility into the whole function, not just an individual BB.

I kind of like option 3, but I'm open to other suggestions.

-bw