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

[James Y Knight via cfe-dev]

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.

Of course that insight does mean it's quite easy to create a test-case with
the exact same problematic store->load mismatch which doesn't use
bit-fields at all. For example:
short f2(short *bfs) {
    *bfs &= ~0x1;
    g();
    return *bfs;
}

creates the same bad sequence:
        movq    %rdi, %rbx
        andb    $-2, (%rdi)
        callq   g()
        movzwl  (%rbx), %eax

On Tue, May 26, 2020 at 9:30 PM John McCall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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