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

[Craig Topper via cfe-dev]

Similar issue with aligned extload widening. The load and store are both
i16 in IR.

struct s {
int x; // force alignment
short y;
};

void g();
short f2(s *a, short b) {
a->y = b;
g();
return a->y - b;
}

produces this
movq %rdi, %rbx
movw %bp, 4(%rdi) // 2 byte store
callq g()
movl 4(%rbx), %eax // 4 byte load
subl %ebp, %eax

~Craig


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.
>
> 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.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
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