[PATCH] D153993: [Headers][doc] Add load/store/cmp/cvt intrinsic descriptions to avx2intrin.h

[Paul Robinson via Phabricator via cfe-commits]

probinson added inline comments.


================
Comment at: clang/lib/Headers/avx2intrin.h:3474
+///   IF __M[j+31] == 1
+///     result[j+31:j] := Load32(__X+(i*4))
+///   ELSE
----------------
pengfei wrote:
> probinson wrote:
> > pengfei wrote:
> > > A more intrinsic guide format is `MEM[__X+j:j]`
> > LoadXX is the syntax in the gather intrinsics, e.g. _mm_mask_i32gather_pd. I'd prefer to be consistent.
> I think the problem here is the measurement is easily confusing.
> From C point of view, `__X` is a `int` pointer, so we should `+ i` rather than `i * 4`
> From the other part of the code, we are measuring in bits, but here `i * 4` is a byte offset.
Well, the pseudo-code is clearly not C. If you look at the gather code, it computes a byte address using an offset multiplied by an explicit scale factor. I am doing exactly the same here.

The syntax `MEM[__X+j:j]` is mixing a byte address with a bit offset, which I think is more confusing. To be fully consistent, using `[]` with bit offsets only, it should be
```
k := __X*8 + i*32
result[j+31:j] := MEM[k+31:k]
```
which I think obscures more than it explains.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153993/new/

https://reviews.llvm.org/D153993