Re: Auto-vectorization speeds up multiplication of large-precision numerics

On Mon, 7 Sep 2020 at 11:23, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I wrote:
> > I made some cosmetic changes to this and committed it.

Thanks!

>
> BTW, poking at this further, it seems that the patch only really
> works for gcc.  clang accepts the -ftree-vectorize switch, but
> looking at the generated asm shows that it does nothing useful.
> Which is odd, because clang does do loop vectorization.
>
> I tried adding -Rpass-analysis=loop-vectorize and got
>
> numeric.c:8341:3: remark: loop not vectorized: could not determine number of loop iterations
[-Rpass-analysis=loop-vectorize]
>                 for (i2 = 0; i2 <= i; i2++)

Hmm, yeah that's unfortunate. My guess is that the compiler would do
vectorization only if 'i' is a constant, which is not true for our
case.

-- 
Thanks,
-Amit Khandekar
Huawei Technologies