On Thu, Jan 28, 2021 at 6:28 AM Alexandre Oliva <ol...@adacore.com> wrote:
>
> On Jan 27, 2021, Richard Biener <richard.guent...@gmail.com> wrote:
>
> > That said, rather than not transforming the loop as you do I'd
> > say we want to re-inline small copies more forcefully during
> > loop distribution code-gen so we turn a loop that sets
> > 3 'short int' to zero into a 'int' store and a 'short' store for example
> > (we have more code-size leeway here because we formerly had
> > a loop).
>
> Ok, that makes sense to me, if there's a chance of growing the access
> stride.
>
> > Since you don't add a testcase
>
> Uhh, sorry, I mentioned TFmode emulation calls, but I wasn't explicit I
> meant the soft-fp ones from libgcc.
>
> ./xgcc -B./ -O2 $srcdir/libgcc/soft-fp/fixtfdi.c \
> -I$srcdir/libgcc/config/riscv -I$srcdir/include \
> -S -o - | grep memset
>
> > I can't see whether the actual case would be fixed by setting SSA
> > pointer alignment on the memset arguments
>
> The dest pointer alignment is known at the builtin expansion time,
> that's not a problem. What isn't known (*) is that the length is a
> multiple of the word size: what gets to the expander is that it's
> between 4 and 12 bytes (targeting 32-bit risc-v), but not that it's
> either 4, 8 or 12. Coming up with an efficient inline expansion becomes
> a bit of a challenge without that extra knowledge.
Ah, yes - that's also useful information. I guess for memset
an enhanced builtin would then have calloc style
size and nmemb arguments where the destination is expected
to have at least 'size' alignment. That would allow turning
back the memset into the original loop (but with optimal IVs, etc.).
So kind of the x86 rep mov{bwlq}. Now think of sth similarly
useful for memcpy/move.
Richard.
>
>
> (*) at least before my related patch for get_nonzero_bits
> https://gcc.gnu.org/pipermail/gcc-patches/2021-January/564344.html
>
> --
> Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
> Free Software Activist GNU Toolchain Engineer
> Vim, Vi, Voltei pro Emacs -- GNUlius Caesar
