optimization problem: ptr not kept in register

[Ian Lance Taylor]

On Tue, Mar 25, 2014 at 12:38 PM, Peter A. Felvegi
<petschy@praire-chicken.com> wrote:
>
> The reduced test case is at the end. It encodes data into a buffer in a loop
> with variable length encoding (not a working real encoding). For some
> reason, the write ptr is not kept in a register, but loaded/stored when
> used/updated. There is a potential function call in the loop, but there are
> __builtin_expect hints, so I think it would be possible to use a register
> for the ptr and store just before the call, and load it back right after the
> call. This would speed up the common code path: less code, less loads and
> stores.I measured around 20-30% more runtime, compared to a version where a
> pointer goes in and the updated ptr is returned. However, passing/returning
> the ptr has other issues, esp for a decoder, that would return the decoded
> value normally, not the ptr.

You marked the encode_noinline function as noinline, and encode can
call encode_noinline.  The encode_noinline function could change any
part of global memory, and in particular could change the value of
n->next.  So the loop has to reload that value, in case it was
changed.

Ian