> On Dec 14, 2020, at 08:24, Nicolai Hähnle via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
> Hi Paul,
>
> On Sat, Dec 12, 2020 at 5:37 PM Paul C. Anagnostopoulos via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
> The Global ISel match table is a sequence of 64-bit entries. For the
AMDGPU, there are about 261,000 entries. for the PowerPC, about 32,000.
>
> A significant amount of memory could be saved by reducing the entries to 32
bits. This would require reworking the matcher to deal with 32-bit entries, and
then re-reworking some of the entries to pick up and assemble two 32-bit entries
into one 64-bit integer.
>
> I was thinking the same thing recently.
>
> By comparison, SelectionDAG's table is even byte-based, it would be
interesting to make the comparison with that as well. Going to byte granularity
surely saves even more space, which is itself a performance benefit, but may
also have a performance cost due to misalignment. (SelectionDAG also uses
dynamic encoding length for integers in places, which seems a more dubious
choice.)
>
> Cheers,
> Nicolai
>
>
>
> I won't be surprised if there are compelling reasons to keep the
entries at 64 bits. I'd like to hear those reasons.
Most of the reasons boil down to not having the time needed to implement
something better. There is one bit I'm aware of that snowballs a bit when
using uint32_t or smaller. Labels and JumpTargets are currently absolute which
allows them to be recorded in a simple lookup table during the first pass to
encode on the second. JumpTargets will need to become relative (and know where
they are in the table) to cope with a large ruleset and reduced range but that
also means you have to start measuring distances between two points in the
table. For fixed-sized commands that's not too bad but if the range drops
below the minimum then you also have to go to variable length commands (or waste
space on padding). That in turn means that you need to determine the encoded
size of commands on the first pass (including JumpTargets which have unknown
encoding until the label is seen later) so that the labels know their position
for the second pass when we encode the table. We'll have to go from the
two-pass approach to a relaxation one.
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> --
> Lerne, wie die Welt wirklich ist,
> aber vergiss niemals, wie sie sein sollte.
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