On 2019-09-19 11:33, Martin Liška wrote:
Hi.
Function reordering has been around for quite some time and a naive
implementation was also part of my diploma thesis some time ago.
Currently, the GCC can reorder function based on first execution, which
happens with PGO and LTO of course. Known limitation is that the order
is preserved only partially as various symbols go into different
LTRANS partitions.
There has been some research in the area and I would point out the
Facebook paper
([1]) and Sony presentation ([2]). Based on that, I decided to make a
new implementation
in the GCC that does the same (in a proper way). First part of the
enablement are patches
to ld.bfd and ld.gold that come up with a new section .text.sorted,
that is always sorted.
Thoughts? I would definitely welcome any interesting measurement on a
bigger load.
Martin
Hi, Martin!
Some time ago I tried to do the same but didn't go that far.
I also used the C3 algorithm, except for the fact that ipa_fn_summary
contains information about size and time (somehow missed it). The linker
option --sort-section=name was used for prototyping. It, obviously,
sorts sections and allows to place functions in the desired order (by
placing them into named sections .text.sorted.NNNN, without patching
linkers or adjusting linker scripts). For testing my implementation I
used several benchmarks from SPEC2006: perlbench, sjeng and gobmk.
Unfortunately, no significant positive changes were obtained.
I've tested the proposed pass on perlbench with train and reference
input (PGO+LTO as a base) but couldn't obtain stable results (still
unclear environment or perlbench specific reasons).
Evgeny.
