ping
> -----Original Message-----
> From: Tamar Christina
> Sent: Wednesday, January 15, 2025 2:08 PM
> To: Alex Coplan <alex.cop...@arm.com>; gcc-patches@gcc.gnu.org
> Cc: Richard Biener <rguent...@suse.de>; Jan Hubicka <hubi...@ucw.cz>
> Subject: RE: [PATCH 2/4] cfgloopmanip: Add infrastructure for scaling of
> multi-exit
> loops [PR117790]
>
> Ping
>
> > -----Original Message-----
> > From: Alex Coplan <alex.cop...@arm.com>
> > Sent: Monday, January 6, 2025 11:35 AM
> > To: gcc-patches@gcc.gnu.org
> > Cc: Richard Biener <rguent...@suse.de>; Jan Hubicka <hubi...@ucw.cz>; Tamar
> > Christina <tamar.christ...@arm.com>
> > Subject: [PATCH 2/4] cfgloopmanip: Add infrastructure for scaling of
> > multi-exit
> > loops [PR117790]
> >
> > As it stands, scale_loop_profile doesn't correctly handle loops with
> > multiple exits. In particular, in the case where the expected niters
> > exceeds iteration_bound, scale_loop_profile attempts to reduce the
> > number of iterations with a call to scale_loop_frequencies, which
> > multiplies the count of each BB by a given probability. This
> > transformation preserves the relationships between the counts of the BBs
> > within the loop (and thus the edge probabilities stay the same) but this
> > cannot possibly work for loops with multiple exits, since in order for
> > the expected niters to reduce (and counts along exit edges to remain the
> > same), the exit edge probabilities must increase, thus decreasing the
> > probabilities of the internal edges, meaning that the ratios of the
> > counts of the BBs inside the loop must change. So we need a different
> > approach (not a straightforward multiplicative scaling) to adjust the
> > expected niters of a loop with multiple exits.
> >
> > This patch introduces a new helper, flow_scale_loop_freqs, which can be
> > used to correctly scale the profile of a loop with multiple exits. It
> > is parameterized by a probability (with which to scale the header and
> > therefore the expected niters) and a lambda which gives the desired
> > counts for the exit edges. In this patch, to make things simpler,
> > flow_scale_loop_freqs only handles loop shapes without internal control
> > flow, and we introduce a predicate can_flow_scale_loop_freqs_p to test
> > whether a given loop meets these criteria. This restriction is
> > reasonable since this patch is motivated by fixing the profile
> > consistency for early break vectorization, and we don't currently
> > vectorize loops with internal control flow. We also fall back to a
> > multiplicative scaling (the status quo) for loops that
> > flow_scale_loop_freqs can't handle, so the patch should be a net
> > improvement.
> >
> > We wrap the call to flow_scale_loop_freqs in a helper
> > scale_loop_freqs_with_exit_counts which handles the above-mentioned
> > fallback. This wrapper is still generic in that it accepts a lambda to
> > allow overriding the desired exit edge counts. We specialize this with
> > another wrapper, scale_loop_freqs_hold_exit_counts (keeping the
> > counts along exit edges fixed), which is then used to implement the
> > niters-scaling case of scale_loop_profile, thus fixing this path through
> > the function for loops with multiple exits.
> >
> > Finally, we expose two new wrapper functions in cfgloopmanip.h for use
> > in subsequent vectorizer patches. scale_loop_profile_hold_exit_counts
> > is a variant of scale_loop_profile which assumes we want to keep the
> > counts along exit edges of the loop fixed through both parts of the
> > transformation (including the initial probability scale).
> > scale_loop_freqs_with_new_exit_count is intended to be used in a
> > subsequent patch when adding a skip edge around the epilog, where the
> > reduction of count entering the loop is mirrored by a reduced count
> > along a given exit edge.
> >
> > Bootstrapped/regtested as a series on aarch64-linux-gnu,
> > x86_64-linux-gnu, and arm-linux-gnueabihf. OK for trunk?
> >
> > Thanks,
> > Alex
> >
> > gcc/ChangeLog:
> >
> > PR tree-optimization/117790
> > * cfgloopmanip.cc (can_flow_scale_loop_freqs_p): New.
> > (flow_scale_loop_freqs): New.
> > (scale_loop_freqs_with_exit_counts): New.
> > (scale_loop_freqs_hold_exit_counts): New.
> > (scale_loop_profile): Refactor to use the newly-added
> > scale_loop_profile_1, and use scale_loop_freqs_hold_exit_counts to
> > correctly handle reducing the expected niters for loops with multiple
> > exits.
> > (scale_loop_freqs_with_new_exit_count): New.
> > (scale_loop_profile_1): New.
> > (scale_loop_profile_hold_exit_counts): New.
> > * cfgloopmanip.h (scale_loop_profile_hold_exit_counts): New.
> > (scale_loop_freqs_with_new_exit_count): New.
> > ---
> > gcc/cfgloopmanip.cc | 309
> ++++++++++++++++++++++++++++++++++++++++--
> > --
> > gcc/cfgloopmanip.h | 7 +
> > 2 files changed, 294 insertions(+), 22 deletions(-)
