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(-)
