On 1 August 2013 18:32, Teresa Johnson <tejohn...@google.com> wrote:
> Patch 3 of 3 split out from the patch I sent in May that fixes problems with
> -freorder-blocks-and-partition, with changes/fixes discussed in that thread.
>
> See http://gcc.gnu.org/ml/gcc-patches/2013-05/threads.html#00388 for context.
>
> This patch sanitizes the partitioning to address issues such as edge
> weight insanities that sometimes occur due to upstream optimizations,
> and ensures that hot blocks are not dominated by cold blocks. This
> needs to be resanitized after certain cfg optimizations that may
> cause hot blocks previously reached via both hot and cold paths to
> only be reached by cold paths.
>
> The verification code in sanitize_dominator_hotness was contributed by
> Steven Bosscher.
>
> Bootstrapped and tested on x86-64-unknown-linux-gnu. Also ensured that
> a profiledbootstrap passed with -freorder-blocks-and-partition enabled
> (and with the dwarf version changed to 2 to work around PR57451).
>
> Ok for trunk?
>
> (I also included the patch as an attachment since my mailer invariably
> messes up the formatting in the pasted version.)
>
> Thanks,
> Teresa
>
> 2013-08-01 Teresa Johnson <tejohn...@google.com>
> Steven Bosscher <ste...@gcc.gnu.org>
>
> * cfgrtl.c (fixup_bb_partition): New routine.
> (commit_edge_insertions): Invoke fixup_partitions.
> (find_partition_fixes): New routine.
> (fixup_partitions): Ditto.
> (verify_hot_cold_block_grouping): Update comments.
> (rtl_verify_edges): Invoke find_partition_fixes.
> (rtl_verify_bb_pointers): Update comments.
> (rtl_verify_bb_layout): Ditto.
> * basic-block.h (fixup_partitions): Declare.
> * cfgcleanup.c (try_optimize_cfg): Invoke fixup_partitions.
> * bb-reorder.c (sanitize_dominator_hotness): New function.
> (find_rarely_executed_basic_blocks_and_crossing_edges): Invoke
> sanitize_dominator_hotness.
>
> Index: cfgrtl.c
> ===================================================================
> --- cfgrtl.c (revision 201281)
> +++ cfgrtl.c (working copy)
> @@ -1341,6 +1341,34 @@ fixup_partition_crossing (edge e)
> }
> }
>
> +/* Called when block BB has been reassigned to a different partition,
> + to ensure that the region crossing attributes are updated. */
> +
> +static void
> +fixup_bb_partition (basic_block bb)
> +{
> + edge e;
> + edge_iterator ei;
> +
> + /* Now need to make bb's pred edges non-region crossing. */
> + FOR_EACH_EDGE (e, ei, bb->preds)
> + {
> + fixup_partition_crossing (e);
> + }
> +
> + /* Possibly need to make bb's successor edges region crossing,
> + or remove stale region crossing. */
> + FOR_EACH_EDGE (e, ei, bb->succs)
> + {
> + if ((e->flags & EDGE_FALLTHRU)
> + && BB_PARTITION (bb) != BB_PARTITION (e->dest)
> + && e->dest != EXIT_BLOCK_PTR)
> + force_nonfallthru (e);
> + else
> + fixup_partition_crossing (e);
> + }
> +}
> +
> /* Attempt to change code to redirect edge E to TARGET. Don't do that on
> expense of adding new instructions or reordering basic blocks.
>
> @@ -1979,6 +2007,14 @@ commit_edge_insertions (void)
> {
> basic_block bb;
>
> + /* Optimization passes that invoke this routine can cause hot blocks
> + previously reached by both hot and cold blocks to become dominated only
> + by cold blocks. This will cause the verification below to fail,
> + and lead to now cold code in the hot section. In some cases this
> + may only be visible after newly unreachable blocks are deleted,
> + which will be done by fixup_partitions. */
> + fixup_partitions ();
> +
> #ifdef ENABLE_CHECKING
> verify_flow_info ();
> #endif
> @@ -2173,6 +2209,101 @@ get_last_bb_insn (basic_block bb)
> return end;
> }
>
> +/* Sanity check partition hotness to ensure that basic blocks in
> + the cold partition don't dominate basic blocks in the hot partition.
> + If FLAG_ONLY is true, report violations as errors. Otherwise
> + re-mark the dominated blocks as cold, since this is run after
> + cfg optimizations that may make hot blocks previously reached
> + by both hot and cold blocks now only reachable along cold paths. */
> +
> +vec<basic_block>
> +find_partition_fixes (bool flag_only)
> +{
> + basic_block bb;
> + vec<basic_block> bbs_in_cold_partition = vNULL;
> + vec<basic_block> bbs_to_fix = vNULL;
> +
> + if (!crtl->has_bb_partition)
> + return vNULL;
I'd push this early return into the callers instead, at most turn it into a
gcc_checking_assert to be safe.
Both callers, fixup_partitions and rtl_verify_edges, look at
ctrl->has_bb_partition already before calling this, so the above should
be dead already.
Did my mailer somehow swallow the static from find_partition_fixes?
> +
> + FOR_EACH_BB (bb)
> + if ((BB_PARTITION (bb) == BB_COLD_PARTITION))
> + bbs_in_cold_partition.safe_push (bb);
> +
> + if (bbs_in_cold_partition.is_empty ())
> + return vNULL;
> +
> + bool dom_calculated_here = !dom_info_available_p (CDI_DOMINATORS);
> +
> + if (dom_calculated_here)
> + calculate_dominance_info (CDI_DOMINATORS);
> +
> + while (! bbs_in_cold_partition.is_empty ())
> + {
> + bb = bbs_in_cold_partition.pop ();
> + /* Any blocks dominated by a block in the cold section
> + must also be cold. */
> + basic_block son;
> + for (son = first_dom_son (CDI_DOMINATORS, bb);
> + son;
> + son = next_dom_son (CDI_DOMINATORS, son))
> + {
> + /* If son is not yet cold, then mark it cold here and
> + enqueue it for further processing. */
> + if ((BB_PARTITION (son) != BB_COLD_PARTITION))
> + {
> + if (flag_only)
> + error ("non-cold basic block %d dominated "
> + "by a block in the cold partition", son->index);
> + else
> + BB_SET_PARTITION (son, BB_COLD_PARTITION);
> + bbs_to_fix.safe_push (son);
> + bbs_in_cold_partition.safe_push (son);
> + }
> + }
> + }
> +
> + if (dom_calculated_here)
> + free_dominance_info (CDI_DOMINATORS);
> +
> + return bbs_to_fix;
> +}
> +
> +/* Perform cleanup on the hot/cold bb partitioning after optimization
> + passes that modify the cfg. */
> +
> +void
> +fixup_partitions (void)
> +{
> + basic_block bb;
> +
> + if (!crtl->has_bb_partition)
> + return;
> +
> + /* Delete any blocks that became unreachable and weren't
> + already cleaned up, for example during edge forwarding
> + and convert_jumps_to_returns. This will expose more
> + opportunities for fixing the partition boundaries here.
> + Also, the calculation of the dominance graph during verification
> + will assert if there are unreachable nodes. */
> + delete_unreachable_blocks ();
> +
> + /* If there are partitions, do a sanity check on them: A basic block in
> + a cold partition cannot dominate a basic block in a hot partition.
> + Fixup any that now violate this requirement, as a result of edge
> + forwarding and unreachable block deletion. */
> + vec<basic_block> bbs_to_fix = find_partition_fixes (false);
> +
> + /* Do the partition fixup after all necessary blocks have been converted to
> + cold, so that we only update the region crossings the minimum number of
> + places, which can require forcing edges to be non fallthru. */
> + while (! bbs_to_fix.is_empty ())
> + {
> + bb = bbs_to_fix.pop ();
> + fixup_bb_partition (bb);
> + }
> +}
> +
> /* Verify, in the basic block chain, that there is at most one switch
> between hot/cold partitions. This condition will not be true until
> after reorder_basic_blocks is called. */
> @@ -2219,7 +2350,8 @@ verify_hot_cold_block_grouping (void)
> /* Perform several checks on the edges out of each block, such as
> the consistency of the branch probabilities, the correctness
> of hot/cold partition crossing edges, and the number of expected
> - successor edges. */
> + successor edges. Also verify that the dominance relationship
> + between hot/cold blocks is sane. */
>
> static int
> rtl_verify_edges (void)
> @@ -2382,6 +2514,14 @@ rtl_verify_edges (void)
> }
> }
>
> + /* If there are partitions, do a sanity check on them: A basic block in
> + a cold partition cannot dominate a basic block in a hot partition. */
> + if (crtl->has_bb_partition && !err)
> + {
> + vec<basic_block> bbs_to_fix = find_partition_fixes (true);
> + err = !bbs_to_fix.is_empty ();
> + }
> +
> /* Clean up. */
> return err;
> }
> @@ -2515,7 +2655,7 @@ rtl_verify_bb_pointers (void)
> and NOTE_INSN_BASIC_BLOCK
> - verify that no fall_thru edge crosses hot/cold partition boundaries
> - verify that there are no pending RTL branch predictions
> - - verify that there is a single hot/cold partition boundary after bbro
> + - verify that hot blocks are not dominated by cold blocks
>
> In future it can be extended check a lot of other stuff as well
> (reachability of basic blocks, life information, etc. etc.). */
> @@ -2761,7 +2901,8 @@ rtl_verify_bb_layout (void)
> - check that all insns are in the basic blocks
> (except the switch handling code, barriers and notes)
> - check that all returns are followed by barriers
> - - check that all fallthru edge points to the adjacent blocks. */
> + - check that all fallthru edge points to the adjacent blocks
> + - verify that there is a single hot/cold partition boundary after bbro */
>
> static int
> rtl_verify_flow_info (void)
> Index: basic-block.h
> ===================================================================
> --- basic-block.h (revision 201281)
> +++ basic-block.h (working copy)
> @@ -797,6 +797,7 @@ extern bool contains_no_active_insn_p (const_basic
> extern bool forwarder_block_p (const_basic_block);
> extern bool can_fallthru (basic_block, basic_block);
> extern void emit_barrier_after_bb (basic_block bb);
> +extern void fixup_partitions (void);
>
> /* In cfgbuild.c. */
> extern void find_many_sub_basic_blocks (sbitmap);
> Index: cfgcleanup.c
> ===================================================================
> --- cfgcleanup.c (revision 201281)
> +++ cfgcleanup.c (working copy)
> @@ -2807,10 +2807,21 @@ try_optimize_cfg (int mode)
> df_analyze ();
> }
>
> + if (changed)
> + {
> + /* Edge forwarding in particular can cause hot blocks
> previously
> + reached by both hot and cold blocks to become dominated only
> + by cold blocks. This will cause the verification
> below to fail,
> + and lead to now cold code in the hot section. This is not
> easy
> + to detect and fix during edge forwarding, and in some cases
> + is only visible after newly unreachable blocks are deleted,
> + which will be done in fixup_partitions. */
> + fixup_partitions ();
> +
> #ifdef ENABLE_CHECKING
> - if (changed)
> - verify_flow_info ();
> + verify_flow_info ();
> #endif
> + }
>
> changed_overall |= changed;
> first_pass = false;
> Index: bb-reorder.c
> ===================================================================
> --- bb-reorder.c (revision 201281)
> +++ bb-reorder.c (working copy)
> @@ -1444,6 +1444,55 @@ fix_up_crossing_landing_pad (eh_landing_pad old_lp
> ei_next (&ei);
> }
>
> +
> +/* Ensure that no cold bbs dominate hot bbs along the dominance or
> + post-dominance DIR, for example as a result of edge weight insanities.
> + Returns the updated value of COLD_BB_COUNT and adds newly-hot bbs
> + to BBS_IN_HOT_PARTITION. */
> +
> +static unsigned int
> +sanitize_dominator_hotness (enum cdi_direction dir, unsigned int
> cold_bb_count,
> + vec<basic_block> *bbs_in_hot_partition)
> +{
> + if (!cold_bb_count)
> + return 0;
Same pattern as above. Callers do not invoke us if !cold_bb_count so the above
check is dead code. Again, remove or checking assert?
> +
> + bool dom_calculated_here = !dom_info_available_p (dir);
> +
> + if (dom_calculated_here)
> + calculate_dominance_info (dir);
> +
> + /* Keep examining hot bbs until we have either checked them all, or
> + re-marked all cold bbs as hot. */
> + vec<basic_block> hot_bbs_to_check = bbs_in_hot_partition->copy ();
> + while (! hot_bbs_to_check.is_empty ()
> + && cold_bb_count)
The comment says "or", which sounds plausible, but the code says "and"?
> + {
> + basic_block bb = hot_bbs_to_check.pop ();
> + basic_block dom_bb = get_immediate_dominator (dir, bb);
> +
> + /* If bb's immediate dominator is also hot then it is ok. */
> + if (BB_PARTITION (dom_bb) != BB_COLD_PARTITION)
Why not follow the comment here and == BB_HOT_PARTITION instead, for clarity?
> + continue;
> +
> + /* We have a hot bb with an immediate dominator that is cold.
> + The dominator needs to be re-marked hot. */
> + BB_SET_PARTITION (dom_bb, BB_HOT_PARTITION);
> + cold_bb_count--;
> +
> + /* Now we need to examine newly-hot dom_bb to see if it is also
> + dominated by a cold bb. */
> + bbs_in_hot_partition->safe_push (dom_bb);
> + hot_bbs_to_check.safe_push (dom_bb);
> + }
> +
> + if (dom_calculated_here)
> + free_dominance_info (dir);
> +
> + return cold_bb_count;
> +}
> +
> +
> /* Find the basic blocks that are rarely executed and need to be moved to
> a separate section of the .o file (to cut down on paging and improve
> cache locality). Return a vector of all edges that cross. */
> @@ -1455,16 +1504,42 @@ find_rarely_executed_basic_blocks_and_crossing_edg
> basic_block bb;
> edge e;
> edge_iterator ei;
> + unsigned int cold_bb_count = 0;
> + vec<basic_block> bbs_in_hot_partition = vNULL;
>
> /* Mark which partition (hot/cold) each basic block belongs in. */
> FOR_EACH_BB (bb)
> {
> if (probably_never_executed_bb_p (cfun, bb))
> - BB_SET_PARTITION (bb, BB_COLD_PARTITION);
> + {
> + BB_SET_PARTITION (bb, BB_COLD_PARTITION);
> + cold_bb_count++;
> + }
> else
> - BB_SET_PARTITION (bb, BB_HOT_PARTITION);
> + {
> + BB_SET_PARTITION (bb, BB_HOT_PARTITION);
> + bbs_in_hot_partition.safe_push (bb);
> + }
> }
>
> + /* Ensure that no cold bbs dominate hot bbs. This could happen as a result
> of
> + several different possibilities. One is that there are edge
> weight insanities
> + due to optimization phases that do not properly update basic block
> profile
> + counts. The second is that the entry of the function may not be
> hot, because
> + it is entered fewer times than the number of profile training
> runs, but there
> + is a loop inside the function that causes blocks within the function to
> be
> + above the threshold for hotness. Then do the same along the
> post-dominator
> + tree (which could have additional changes required after fixing up
> + dominators). */
> + if (cold_bb_count)
> + cold_bb_count = sanitize_dominator_hotness (CDI_DOMINATORS,
> + cold_bb_count,
> + &bbs_in_hot_partition);
> + if (cold_bb_count)
> + cold_bb_count = sanitize_dominator_hotness (CDI_POST_DOMINATORS,
> + cold_bb_count,
> + &bbs_in_hot_partition);
I take it this last store to cold_bb_count is eliminated anyway.
Thanks,
> +
> /* The format of .gcc_except_table does not allow landing pads to
> be in a different partition as the throw. Fix this by either
> moving or duplicating the landing pads. */
>
>
> --
> Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
