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Re: [PATCH] Take branch misprediction effects into account when RTL loop unrolling (issue6099055)


Ping?
Teresa

On Fri, May 4, 2012 at 3:41 PM, Teresa Johnson <tejohnson@google.com> wrote:
>
> On David's suggestion, I have removed the changes that rename niter_desc
> to
> loop_desc from this patch to focus the patch on the unrolling changes. I
> can
> submit a cleanup patch to do the renaming as soon as this one goes in.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu. ?Ok for trunk?
>
> Thanks,
> Teresa
>
> Here is the new description of improvements from the original patch:
>
> Improved patch based on feedback. Main changes are:
>
> 1) Improve efficiency by caching loop analysis results in the loop
> auxiliary
> info structure hanging off the loop structure. Added a new routine,
> analyze_loop_insns, to fill in information about the average and total
> number
> of branches, as well as whether there are any floating point set and call
> instructions in the loop. The new routine is invoked when we first create
> a
> loop's niter_desc struct, and the caller (get_simple_loop_desc) has been
> modified to handle creating a niter_desc for the fake outermost loop.
>
> 2) Improvements to max_unroll_with_branches:
> - Treat the fake outermost loop (the procedure body) as we would a hot
> outer
> loop, i.e. compute the max unroll looking at its nested branches, instead
> of
> shutting off unrolling when we reach the fake outermost loop.
> - Pull the checks previously done in the caller into the routine (e.g.
> whether the loop iterates frequently or contains fp instructions).
> - Fix a bug in the previous version that sometimes caused overflow in the
> new unroll factor.
>
> 3) Remove float variables, and use integer computation to compute the
> average number of branches in the loop.
>
> 4) Detect more types of floating point computations in the loop by walking
> all set instructions, not just single sets.
>
> 2012-05-04 ? Teresa Johnson ?<tejohnson@google.com>
>
> ? ? ? ?* doc/invoke.texi: Update the documentation with new params.
> ? ? ? ?* loop-unroll.c (max_unroll_with_branches): New function.
> ? ? ? ?(decide_unroll_constant_iterations,
> decide_unroll_runtime_iterations):
> ? ? ? ?Add heuristic to avoid increasing branch mispredicts when
> unrolling.
> ? ? ? ?(decide_peel_simple, decide_unroll_stupid): Retrieve number of
> ? ? ? ?branches from niter_desc instead of via function that walks loop.
> ? ? ? ?* loop-iv.c (get_simple_loop_desc): Invoke new analyze_loop_insns
> ? ? ? ?function, and add guards to enable this function to work for the
> ? ? ? ?outermost loop.
> ? ? ? ?* cfgloop.c (insn_has_fp_set, analyze_loop_insns): New functions.
> ? ? ? ?(num_loop_branches): Remove.
> ? ? ? ?* cfgloop.h (struct loop_desc): Added new fields to cache
> additional
> ? ? ? ?loop analysis information.
> ? ? ? ?(num_loop_branches): Remove.
> ? ? ? ?(analyze_loop_insns): Declare.
> ? ? ? ?* params.def (PARAM_MIN_ITER_UNROLL_WITH_BRANCHES): New param.
> ? ? ? ?(PARAM_UNROLL_OUTER_LOOP_BRANCH_BUDGET): Ditto.
>
> Index: doc/invoke.texi
> ===================================================================
> --- doc/invoke.texi ? ? (revision 187013)
> +++ doc/invoke.texi ? ? (working copy)
> @@ -8842,6 +8842,12 @@ The maximum number of insns of an unswitched loop.
> ?@item max-unswitch-level
> ?The maximum number of branches unswitched in a single loop.
>
> +@item min-iter-unroll-with-branches
> +Minimum iteration count to ignore branch effects when unrolling.
> +
> +@item unroll-outer-loop-branch-budget
> +Maximum number of branches allowed in hot outer loop region after unroll.
> +
> ?@item lim-expensive
> ?The minimum cost of an expensive expression in the loop invariant motion.
>
> Index: loop-unroll.c
> ===================================================================
> --- loop-unroll.c ? ? ? (revision 187013)
> +++ loop-unroll.c ? ? ? (working copy)
> @@ -152,6 +152,99 @@ static void combine_var_copies_in_loop_exit (struc
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? basic_block);
> ?static rtx get_expansion (struct var_to_expand *);
>
> +/* Compute the maximum number of times LOOP can be unrolled without
> exceeding
> + ? a branch budget, which can increase branch mispredictions. The number
> of
> + ? branches is computed by weighting each branch with its expected
> execution
> + ? probability through the loop based on profile data. If no profile
> feedback
> + ? data exists, simply return the current NUNROLL factor. ?*/
> +
> +static unsigned
> +max_unroll_with_branches(struct loop *loop, unsigned nunroll)
> +{
> + ?struct loop *outer;
> + ?struct niter_desc *outer_desc = 0;
> + ?int outer_niters = 1;
> + ?int frequent_iteration_threshold;
> + ?unsigned branch_budget;
> + ?struct niter_desc *desc = get_simple_loop_desc (loop);
> +
> + ?/* Ignore loops with FP computation as these tend to benefit much more
> + ? ? consistently from unrolling. ?*/
> + ?if (desc->has_fp)
> + ? ?return nunroll;
> +
> + ?frequent_iteration_threshold = PARAM_VALUE
> (PARAM_MIN_ITER_UNROLL_WITH_BRANCHES);
> + ?if (expected_loop_iterations (loop) >= (unsigned)
> frequent_iteration_threshold)
> + ? ?return nunroll;
> +
> + ?/* If there was no profile feedback data, av_num_branches will be 0
> + ? ? and we won't limit unrolling. If the av_num_branches is at most 1,
> + ? ? also don't limit unrolling as the back-edge branch will not be
> duplicated. ?*/
> + ?if (desc->av_num_branches <= 1)
> + ? ?return nunroll;
> +
> + ?/* Walk up the loop tree until we find a hot outer loop in which the
> current
> + ? ? loop is nested. At that point we will compute the number of times
> the
> + ? ? current loop can be unrolled based on the number of branches in the
> hot
> + ? ? outer loop. ?*/
> + ?outer = loop_outer (loop);
> + ?/* The loop structure contains a fake outermost loop, so this should
> always
> + ? ? be non-NULL for our current loop. ?*/
> + ?gcc_assert (outer);
> +
> + ?/* Walk up the loop tree until we either find a hot outer loop or hit
> the
> + ? ? fake outermost loop at the root. ?*/
> + ?while (true)
> + ? ?{
> + ? ? ?outer_desc = get_simple_loop_desc (outer);
> +
> + ? ? ?/* Stop if we hit the fake outermost loop at the root of the tree,
> + ? ? ? ? which includes the whole procedure. ?*/
> + ? ? ?if (!loop_outer (outer))
> + ? ? ? ?break;
> +
> + ? ? ?if (outer_desc->const_iter)
> + ? ? ? ?outer_niters *= outer_desc->niter;
> + ? ? ?else if (outer->header->count)
> + ? ? ? ?outer_niters *= expected_loop_iterations (outer);
> +
> + ? ? ?/* If the outer loop has enough iterations to be considered hot,
> then
> + ? ? ? ? we can stop our upwards loop tree traversal and examine the
> current
> + ? ? ? ? outer loop. ?*/
> + ? ? ?if (outer_niters >= frequent_iteration_threshold)
> + ? ? ? ?break;
> +
> + ? ? ?outer = loop_outer (outer);
> + ? ?}
> +
> + ?gcc_assert(outer);
> +
> + ?/* Assume that any call will cause the branch budget to be exceeded,
> + ? ? and that we can't unroll the current loop without increasing
> + ? ? mispredicts. ?*/
> + ?if (outer_desc->has_call)
> + ? ?return 0;
> +
> + ?/* Otherwise, compute the maximum number of times current loop can be
> + ? ? unrolled without exceeding our branch budget. First we subtract
> + ? ? off the outer loop's average branch count from the budget. Note
> + ? ? that this includes the branches in the current loop. This yields
> + ? ? the number of branches left in the budget for the unrolled copies.
> + ? ? We divide this by the number of branches in the current loop that
> + ? ? must be duplicated when we unroll, which is the total average
> + ? ? number of branches minus the back-edge branch. This yields the
> + ? ? number of new loop body copies that can be created by unrolling
> + ? ? without exceeding the budget, to which we add 1 to get the unroll
> + ? ? factor. Note that the "outermost loop" may be the whole procedure
> + ? ? if we did not find a hot enough enclosing loop. ?*/
> + ?branch_budget = PARAM_VALUE (PARAM_UNROLL_OUTER_LOOP_BRANCH_BUDGET);
> + ?if (outer_desc->av_num_branches > branch_budget)
> + ? ?return 0;
> + ?/* We already returned early if desc->av_num_branches <= 1. ?*/
> + ?return (branch_budget - outer_desc->av_num_branches)
> + ? ? ?/ (desc->av_num_branches - 1) + 1;
> +}
> +
> ?/* Unroll and/or peel (depending on FLAGS) LOOPS. ?*/
> ?void
> ?unroll_and_peel_loops (int flags)
> @@ -522,6 +615,7 @@ static void
> ?decide_unroll_constant_iterations (struct loop *loop, int flags)
> ?{
> ? unsigned nunroll, nunroll_by_av, best_copies, best_unroll = 0, n_copies,
> i;
> + ?unsigned nunroll_branches;
> ? struct niter_desc *desc;
>
> ? if (!(flags & UAP_UNROLL))
> @@ -565,6 +659,21 @@ decide_unroll_constant_iterations (struct loop *lo
> ? ? ? return;
> ? ? }
>
> + ?/* Be careful when unrolling loops with branches inside -- it can
> increase
> + ? ? the number of mispredicts. ?*/
> + ?if (desc->num_branches > 1)
> + ? ?{
> + ? ? ?nunroll_branches = max_unroll_with_branches (loop, nunroll);
> + ? ? ?if (nunroll > nunroll_branches)
> + ? ? ? ?nunroll = nunroll_branches;
> + ? ? ?if (nunroll <= 1)
> + ? ? ? ?{
> + ? ? ? ? ?if (dump_file)
> + ? ? ? ? ? fprintf (dump_file, ";; Not unrolling, contains branches\n");
> + ? ? ? ? ?return;
> + ? ? ? ?}
> + ? ?}
> +
> ? /* Check whether the loop rolls enough to consider. ?*/
> ? if (desc->niter < 2 * nunroll)
> ? ? {
> @@ -802,7 +911,7 @@ unroll_loop_constant_iterations (struct loop *loop
> ?static void
> ?decide_unroll_runtime_iterations (struct loop *loop, int flags)
> ?{
> - ?unsigned nunroll, nunroll_by_av, i;
> + ?unsigned nunroll, nunroll_by_av, nunroll_branches, i;
> ? struct niter_desc *desc;
>
> ? if (!(flags & UAP_UNROLL))
> @@ -856,6 +965,21 @@ decide_unroll_runtime_iterations (struct loop *loo
> ? ? ? return;
> ? ? }
>
> + ?/* Be careful when unrolling loops with branches inside -- it can
> increase
> + ? ? the number of mispredicts. ?*/
> + ?if (desc->num_branches > 1)
> + ? ?{
> + ? ? ?nunroll_branches = max_unroll_with_branches (loop, nunroll);
> + ? ? ?if (nunroll > nunroll_branches)
> + ? ? ? ?nunroll = nunroll_branches;
> + ? ? ?if (nunroll <= 1)
> + ? ? ? ?{
> + ? ? ? ? ?if (dump_file)
> + ? ? ? ? ? ?fprintf (dump_file, ";; Not unrolling, contains branches\n");
> + ? ? ? ? ?return;
> + ? ? ? ?}
> + ? ?}
> +
> ? /* If we have profile feedback, check whether the loop rolls. ?*/
> ? if ((loop->header->count
> ? ? ? ?&& expected_loop_iterations (loop) < 2 * nunroll)
> @@ -1233,7 +1357,7 @@ decide_peel_simple (struct loop *loop, int flags)
>
> ? /* Do not simply peel loops with branches inside -- it increases number
> ? ? ?of mispredicts. ?*/
> - ?if (num_loop_branches (loop) > 1)
> + ?if (desc->num_branches > 1)
> ? ? {
> ? ? ? if (dump_file)
> ? ? ? ?fprintf (dump_file, ";; Not peeling, contains branches\n");
> @@ -1394,7 +1518,7 @@ decide_unroll_stupid (struct loop *loop, int flags
>
> ? /* Do not unroll loops with branches inside -- it increases number
> ? ? ?of mispredicts. ?*/
> - ?if (num_loop_branches (loop) > 1)
> + ?if (desc->num_branches > 1)
> ? ? {
> ? ? ? if (dump_file)
> ? ? ? ?fprintf (dump_file, ";; Not unrolling, contains branches\n");
> Index: loop-iv.c
> ===================================================================
> --- loop-iv.c ? (revision 187013)
> +++ loop-iv.c ? (working copy)
> @@ -2948,8 +2948,12 @@ get_simple_loop_desc (struct loop *loop)
> ? /* At least desc->infinite is not always initialized by
> ? ? ?find_simple_loop_exit. ?*/
> ? desc = XCNEW (struct niter_desc);
> - ?iv_analysis_loop_init (loop);
> - ?find_simple_exit (loop, desc);
> + ?if (loop->latch != EXIT_BLOCK_PTR)
> + ? ?{
> + ? ? ?iv_analysis_loop_init (loop);
> + ? ? ?find_simple_exit (loop, desc);
> + ? ?}
> + ?analyze_loop_insns (loop, desc);
> ? loop->aux = desc;
>
> ? if (desc->simple_p && (desc->assumptions || desc->infinite))
> Index: cfgloop.c
> ===================================================================
> --- cfgloop.c ? (revision 187013)
> +++ cfgloop.c ? (working copy)
> @@ -1156,24 +1156,98 @@ get_loop_exit_edges (const struct loop *loop)
> ? return edges;
> ?}
>
> -/* Counts the number of conditional branches inside LOOP. ?*/
> +/* Determine if INSN is a floating point set. ?*/
>
> -unsigned
> -num_loop_branches (const struct loop *loop)
> +static bool
> +insn_has_fp_set(rtx insn)
> ?{
> - ?unsigned i, n;
> - ?basic_block * body;
> + ?int i;
> + ?rtx pat = PATTERN(insn);
> + ?if (GET_CODE (pat) == SET)
> + ? ?return (FLOAT_MODE_P (GET_MODE (SET_DEST (pat))));
> + ?else if (GET_CODE (pat) == PARALLEL)
> + ? ?{
> + ? ? ?for (i = 0; i < XVECLEN (pat, 0); i++)
> + ? ? ? ?{
> + ? ? ? ? ?rtx sub = XVECEXP (pat, 0, i);
> + ? ? ? ? ?if (GET_CODE (sub) == SET)
> + ? ? ? ? ? ?return (FLOAT_MODE_P (GET_MODE (SET_DEST (sub))));
> + ? ? ? ?}
> + ? ?}
> + ?return false;
> +}
>
> - ?gcc_assert (loop->latch != EXIT_BLOCK_PTR);
> +/* Analyzes the instructions inside LOOP, updating the DESC. Currently
> counts
> + ? the number of conditional branch instructions, calls and fp
> instructions,
> + ? as well as the average number of branches executed per iteration. ?*/
>
> +void
> +analyze_loop_insns (const struct loop *loop, struct niter_desc *desc)
> +{
> + ?unsigned i, nbranch;
> + ?gcov_type weighted_nbranch;
> + ?bool has_call, has_fp;
> + ?basic_block * body, bb;
> + ?rtx insn;
> + ?gcov_type header_count = loop->header->count;
> +
> + ?nbranch = weighted_nbranch = 0;
> + ?has_call = has_fp = false;
> +
> ? body = get_loop_body (loop);
> - ?n = 0;
> ? for (i = 0; i < loop->num_nodes; i++)
> - ? ?if (EDGE_COUNT (body[i]->succs) >= 2)
> - ? ? ?n++;
> + ? ?{
> + ? ? ?bb = body[i];
> +
> + ? ? ?if (EDGE_COUNT (bb->succs) >= 2)
> + ? ? ? ?{
> + ? ? ? ? ?nbranch++;
> +
> + ? ? ? ? ?/* If this block is executed less frequently than the header
> (loop
> + ? ? ? ? ? ? entry), then it is weighted based on its execution count,
> which
> + ? ? ? ? ? ? will be turned into a ratio compared to the loop header
> below. */
> + ? ? ? ? ?if (bb->count < header_count)
> + ? ? ? ? ? ?weighted_nbranch += bb->count;
> +
> + ? ? ? ? ?/* When it is executed more frequently than the header (i.e. it
> is
> + ? ? ? ? ? ? in a nested inner loop), simply weight the branch the same
> as the
> + ? ? ? ? ? ? header execution count, so that it will contribute 1 branch
> to
> + ? ? ? ? ? ? the ratio computed below. */
> + ? ? ? ? ?else
> + ? ? ? ? ? ?weighted_nbranch += header_count;
> + ? ? ? ?}
> +
> + ? ? ?/* No need to iterate through the instructions below if
> + ? ? ? ? both flags have already been set. ?*/
> + ? ? ?if (has_call && has_fp)
> + ? ? ? ?continue;
> +
> + ? ? ?FOR_BB_INSNS (bb, insn)
> + ? ? ? ?{
> + ? ? ? ? ?if (!INSN_P (insn))
> + ? ? ? ? ? ?continue;
> +
> + ? ? ? ? ?if (!has_call)
> + ? ? ? ? ? ?has_call = CALL_P (insn);
> +
> + ? ? ? ? ?if (!has_fp)
> + ? ? ? ? ? ?has_fp = insn_has_fp_set (insn);
> + ? ? ? ?}
> + ? ?}
> ? free (body);
>
> - ?return n;
> + ?desc->num_branches = nbranch;
> + ?/* Now divide the weights computed above by the loop header execution
> count,
> + ? ? to compute the average number of branches through the loop. By
> adding
> + ? ? header_count/2 to the numerator we round to nearest with integer
> + ? ? division. ?*/
> + ?if (header_count ?!= 0)
> + ? ?desc->av_num_branches
> + ? ? ? ?= (weighted_nbranch + header_count/2) / header_count;
> + ?else
> + ? ?desc->av_num_branches = 0;
> + ?desc->has_call = has_call;
> + ?desc->has_fp = has_fp;
> ?}
>
> ?/* Adds basic block BB to LOOP. ?*/
> Index: cfgloop.h
> ===================================================================
> --- cfgloop.h ? (revision 187013)
> +++ cfgloop.h ? (working copy)
> @@ -249,7 +249,6 @@ extern basic_block *get_loop_body_in_custom_order
>
> ?extern VEC (edge, heap) *get_loop_exit_edges (const struct loop *);
> ?edge single_exit (const struct loop *);
> -extern unsigned num_loop_branches (const struct loop *);
>
> ?extern edge loop_preheader_edge (const struct loop *);
> ?extern edge loop_latch_edge (const struct loop *);
> @@ -359,7 +358,8 @@ struct rtx_iv
> ?};
>
> ?/* The description of an exit from the loop and of the number of
> iterations
> - ? till we take the exit. ?*/
> + ? till we take the exit. Also includes other information used primarily
> + ? by the loop unroller. ?*/
>
> ?struct niter_desc
> ?{
> @@ -400,6 +400,18 @@ struct niter_desc
>
> ? /* The number of iterations of the loop. ?*/
> ? rtx niter_expr;
> +
> + ?/* The number of branches in the loop. ?*/
> + ?unsigned num_branches;
> +
> + ?/* The number of executed branches per iteration. ?*/
> + ?unsigned av_num_branches;
> +
> + ?/* Whether the loop contains a call instruction. ?*/
> + ?bool has_call;
> +
> + ?/* Whether the loop contains fp instructions. ?*/
> + ?bool has_fp;
> ?};
>
> ?extern void iv_analysis_loop_init (struct loop *);
> @@ -413,6 +425,7 @@ extern void iv_analysis_done (void);
>
> ?extern struct niter_desc *get_simple_loop_desc (struct loop *loop);
> ?extern void free_simple_loop_desc (struct loop *loop);
> +void analyze_loop_insns (const struct loop *, struct niter_desc *desc);
>
> ?static inline struct niter_desc *
> ?simple_loop_desc (struct loop *loop)
> Index: params.def
> ===================================================================
> --- params.def ?(revision 187013)
> +++ params.def ?(working copy)
> @@ -312,6 +312,15 @@ DEFPARAM(PARAM_MAX_UNROLL_ITERATIONS,
> ? ? ? ? "The maximum depth of a loop nest we completely peel",
> ? ? ? ? 8, 0, 0)
>
> +DEFPARAM(PARAM_MIN_ITER_UNROLL_WITH_BRANCHES,
> + ? ? ? "min-iter-unroll-with-branches",
> + ? ? ? "Minimum iteration count to ignore branch effects when unrolling",
> + ? ? ? 50, 0, 0)
> +DEFPARAM(PARAM_UNROLL_OUTER_LOOP_BRANCH_BUDGET,
> + ? ? ? "unroll-outer-loop-branch-budget",
> + ? ? ? "Maximum number of branches allowed in hot outer loop region after
> unroll",
> + ? ? ? 25, 0, 0)
> +
> ?/* The maximum number of insns of an unswitched loop. ?*/
> ?DEFPARAM(PARAM_MAX_UNSWITCH_INSNS,
> ? ? ? ?"max-unswitch-insns",
>
> --
> This patch is available for review at
> http://codereview.appspot.com/6099055




--
Teresa Johnson?|?Software Engineer?|?tejohnson@google.com?|?408-460-2413


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