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

[Teresa Johnson <tejohnson at google dot com>]

Ping.
Teresa

On Fri, May 18, 2012 at 7:21 AM, Teresa Johnson <tejohnson@google.com> wrote:
> Ping?
> Teresa
>
> On Fri, May 11, 2012 at 6:11 AM, Teresa Johnson <tejohnson@google.com> wrote:
>> 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
>
>
>
> --
> Teresa Johnson?|?Software Engineer?|?tejohnson@google.com?|?408-460-2413



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