Ping.
Teresa
On Fri, May 18, 2012 at 7:21 AM, Teresa Johnson <tejohn...@google.com> wrote:
> Ping?
> Teresa
>
> On Fri, May 11, 2012 at 6:11 AM, Teresa Johnson <tejohn...@google.com> wrote:
>> Ping?
>> Teresa
>>
>> On Fri, May 4, 2012 at 3:41 PM, Teresa Johnson <tejohn...@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 <tejohn...@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 | tejohn...@google.com | 408-460-2413
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
--
Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
