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
