On Wed, Mar 30, 2016 at 9:09 AM, Richard Biener
<richard.guent...@gmail.com> wrote:
> On Thu, Mar 24, 2016 at 6:26 PM, Bin Cheng <bin.ch...@arm.com> wrote:
>> Hi,
>> Quite lot of time is used when IVOPT computes cost for <use, cand> pairs.
>> As a matter of fact, some pairs are very similar to each other, and we can
>> abstract and compute cost only once for these pairs. This is a patch doing
>> so, the idea is skipping cost computation for sub-uses in each group, of
>> course it may result in different assembly code for some complicated cases
>> because it estimates cost rather than doing real computation. I did double
>> check one of such case that the change in generated assembly is not
>> degeneration. For an IVOPT heavy program (spec2k/173), this patch reduces
>> IVOPT's compilation time by 7~8%, as well as the memory consumption on my
>> developing machine.
>>
>> Bootstrap & test on x86_64.
>>
>> For spec2k6 data on x86_64. Maybe because I ran spec2k6 compiled with
>> patched GCC in unclean environment, some cases are regressed by small amount
>> (< %1). I manually compared assembly code for several cases, including ones
>> with the largest regression (still within <1%). I could confirm that
>> generated assembly code is exact the same as unpatched GCC, except for
>> function emit_library_call_value_1 in 403.gcc/calls.c.
>>
>> In this case, difference of IVOPT dumps is as below:
>>
>> $ diff -y trunk/calls.c.154t.ivopts patch/calls.c.154t.ivopts
>>
>> <bb 44>: <bb 44>:
>> # val_21 = PHI <val_175(168), val_650(43)> # val_21 =
>> PHI <val_175(168), val_650(43)>
>> _811 = (void *) ivtmp.322_829; _811 =
>> (void *) ivtmp.322_829;
>> MEM[base: _811, offset: -48B] = val_21; | MEM[base:
>> _811, offset: -32B] = val_21;
>> _810 = (void *) ivtmp.322_829; _810 =
>> (void *) ivtmp.322_829;
>> MEM[base: _810, offset: -40B] = mode_163; | MEM[base:
>> _810, offset: -24B] = mode_163;
>> _182 = function_arg (&args_so_far, mode_163, 0B, 1); _182 =
>> function_arg (&args_so_far, mode_163, 0B, 1);
>> _809 = (void *) ivtmp.322_829; _809 =
>> (void *) ivtmp.322_829;
>> MEM[base: _809, offset: -32B] = _182; | MEM[base:
>> _809, offset: -16B] = _182;
>> _807 = (void *) ivtmp.322_829; _807 =
>> (void *) ivtmp.322_829;
>> MEM[base: _807, offset: -24B] = 0; | MEM[base:
>> _807, offset: -8B] = 0;
>> _185 = (struct args_size *) ivtmp.322_829; | _801 =
>> ivtmp.322_829 + 16;
>> _801 = ivtmp.322_829 + 18446744073709551600; <
>> _800 = (struct args_size *) _801; _800 =
>> (struct args_size *) _801;
>> _186 = _800; | _185 =
>> _800;
>> > _186 =
>> (struct args_size *) ivtmp.322_829;
>> _187 = _182 != 0B; _187 =
>> _182 != 0B;
>> _188 = (int) _187; _188 =
>> (int) _187;
>> locate_and_pad_parm (mode_163, 0B, _188, 0B, &args_size, _1
>> locate_and_pad_parm (mode_163, 0B, _188, 0B, &args_size, _1
>> _802 = (void *) ivtmp.322_829; _802 =
>> (void *) ivtmp.322_829;
>> _190 = MEM[base: _802, offset: 8B]; | _190 =
>> MEM[base: _802, offset: 24B];
>> if (_190 != 0B) if (_190
>> != 0B)
>> goto <bb 45>; goto <bb
>> 45>;
>> else else
>> goto <bb 46>; goto <bb
>> 46>;
>>
>> <bb 45>: <bb 45>:
>> fancy_abort ("calls.c", 3724, &__FUNCTION__);
>> fancy_abort ("calls.c", 3724, &__FUNCTION__);
>>
>> It's only an offset difference in IV. And below is difference of generated
>> assembly:
>> $ diff -y trunk/calls.S patch/calls.S
>> .L489: .L489:
>> leaq -80(%rbp), %rdi leaq
>> -80(%rbp), %rdi
>> xorl %edx, %edx xorl
>> %edx, %edx
>> movl $1, %ecx movl
>> $1, %ecx
>> movl %r13d, %esi movl
>> %r13d, %esi
>> movq %rax, -48(%r15) <
>> movl %r13d, -40(%r15) <
>> call function_arg <
>> movl $0, -24(%r15) <
>> movq %rax, -32(%r15) movq
>> %rax, -32(%r15)
>> > movl
>> %r13d, -24(%r15)
>> > call
>> function_arg
>> xorl %edx, %edx xorl
>> %edx, %edx
>> pushq %r12 | movq
>> %rax, -16(%r15)
>> testq %rax, %rax
>> testq %rax, %rax
>> pushq %r15 | leaq
>> 16(%r15), %rax <--I1
>> leaq -16(%r15), %r9 | movl
>> $0, -8(%r15)
>> leaq -112(%rbp), %r8 leaq
>> -112(%rbp), %r8
>> >
>> pushq %r12
>> setne %dl
>> setne %dl
>> movl %r13d, %edi | movq
>> %r15, %r9 <--I2
>> >
>> pushq %rax <--I3
>> xorl %ecx, %ecx xorl
>> %ecx, %ecx
>> xorl %esi, %esi xorl
>> %esi, %esi
>> > movl
>> %r13d, %edi
>> call locate_and_pad_parm call
>> locate_and_pad_parm
>> cmpq $0, 8(%r15) | cmpq
>> $0, 24(%r15)
>> popq %rax popq
>> %rax
>> popq %rdx popq
>> %rdx
>> jne .L602 jne
>> .L602
>>
>> There is one additional move instruction (I2) after patching, but I believe
>> it's a choice of RA. If we switch %rax/%r9 in instructions I1/I2 as below:
>> ...
>> leaq 16(%r15), %r9
>> ...
>> movq %r15, %rax
>> pushq %r15
>>
>> Then I2 becomes redundant and can be removed.
>>
>> I will collect performance data on AArch64 to make sure there is no breakage
>> either. So is it OK?
>
> I think the patch is ok - note that I have a hard time following the
> code, esp. the 'first' flag.
>
> + /* Add cost for sub uses in group. */
> + do
> + {
> + /* Compute cost for the first sub_use with different offset to
> + the first one and use it afterwards, because the cost could
> + be very different if the offset is different. */
> + if (first && use->addr_offset != sub_use->addr_offset)
> + {
> + first = false;
> + sub_cost = get_computation_cost (data, sub_use, cand, true,
> + NULL, &can_autoinc, NULL);
> + if (infinite_cost_p (sub_cost))
> + {
> + cost = infinite_cost;
> + break;
> + }
> + }
> +
> + cost = add_costs (cost, sub_cost);
> + sub_use = sub_use->next;
> + }
> + while (sub_use);
>
> we start this loop with first = true, so for each sub-use we compute
> no new sub-cost until
> use->addr_offset changes for the first time after which we will never
> compute sub-cost
> again. So we call get_computation_cost at most twice for al sub-uses.
>
> I suppose all sub-uses have equal ->addr_base. Suppose sub-uses are then
> sorted
> after ->addr_offset what keeps that list from having three different
> addr_offset but
> with "very different cost"? There seems to be group_address_uses but
> that suggests
> the cost might be actually the same for all sub-uses.
>
> So adding a little explaining before the loop over sub-uses would be
> appreciated.
Thanks for reviewing. Here is the patch with trivially revised comments.
I also collected benchmark data for spec2k6 on AArch64, there is no
surprise except for case 456.hmmer. I double checked generated
assembly and can confirm it's not real.
Will apply the patch later.
Thanks,
bin
>
> Thanks,
> Richard.
>
>> Thanks,
>> bin
>>
>> 2016-03-23 Bin Cheng <bin.ch...@arm.com>
>>
>> * tree-ssa-loop-ivopts.c (struct comp_cost): New scrach field.
>> (no_cost, infinite_cost): Initialize the new field.
>> (get_computation_cost_at): Record setup cost.
>> (determine_use_iv_cost_address): Skip cost computation for sub
>> uses if we can estimate it without losing accuracy.
>>
diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index 5302edf..3c81bf7 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -168,10 +168,11 @@ struct comp_cost
the computation (in no concrete units --
complexity field should be larger for more
complex expressions and addressing modes). */
+ int scratch; /* Scratch used during cost computation. */
};
-static const comp_cost no_cost = {0, 0};
-static const comp_cost infinite_cost = {INFTY, INFTY};
+static const comp_cost no_cost = {0, 0, 0};
+static const comp_cost infinite_cost = {INFTY, INFTY, INFTY};
/* The candidate - cost pair. */
struct cost_pair
@@ -4947,6 +4948,8 @@ get_computation_cost_at (struct ivopts_data *data,
cost.cost += add_cost (data->speed, TYPE_MODE (ctype));
}
+ /* Record setup cost in scrach field. */
+ cost.scratch = cost.cost;
/* Set of invariants depended on by sub use has already been computed
for the first use in the group. */
if (use->sub_id)
@@ -5082,12 +5085,12 @@ determine_use_iv_cost_address (struct ivopts_data *data,
struct iv_use *use, struct iv_cand *cand)
{
bitmap depends_on;
- bool can_autoinc;
+ bool can_autoinc, first;
int inv_expr_id = -1;
struct iv_use *sub_use;
- comp_cost sub_cost;
comp_cost cost = get_computation_cost (data, use, cand, true, &depends_on,
&can_autoinc, &inv_expr_id);
+ comp_cost sub_cost = cost;
if (cand->ainc_use == use)
{
@@ -5099,13 +5102,47 @@ determine_use_iv_cost_address (struct ivopts_data *data,
else if (cand->pos == IP_AFTER_USE || cand->pos == IP_BEFORE_USE)
cost = infinite_cost;
}
- for (sub_use = use->next;
- sub_use && !infinite_cost_p (cost);
- sub_use = sub_use->next)
+
+ if (!infinite_cost_p (cost) && use->next)
{
- sub_cost = get_computation_cost (data, sub_use, cand, true, NULL,
- &can_autoinc, NULL);
- cost = add_costs (cost, sub_cost);
+ first = true;
+ sub_use = use->next;
+ /* We don't want to add setup cost for sub-uses. */
+ sub_cost.cost -= sub_cost.scratch;
+ /* Add cost for sub uses in group. */
+ do
+ {
+ /* Compute cost for the first sub use with different offset to
+ the main use and add it afterwards. Costs for these uses
+ could be quite different. Given below uses in a group:
+ use 0 : {base + A + offset_0, step}
+ use 0.1: {base + A + offset_0, step}
+ use 0.2: {base + A + offset_1, step}
+ use 0.3: {base + A + offset_2, step}
+ when we need to compute costs with candidate:
+ cand 1 : {base + B + offset_0, step}
+
+ The first sub use with different offset is use 0.2, its cost
+ is larger than cost of use 0/0.1 because we need to compute:
+ A - B + offset_1 - offset_0
+ rather than:
+ A - B. */
+ if (first && use->addr_offset != sub_use->addr_offset)
+ {
+ first = false;
+ sub_cost = get_computation_cost (data, sub_use, cand, true,
+ NULL, &can_autoinc, NULL);
+ if (infinite_cost_p (sub_cost))
+ {
+ cost = infinite_cost;
+ break;
+ }
+ }
+
+ cost = add_costs (cost, sub_cost);
+ sub_use = sub_use->next;
+ }
+ while (sub_use);
}
set_use_iv_cost (data, use, cand, cost, depends_on, NULL_TREE, ERROR_MARK,
