[PATCH] Enhance loop rotation with existence of profile data in MachineBlockPlacement pass.

[Xinliang David Li]

The already rotated inner loop needs to be treated as a single node
when participating in parent loop's rotation, otherwise it may end up
with wasting compile time and a suboptimal solution.

Consider the loop nest:

Entry
do { // outer loop

 B0
 if (...) {
  do {    // inner loop
    B1
    if (..) {
     B2
    } else {
     B3
    }
    B4
  } while (..);  // inner loop
 }
else {
  B6
}
B5;
} while (...); // outer loop

B7


The optimal inner loop layout is B3 B4 B1 B2;
The original outloop layout is :   B0 B6 (B3 B4 B1 B2) B5

The optimal rotation of the outer loop should produce:    (B3 B4 B1
B2) B5 B0 B6, with the final layout be:

Entry ((B3 B4 B1 B2) B5 B0 B6) B7

However the current algorithm may produce  Entry (B5 B0 B6 (B3 B4 B1
B2)) B7  because it will have the same cost as the the optimal one.
The problem is that cost analysis needs to consider the edge from
inside of the inner loop to the top of the outer loop chain -- in the
bad solution, there is an edge from B4 to B5 whose cost should be
considered as part-3 cost.

David





On Mon, Jul 6, 2015 at 10:22 AM, Cong Hou <congh at google.com> wrote:
> When the outer loop is rotated, the inner loop is already linked to other
> CFG nodes in the outer loop. So I think we won't have to adjust the current
> algorithm as we have already considered all costs the rotation may bring or
> reduce.
>
>
> thanks,
> Cong
>
> On Mon, Jul 6, 2015 at 9:57 AM, Xinliang David Li <xinliangli at gmail.com>
> wrote:
>>
>> Does the cost analysis work well for loop nest? After the inner loop chain
>> is formed and rotated, it will be later be merged into the parent loop
>> chain. The cost analysis for the parent loop may need to be adjusted to
>> consider the inner loops that are already rotated.
>>
>> David
>>
>> On Tue, Jun 30, 2015 at 2:29 PM, Cong Hou <congh at google.com> wrote:
>>>
>>> Update the patch by adding two opt parameters that define the cost of
>>> misfetch and jump instruction, and use them when rotating loops.
>>>
>>>
>>> http://reviews.llvm.org/D10717
>>>
>>> Files:
>>>   lib/CodeGen/MachineBlockPlacement.cpp
>>>   test/CodeGen/X86/code_placement_loop_rotation.ll
>>>
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>>>
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>>
>