[llvm-commits] [LLVM, loop-unswitch, bugfix for #11429] Wrong behaviour for switches.

[Stepan Dyatkovskiy]

ping.

-Stepan.

Stepan Dyatkovskiy wrote:
> Hi Dan. This bug is described in details (with examples) here:
> http://llvm.org/bugs/show_bug.cgi?id=11429
>
> Regarding to your questions..
>
>> It's even possible that this bug is
>> accidentally helping the code by preventing it from unswitching too
>> much in the presence of switches.
>> Do you have an idea on what impact
>> this patch has on code size, and performance, in general?
>
> Happily this bug helps to keep the code size small. But
> LoopUnswitch::UnswitchIfProfitable method already controls the produced
> code size. Please, see LoopUnswitch.cpp, string #446 for more details. I
> think that if we need to improve the "restrictioning" of produced code
> size we need to implement this improvement instead of keeping some
> strange code.
>
> About impact on code size.
> For switch with N cases (+ 1 default) we got N new loops. If you wish I
> can present the .ll code that should be produced after optimization.
>
> Impact on performance.
> The main purpose of this optimization is to move out of loop the
> switches. Each unswitched case increases the performance.
>
> About releaseMemory and CloneUnswitchedVals.
> This methods the part of unswitch info cloning for new loops. It is also
> described in bug #11429.
>
>  > CloneUnswitchedVals doesn't actually need to iterate over the
>  > instructions in the block to find the SwitchInst. If there's a
>  > SwitchInst present, it'll be the Terminator instruction.
> OK. You're right. Please find the fixed patch.
>
> Thanks!
> -Stepan.
>
>>
>> Thanks,
>>
>> Dan
>>
>> On Dec 2, 2011, at 12:31 AM, Stepan Dyatkovskiy wrote:
>>
>>>
>>> ping.
>>>
>>> -Stepan.
>>> Stepan Dyatkovskiy wrote:
>>>> ping.
>>>>
>>>> -Stepan.
>>>>
>>>> Stepan Dyatkovskiy wrote:
>>>>> ping.
>>>>>
>>>>> -Stepan.
>>>>>
>>>>> Stepan Dyatkovskiy wrote:
>>>>>> ping.
>>>>>>
>>>>>> -Stepan
>>>>>>
>>>>>> Stepan Dyatkovskiy wrote:
>>>>>>> Hi all. Please find the patch in attachment for review.
>>>>>>> Regression tests
>>>>>>> are attached in separated patch.
>>>>>>>
>>>>>>> Short tests description:
>>>>>>>
>>>>>>> 2011-11-18-SimpleSwitch.ll
>>>>>>> Check simple that simple switch will totally unswitched:
>>>>>>>
>>>>>>> for (...)
>>>>>>> switch(c) {
>>>>>>> case 0: inc(); break;
>>>>>>> case 1: dec(); break;
>>>>>>> default: goto loop_exit;
>>>>>>> }
>>>>>>>
>>>>>>> loop_exit:
>>>>>>> ...
>>>>>>>
>>>>>>> Result of processing should be 2 additional loops for c == 0 and
>>>>>>> for c
>>>>>>> == 1.
>>>>>>>
>>>>>>>
>>>>>>> 2011-11-18-TwoSwitches.ll
>>>>>>> Check that second switch will unswitched too. Check that switches
>>>>>>> will
>>>>>>> not unswitched again in new loop:
>>>>>>>
>>>>>>> Initially we have the next:
>>>>>>>
>>>>>>> for (...) {
>>>>>>> switch(c) {
>>>>>>> case 0: inc(); break;
>>>>>>> }
>>>>>>> switch(d) {
>>>>>>> case 0: inc(); break;
>>>>>>> }
>>>>>>> }
>>>>>>>
>>>>>>> After optimization we should got 3 additional loops: when (c ==
>>>>>>> 0&& d
>>>>>>> == 0), when (c == 0&& d != 0) and when (c != 0&& d == 0). Original
>>>>>>> loop will activated for (c != 0&& d != 0):
>>>>>>>
>>>>>>> if (c == 0&& d == 0) {
>>>>>>> for (...) ... // All is clear here. Two "switch(0)" instructions.
>>>>>>> } else if (c == 0&& d != 0) {
>>>>>>> for (...) {
>>>>>>> switch (0) { // c == 0
>>>>>>> case 0: inc(); break;
>>>>>>> }
>>>>>>> switch (d) {
>>>>>>> case 0: goto unreachable; // CHECK: That it will not unswitched
>>>>>>> // again, since it looks "trivial",
>>>>>>> // see LoopUnswitch::IsTrivialUnswitchCondition
>>>>>>> }
>>>>>>> }
>>>>>>> } else if (c != 0&& d == 0) {
>>>>>>> // the same...
>>>>>>> ...
>>>>>>> } else { // if (c != 0&& d != 0)
>>>>>>> // Original totally unswitched loop:
>>>>>>> switch (c) {
>>>>>>> case 0: goto unreachable;
>>>>>>> }
>>>>>>> switch (d) {
>>>>>>> case 0: goto unreachable;
>>>>>>> }
>>>>>>> }
>>>>>>>
>>>>>>> Stepan.
>>>>>>>
>>>>>>>
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