[llvm-commits] [Patch] Use bitmaps for switch lookup tables that fit in registers

David Blaikie dblaikie at gmail.com
Fri Sep 21 10:50:03 PDT 2012 

On Thu, Sep 20, 2012 at 9:35 PM, Sean Silva <silvas at purdue.edu> wrote:
> Yeah, it makes sense. It's just that literally every case I can
> remember seeing these kinds of switches have an unreachable default
> case. Whatever. Good to know. Maybe a little blurb should be added to
> the coding standards (which has been getting quite a bit of love
> lately, btw)?

Committed in r164389 - post commit review/wordsmithing welcome.

(I was going to document LLVM_OVERRIDE too, but it seems that it's not
actually in-use yet, I guess Craig'll get to that at some point)

- David

>
> --Sean Silva
>
> On Thu, Sep 20, 2012 at 10:12 PM, David Blaikie <dblaikie at gmail.com> wrote:
>> On Thu, Sep 20, 2012 at 6:37 PM, Sean Silva <silvas at purdue.edu> wrote:
>>>> Generally not because the presence of a default in a fully-covered
>>>> switch would suppress -Wswitch. (& in fact -Wcovered-switch-default
>>>> will warn if you have a default in a fully-covered switch)
>>>
>>> Even with the llvm_unreachable?
>>
>> That's correct. -Wswitch cannot fire on an incompletely covered switch
>> with an unreachable default - that would be a reasonable piece of code
>> (eg: if you knew in this code path only some subset of the enum values
>> were expected), so writing a covered switch with a default means you
>> won't get the -Wswitch diagnostics if you add new values to the enum.
>>
>> Does that make sense? (Sorry, not sure the explanation was a great one)
>>
>> - David
>>
>>>
>>> --Sean Silva
>>>
>>> On Thu, Sep 20, 2012 at 4:57 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>> On Thu, Sep 20, 2012 at 10:01 AM, Sean Silva <silvas at purdue.edu> wrote:
>>>>> +  switch (Kind) {
>>>>> +    case SingleValueKind:
>>>>> +      return SingleValue;
>>>>> +    case ArrayKind: {
>>>>> +      Value *GEPIndices[] = { Builder.getInt32(0), Index };
>>>>> +      Value *GEP = Builder.CreateInBoundsGEP(Array, GEPIndices,
>>>>> +                                             "switch.gep");
>>>>> +      return Builder.CreateLoad(GEP, "switch.load");
>>>>> +    }
>>>>> +  }
>>>>> +  llvm_unreachable("Unknown lookup table kind!");
>>>>>
>>>>> I may be wrong, but my impression is that for fully-determined
>>>>> switches the unreachable typically gets put in a default case.
>>>>
>>>> Generally not because the presence of a default in a fully-covered
>>>> switch would suppress -Wswitch. (& in fact -Wcovered-switch-default
>>>> will warn if you have a default in a fully-covered switch)
>>>>
>>>>>    // If all values in the table are equal, this is that value.
>>>>> -  Constant *SameResult = Results.begin()->second;
>>>>> +  SingleValue = Values.begin()->second;
>>>>>
>>>>> I know this is unrelated to this patch, which is just a bit of
>>>>> refactoring, but is it really necessary to single out the 1-case
>>>>> switch like this? Will a 1-case lookup table not get optimized away
>>>>> somewhere else? It just seems weird to single it out like this:
>>>>> wouldn't it be equally reasonable to single out when there are 2 cases
>>>>> (convert to `select`)? IIRC in the original thread for the inception
>>>>> of this switch-to-lookup-table, someone said that maybe we need to
>>>>> explicitly model this with a "switchselect", the same way that we have
>>>>> "if" and "select". I think that should be investigated further, so
>>>>> that these constructs get optimized well.
>>>>>
>>>>>
>>>>> +      else if (IndexTy->getBitWidth() > MapTy->getBitWidth())
>>>>> +        ShiftAmt = Builder.CreateTrunc(ShiftAmt, MapTy, "switch.trunc");
>>>>>
>>>>> Please comment why it is safe to truncate the index here.
>>>>>
>>>>> +      // Multiply the shift amount by the element width.
>>>>> +      ShiftAmt = Builder.CreateMul(ShiftAmt,
>>>>> +                      ConstantInt::get(MapTy, BitMapElementTy->getBitWidth()),
>>>>> +                                   "switch.shiftamt");
>>>>>
>>>>> If the bitmap type has "room to spare" for widening the element types,
>>>>> it will probably be profitable to pad them to a power-of-2 size so
>>>>> that this multiply will become a shift. Just something to think about
>>>>> for a future patch. I doubt that it's a big deal, but maybe
>>>>>
>>>>> +bool SwitchLookupTable::WouldFitInRegister(const TargetData *TD,
>>>>> +                                           uint64_t TableSize,
>>>>> +                                           const Type *ElementType) {
>>>>> +  if (!TD)
>>>>> +    return false;
>>>>> +  const IntegerType *IT = dyn_cast<IntegerType>(ElementType);
>>>>> +  if (!IT)
>>>>> +    return false;
>>>>> +  return TD->fitsInLegalInteger(TableSize * IT->getBitWidth());
>>>>> +}
>>>>>
>>>>> You have a FIXME about possibly narrowing the types. I think that that
>>>>> FIXME should be here. Basically you can take the minimum of the bits
>>>>> needed to represent each value. I'm pretty sure that in practically
>>>>> all cases, that minimum is going to be smaller than the actual integer
>>>>> type of the elements, possibly by a lot.
>>>>>
>>>>> In fact, computing that minimum is sufficiently simple that I think it
>>>>> could be included in this patch as well (it might require some code
>>>>> reorganization though, but I think that computing this minimum is a
>>>>> sufficiently important thing to do that you might as well reorganize
>>>>> now).
>>>>>
>>>>> --Sean Silva
>>>>>
>>>>> On Thu, Sep 20, 2012 at 5:31 AM, Hans Wennborg <hans at chromium.org> wrote:
>>>>>> Thanks for the comments! New version attached, split into two patches.
>>>>>>
>>>>>>  - Hans
>>>>>>
>>>>>> On Thu, Sep 20, 2012 at 2:08 AM, Sean Silva <silvas at purdue.edu> wrote:
>>>>>>>> The patch also refactors the code to put the table-building
>>>>>>>> functionality in a separate class. I'm happy to break that out into a
>>>>>>>> separate patch if preferred.
>>>>>>>
>>>>>>> Please. It's extremely hard to pick out just the new bitmask-table
>>>>>>> building code or just the refactoring to a class. It's OK to send two
>>>>>>> patches in an email (just make sure they have numbered prefixes so
>>>>>>> that it is unambiguous in what order to apply them).
>>>>>>
>>>>>> Done.
>>>>>>
>>>>>>> A couple comments:
>>>>>>>
>>>>>>> +            const SmallVector<std::pair<ConstantInt*, Constant*>, 4>& Results,
>>>>>>>
>>>>>>> With a name like "Results" this could easily be confused (read: it
>>>>>>> confused me) with being an out parameter.
>>>>>>
>>>>>> RIght, let's call it Values instead.
>>>>>>
>>>>>>> +/// SwitchLookupTable - This class represents a lookup table that can
>>>>>>> be used to
>>>>>>> +/// replace a switch.
>>>>>>> +class SwitchLookupTable {
>>>>>>>
>>>>>>> Please wrap this class in an anonymous namespace as per the coding
>>>>>>> standards <http://llvm.org/docs/CodingStandards.html#anonymous-namespaces>.
>>>>>>
>>>>>> Done.
>>>>>>
>>>>>>> +    std::vector<Constant*> TableContents(TableSize);
>>>>>>>
>>>>>>> Why vector and not SmallVector?
>>>>>>
>>>>>> You're right, SmallVector will work fine here.
>>>>>>
>>>>>>> +    SwitchLookupTable Table;
>>>>>>> +    Table.Init(Mod, TableSize, MinCaseVal, ResultLists[PHI],
>>>>>>> +               DefaultResults[PHI], TD);
>>>>>>> [...]
>>>>>>> +    Value *Result = Table.BuildLookup(TableIndex, Builder);
>>>>>>>
>>>>>>> What's the point of having Init? It just seems like an error-prone
>>>>>>> constructor that might not be called. Maybe better to factor it as one
>>>>>>> class that "analyzes" the switch, and another that does the building;
>>>>>>> that way, the type system enforces things that the current approach
>>>>>>> leaves up to the programmer.
>>>>>>
>>>>>> Right, the Init function looks pretty unnecessary here. Removed.
>>>>>>
>>>>>>
>>>>>>
>>>>>>> On Wed, Sep 19, 2012 at 1:32 PM, Hans Wennborg <hans at chromium.org> wrote:
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> The attached batch updates the switch-to-lookup table transformation
>>>>>>>> in SimplifyCFG to put the lookup tables in a bitmap when they would
>>>>>>>> fit in a register on the target. This saves some space, and it also
>>>>>>>> allows for building tables that would otherwise be too sparse.
>>>>>>>>
>>>>>>>> One interesting case that this hits is example 7 from
>>>>>>>> http://blog.regehr.org/archives/320. We currently generate good code
>>>>>>>> for this when lowering the switch to the selection DAG: we build a
>>>>>>>> bitmask to decide whether to jump to one block or the other. My patch
>>>>>>>> will result in the same bitmask, but it removes the need for the jump,
>>>>>>>> as the return value can just be retrieved from the mask.
>>>>>>>>
>>>>>>>> This was part of my original lookup table patch [1], but it was
>>>>>>>> suggested in review that we should do it separately.
>>>>>>>>
>>>>>>>> I tried this on a bootstrap build of Clang, and it reduced the binary
>>>>>>>> size with 8328 bytes (0.03%).
>>>>>>>>
>>>>>>>> The patch also refactors the code to put the table-building
>>>>>>>> functionality in a separate class. I'm happy to break that out into a
>>>>>>>> separate patch if preferred.
>>>>>>>>
>>>>>>>> Please take a look.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Hans
>>>>>>>>
>>>>>>>>  [1]. http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120903/149605.html
>>>>> _______________________________________________
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