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

[Sean Silva]

> 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?

--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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