[llvm-commits] [Patch] Use bitmaps for switch lookup tables that fit in registers
Sean Silva
silvas at purdue.edu
Thu Sep 20 21:35:12 PDT 2012
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)?
--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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