[llvm-commits] [Patch] Use bitmaps for switch lookup tables that fit in registers
David Blaikie
dblaikie at gmail.com
Thu Sep 20 19:12:00 PDT 2012
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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