[llvm-dev] RFC: A change in InstCombine canonical form
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Tue Mar 22 12:36:28 PDT 2016
I'd phrase this differently: being pointer-bitcast agnostic is a step
towards support typeless pointers. :) We can either become bitcast
agnostic all in one big change or incrementally. Personally, I'd prefer
the later since it reduces the risk associated with enabling typeless
pointers in the end.
Philip
On 03/22/2016 12:16 PM, Mehdi Amini via llvm-dev wrote:
> I don't know enough about the tradeoff for 1, but 2 seems like a
> bandaid for something that is not a correctness issue neither a
> regression. I'm not sure it justifies "bandaid patches" while there is
> a clear path forward, i.e. typeless pointers, unless there is an
> acknowledgement that typeless pointers won't be there before a couple
> of years.
>
> --
> Mehdi
>
>> On Mar 22, 2016, at 11:32 AM, Ehsan Amiri <ehsanamiri at gmail.com
>> <mailto:ehsanamiri at gmail.com>> wrote:
>>
>> Back to the discussion on the RFC, I still see some advantage in
>> following the proposed solution. I see two paths forward:
>>
>> 1- Change canonical form, possibly lower memcpy to non-integer load
>> and store in InstCombine. Then teach the backends to convert that to
>> integer load and store if that is more profitable. Notice that we are
>> talking about loads that have no use other than store. So it is a
>> fairly simple change in the backends.
>>
>> 2- Do not change the canonical form. Then for this bug, we need to
>> teach select speculation to see through bitcasts. We will probably
>> need to teach other optimizations to see though bitcasts in the
>> future as problems are uncovered. That is until typeless pointer work
>> is complete. Once the typeless pointer work is complete, we have some
>> extra code in each optimization for seeing through bitcasts which is
>> possibly no longer needed.
>>
>> Based on this I think (1) is the right thing to do. But there might
>> be other reasons for the current canonical form that I am not aware
>> of. Please let me know what you think.
>>
>> Thanks
>> Ehsan
>>
>> On Wed, Mar 16, 2016 at 2:13 PM, David Blaikie <dblaikie at gmail.com
>> <mailto:dblaikie at gmail.com>> wrote:
>>
>>
>>
>> On Wed, Mar 16, 2016 at 11:00 AM, Ehsan Amiri
>> <ehsanamiri at gmail.com <mailto:ehsanamiri at gmail.com>> wrote:
>>
>> David,
>>
>> Could you give us an update on the status of typeless pointer
>> work? How much work is left and when you think it might be ready?
>>
>>
>> It's a bit of an onion peel, really - since it will eventually
>> involve generalizing/fixing every optimization that's currently
>> leaning on typed pointers to keep the performance while removing
>> the crutch they're currently leaning on. (in cases where bitcasts
>> are literally just getting in the way, those won't require
>> cleaning up & should just become "free performance wins" once we
>> remove them, though)
>>
>> At the moment we can roundtrip every LLVM IR test case through
>> bitcode and textual IR (reading/writing both formats) while using
>> only a narrow whitelist of places that request the type of a
>> pointer (things like the verifier, the parser/printer where it
>> actually needs the typed pointer to verify it matches the
>> explicit type, etc).
>>
>> The next thing on the list is probably figuring out the
>> byval/inalloca representation (add an explicit pointee type? just
>> make the number of bytes explicit with no type information?).
>>
>> Then start migrating optimizations over - doing the same sort of
>> testing I did for the IR/bitcode roundtripping - assert that the
>> pointee type is not accessed, whitelist places that need it until
>> the bitcasts go away, fix anything else... it'll still be a fair
>> bit of work & I don't really know how much. It should parallelize
>> pretty well (doing any of this work is really helpful, each
>> optimization is indepednent, etc) if anyone wants to/is able to help.
>>
>> - Dave
>>
>>
>> Thanks
>> Ehsan
>>
>>
>> On Wed, Mar 16, 2016 at 1:12 PM, David Blaikie
>> <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>
>>
>>
>> On Wed, Mar 16, 2016 at 8:34 AM, Mehdi Amini via llvm-dev
>> <llvm-dev at lists.llvm.org
>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Hi,
>>
>> How do it interact with the "typeless pointers" work?
>>
>>
>> Right - the goal of the typeless pointer work is to fix
>> all these bugs related to "didn't look through bitcasts"
>> in optimizations. Sometimes that's going to mean more
>> work (because the code is leaning on the absence of
>> bitcasts & the presence of convenient (but not
>> guaranteed) type information to inform optimization
>> decisions) but if we remove typed pointers while keeping
>> optimization quality in the cases we have today, then we
>> should've also fixed the cases that were broken because
>> the type information didn't end up aligning to produce
>> the optimal output.
>>
>> & I know I've been off the typeless pointer stuff for a
>> few months working on llvm-dwp - but happy for any help
>> (the next immediate piece is probably figuring out teh
>> right representation for byval and inalloca - there were
>> some code reviews sent out for that that I'll need to
>> come back around to - but also any optimizations people
>> want to help rework/improve would be great too & I can
>> provide some techniques/tools to help people approach those)
>>
>> - Dave
>>
>>
>> Thanks,
>>
>> --
>> Mehdi
>>
>>> On Mar 16, 2016, at 6:41 AM, Ehsan Amiri via
>>> llvm-dev <llvm-dev at lists.llvm.org
>>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>
>>> === PROBLEM === (See this bug
>>> https://llvm.org/bugs/show_bug.cgi?id=26445)
>>>
>>> IR contains code for loading a float from float *
>>> and storing it to a float * address. After
>>> canonicalization of load in InstCombine [1], new
>>> bitcasts are added to the IR (see bottom of the
>>> email for code samples). This prevents select
>>> speculation in SROA to work. Also after SROA we have
>>> bitcasts from int32 to float. (Whereas originally
>>> after instCombine, bitcasts are only done on pointer
>>> types).
>>>
>>> === PROPOSED SOLUTION===
>>>
>>> [1] implies that we need load canonicalization when
>>> we load a value only to store it again. The reason
>>> is to avoid generating slightly different code (due
>>> to different ways of adding bitcasts), in different
>>> situations. In all examples presented in [1] there
>>> is a non-zero number of bitcasts. I think when we
>>> load a value of type T from a T* address and store
>>> it as a type T value to one or more T* address (and
>>> there is no other use or store), we can redefine
>>> canonical form to mean there should not be any
>>> bitcasts. So we still have a canonical form, but its
>>> definition is slightly different.
>>>
>>> === REASONS FOR / AGAINST===
>>>
>>> Hal Finkel warns that while this may be useful for
>>> power pc, this may hurt more than one other platform
>>> and become a very large project. Despite this he is
>>> fine with bringing up the issue to the mailing list
>>> to get feedback, mostly because this seems inline
>>> with our future direction of having a unique type
>>> for all pointers. (Hal please correct me if I
>>> misunderstood your comment)
>>>
>>> This is a much simpler fix compared to alternatives.
>>> (ignoring potential regressions)
>>>
>>> === ALTERNATIVE SOLUTION ===
>>>
>>> Fix select speculation in SROA to see through
>>> bitcasts. Handle remaining bitcasts during code gen.
>>> Other alternative solutions are welcome.
>>>
>>> Should I implement the proposed solution or is it
>>> too risky? I understand that we may need to undo it
>>> if it breaks too many things. Comments are welcome.
>>>
>>>
>>> [1]
>>> http://lists.llvm.org/pipermail/llvm-dev/2015-January/080956.html
>>> r226781 git commit id: b778cbc0c8
>>>
>>>
>>>
>>> Code Samples (only relevant part is copied):
>>>
>>> -------------------- Before Canonicalization
>>> (contains call to std::max): --------------------
>>> entry:
>>> %max_value = alloca float, align 4
>>> %1 = load float, float* %input, align 4, !tbaa !1
>>> store float %1, float* %max_value, align 4, !tbaa !1
>>>
>>> for.body:
>>> %call = call dereferenceable(4) float*
>>> @_ZSt3maxIfERKT_S2_S2_(float* dereferenceable(4)
>>> %max_value, float* dereferenceable(4) %arrayidx1)
>>> %3 = load float, float* %call, align 4, !tbaa !1
>>> store float %3, float* %max_value, align 4, !tbaa !1
>>>
>>> -------------------- After Canonicalization
>>> (contains call to std::max):--------------------
>>>
>>> entry:
>>> %max_value = alloca float, align 4
>>> %1 = bitcast float* %input to i32*
>>> %2 = load i32, i32* %1, align 4, !tbaa !1
>>> %3 = bitcast float* %max_value to i32*
>>> store i32 %2, i32* %3, align 4, !tbaa !1
>>>
>>> for.body:
>>> %call = call dereferenceable(4) float*
>>> @_ZSt3maxIfERKT_S2_S2_(float* nonnull
>>> dereferenceable(4) %max_value, float*
>>> dereferenceable(4) %arrayidx1)
>>> %5 = bitcast float* %call to i32*
>>> %6 = load i32, i32* %5, align 4, !tbaa !1
>>> %7 = bitcast float* %max_value to i32*
>>> store i32 %6, i32* %7, align 4, !tbaa !1
>>>
>>> -------------------- After SROA (the call to
>>> std::max is inlined now):--------------------
>>> entry:
>>> %max_value.sroa.0 = alloca i32
>>> %0 = bitcast float* %input to i32*
>>> %1 = load i32, i32* %0, align 4, !tbaa !1
>>> store i32 %1, i32* %max_value.sroa.0
>>>
>>> for.body:
>>> %max_value.sroa.0.0.max_value.sroa.0.0.6 = load i32,
>>> i32* %max_value.sroa.0
>>> %3 = bitcast i32
>>> %max_value.sroa.0.0.max_value.sroa.0.0.6 to float
>>> %max_value.sroa.0.0.max_value.sroa_cast8 = bitcast
>>> i32* %max_value.sroa.0 to float*
>>> %__b.__a.i = select i1 %cmp.i, float* %arrayidx1,
>>> float* %max_value.sroa.0.0.max_value.sroa_cast8
>>> %5 = bitcast float* %__b.__a.i to i32*
>>> %6 = load i32, i32* %5, align 4, !tbaa !1
>>> store i32 %6, i32* %max_value.sroa.0
>>>
>>> -------------------- After SROA when
>>> Canonicalization is turned off--------------------
>>> entry:
>>> %0 = load float, float* %input, align 4, !tbaa !1
>>>
>>> for.cond: ; preds = %for.body, %entry
>>> %max_value.0 = phi float [ %0, %entry ], [
>>> %.sroa.speculated, %for.body ]
>>>
>>> for.body:
>>> %1 = load float, float* %arrayidx1, align 4, !tbaa !1
>>> %cmp.i = fcmp olt float %max_value.0, %1
>>> %.sroa.speculate.load.true = load float, float*
>>> %arrayidx1, align 4, !tbaa !1
>>> %.sroa.speculated = select i1 %cmp.i, float
>>> %.sroa.speculate.load.true, float %max_value.0
>>> _______________________________________________
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>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
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