[llvm-dev] Reducing the number of ptrtoint/inttoptrs that are generated by LLVM

Tue Jan 15 08:11:02 PST 2019

Hello Chandler,

> ```
> void f(int *arr1, int *arr2, int length) {
>   intptr_t rel_offset = arr2 - arr1;
>   int *arr1_end = arr1 + length;
>   for (int *p = arr1; p < arr1_end; p += sizeof(int)) {
>     do_something(p, p + rel_offset);
>   }
> }
> ```
>
> For example, a common loop optimization technique is something like the
following in *pseudocode* (note that this is
> not necessarily valid C but we might reasonably want to express this kind
of optimization within LLVM:

When LLVM need to insert subtraction between two pointers, like the example
you wrote, I believe we can use 'sub(ptrtoint p, ptrtoint q)' as before.
This addresses concerns regarding correctness of existing pointer analysis.
So you don't lose expressiveness, while you gain precision when possible.

A concrete suggestion is to add a parameter like 'bool use_psub' to
IRBuilder::CreatePtrDiff. When Clang inserts  a pointer subtraction,
use_psub is set to true. When LLVM inserts it, it is set as false. Default
value of use_psub is false, so existing LLVM passes will still insert sub
(ptrtoint, ptrtoint) but most of ptrtoints are inserted from Clang, so # of
ptrtoint will significantly decrease as well.

Adding an "inbounds" flag to psub doesn't seem necessary since you can
achieve the same thing with sub(ptrtoint, ptrtoint).

> This doesn't make a lot of sense to me... the pointer subtractions
semantics depend on whether the
> input was rinsed through `inttoptr`? It also doesn't handle the case I
describe above.

The motivation is that (as far as I understand) LLVM already treat pointers
casted from inttoptr differently from pointers from gep.
We tentatively call these "physical pointers" (as opposed to "logical
pointers").
For example, GEP document says that inttoptr(i + ptrtoint p) is different
from gep(p, i) - it says 'most of GEP’s special aliasing rules
do not apply to pointers computed from ptrtoint, arithmetic, and inttoptr
sequences', implying that it can touch any object in LLVM.

Juneyoung Lee

On Mon, Jan 14, 2019 at 10:59 PM Chandler Carruth <chandlerc at gmail.com>
wrote:

> Note that we should keep the discussion of instcombine and
> canonicalization completely separate. It is an independent question IMO.
> Only responding to the pointer subtraction point here.
>
> On Mon, Jan 14, 2019 at 12:56 PM Juneyoung Lee <juneyoung.lee at sf.snu.ac.kr>
> wrote:
>
>> > First and foremost - how do you address correctness issues here?
>> Because the subtraction `A - B` can escape/capture more things.
>> Specifically, if one of `A` or `B` is escaped/captured, the
>> > subtraction can be used to escape or capture the other pointer. So
>> *some* of the conservative treatment is necessary. What is the plan to
>> update all the analyses to remain correct? What
>> > correctness testing have you done?
>>
>> Correctness of psub is guaranteed by the specification of pointer
>> subtraction of C/C++.
>>
>
> LLVM is not a C/C++ compiler backend. We cannot rely on frontend language
> semantics to make the optimizer correct.
>
> LLVM has its own semantic model and we need *it* to be correct.
>
> Also, the reality is that we *do* have the case I described. Even if the
> source language does not form this directly, LLVM itself can form the exact
> pattern I describe? We would need rules in LLVM's IR that preclude this at
> a semantic level and to find and remove optimizations that violate that.
> Have we done this?
>
> I think this may be much harder than it seems due to fundamental
> transformations. If we prove that `C = (A - B)`, even though we originally
> got `C` directly, we might replace it with `A - B`. This subtraction
> doesn't even have to be something we can compute, we just need a proof that
> it is equivalent.
>
> For example, a common loop optimization technique is something like the
> following in *pseudocode* (note that this is not necessarily valid C but we
> might reasonably want to express this kind of optimization within LLVM:
>
> ```
> void f(int *arr1, int *arr2, int length) {
>   intptr_t rel_offset = arr2 - arr1;
>   int *arr1_end = arr1 + length;
>   for (int *p = arr1; p < arr1_end; p += sizeof(int)) {
>     do_something(p, p + rel_offset);
>   }
> }
> ```
>
> This kind of thing should be representable in LLVM's IR in my opinion.
>
> An analogous situation is with `getelementptr` vs `getelementptr
> inbounds`. I think you'll need something similar, and you'll want to update
> the optimizer to track the non-inbounds version as potentially escaping
> (much as ptrtoint is potentially escaping). This is still distinct from
> ptrtoint as things like address spaces and other properties of the pointers
> are not necessarily lost by this operation.
>
> When two pointers are subtracted, both shall point to elements of the same
>> array object, or one past the last element of the array object (6.5.6.9).
>> So, if the two pointers p and q point to different objects, we can define
>> llvm.psub(p,q) as poison.
>> Other than meeting C specification, correctness of llvm.psub is tested
>> with SPEC CPU2017 and LLVM Nightly Tests as well.
>>
>
> SPEC and the nightly test suite are great at finding problems, but quite
> bad at giving confindence that there are not problems. For subtle things
> like this, I would expect you need to look an a much larger sampling of
> real-world application code.
>
>
>>
>> But it is true that sometimes pointer subtraction is used to get distance
>> between two objects.
>> Most common case is doing something like 'p - NULL', and this pattern
>> exists in SPEC CPU2017, for example spec_qsort.c in mcf_r .
>> Our suggestion is to define 'p - q' correctly return the distance between
>> p and q if either p or q is based on inttoptr(i). This naturally explains
>> 'p - NULL' because NULL is equivalent to inttoptr(0).
>>
>
> This doesn't make a lot of sense to me... the pointer subtractions
> semantics depend on whether the input was rinsed through `inttoptr`? It
> also doesn't handle the case I describe above.
>
>
>> Regarding analysis - what I've observed is that analysis was done after
>> pointer subtraction was optimized into another form.
>> For example, if '(p - q) == 0' was given, this is transformed into 'p ==
>> q', and then some analysis was done.
>> Good thing is that these transformations can be simply applied to
>> llvm.psub as well, which will reenable analysis.
>> Also we're adding a new operation here, so existing analysis wouldn't be
>> incorrect, but wouldn't fire.
>> Fortunately, the performance impact after changing llvm.psub wasn't big.
>>
>> > Second - an intrinsic seems a poor fit here given the significance of
>> this operation. We have an instruction that covers most pointer arithmetic
>> (`getelementptr`), and I can imagine growing
>> > pointer subtraction, but it seems like it should be an instruction if
>> we're going to have it. Based on the above, we will need to use it very
>> often in analysis.
>>
>> I also think that defining psub as instruction is fine. :)
>>
>> > Regarding the instcombine, it should be very easy to keep loads and
>> stores of pointers as pointer typed in instcombine. Likely just a missing
>> case in the code I added/touched there.
>>
>> That's really good. :) I found that  combineLoadToOperationType from
>> InstCombineLoadStoreAlloca was responsible for the transformation.
>> I can upload a patch for that if ok.
>>
>> Best Regards,
>> Juneyoung Lee
>>
>> On Mon, Jan 14, 2019 at 5:36 PM Chandler Carruth <chandlerc at gmail.com>
>> wrote:
>>
>>> While I'm very interested in the end result here, I have some questions
>>> that don't seem well answered yet around pointer subtraction...
>>>
>>> First and foremost - how do you address correctness issues here? Because
>>> the subtraction `A - B` can escape/capture more things. Specifically, if
>>> one of `A` or `B` is escaped/captured, the subtraction can be used to
>>> escape or capture the other pointer. So *some* of the conservative
>>> treatment is necessary. What is the plan to update all the analyses to
>>> remain correct? What correctness testing have you done?
>>>
>>> Second - an intrinsic seems a poor fit here given the significance of
>>> this operation. We have an instruction that covers most pointer arithmetic
>>> (`getelementptr`), and I can imagine growing pointer subtraction, but it
>>> seems like it should be an instruction if we're going to have it. Based on
>>> the above, we will need to use it very often in analysis.
>>>
>>>
>>> Regarding the instcombine, it should be very easy to keep loads and
>>> stores of pointers as pointer typed in instcombine. Likely just a missing
>>> case in the code I added/touched there.
>>>
>>> On Mon, Jan 14, 2019 at 3:23 AM Juneyoung Lee via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hello all,
>>>>
>>>> This is a proposal for reducing # of ptrtoint/inttoptr casts which are
>>>> not
>>>> written by programmers but rather generated by LLVM passes.
>>>> Currently the majority of ptrtoint/inttoptr casts are generated by LLVM;
>>>> when compiling SPEC 2017 with LLVM r348082 (Dec 2 2018) with -O3,
>>>> the output IR contains 22,771 inttoptr instructions. However, when
>>>> compiling it with -O0, there are only 1048 inttoptrs, meaning that 95.4%
>>>> of them are generated by LLVM passes.
>>>>
>>>> This trend is similar in ptrtoint instruction as well. When compiling
>>>> SPEC 2017
>>>> with -O0, there are 23,208 ptrtoint instructions, but among them 22,016
>>>> (94.8%)
>>>> are generated by Clang frontend to represent pointer subtraction.
>>>> They aren't effectively optimized out because there are even more
>>>> ptrtoints (31,721) after -O3.
>>>> This is bad for performance because existence of ptrtoint makes
>>>> analysis return conservative
>>>> result as a pointer can be escaped through the cast.
>>>> Memory accesses to a pointer came from inttoptr is assumed
>>>> to possibly access anywhere, therefore it may block
>>>> store-to-load forwarding, merging two same loads, etc.
>>>>
>>>> I believe this can be addressed by applying two patches - first one is
>>>> representing pointer subtraction with a dedicated intrinsic function,
>>>> llvm.psub, and second one is disabling InstCombine transformation
>>>>
>>>>     %q = load i8*, i8** %p1
>>>>     store i8* %q, i8** %p2
>>>> =>
>>>>   %1 = bitcast i8** %p1 to i64*
>>>>   %q1 = load i64, i64* %1, align 8
>>>>   %2 = bitcast i8** %p2 to i64*
>>>>   store i64 %q1, i64* %2, align 8
>>>>
>>>> This transformation can introduce inttoptrs later if loads are followed
>>>> (https://godbolt.org/z/wsZ3II ). Both are discussed in
>>>> https://bugs.llvm.org/show_bug.cgi?id=39846 as well.
>>>> After llvm.psub is used & this transformation is disabled, # of
>>>> inttoptrs decreases from 22,771 to 1,565 (6.9%), and # of ptrtoints
>>>> decreases from 31,721 to 7,772 (24.5%).
>>>>
>>>> I'll introduce llvm.psub patch first.
>>>>
>>>>
>>>> --- Adding llvm.psub ---
>>>>
>>>> By defining pointer subtraction intrinsic, we can get performance gain
>>>> because it gives more undefined behavior than just subtracting two
>>>> ptrtoints.
>>>>
>>>> Patch https://reviews.llvm.org/D56598 adds llvm.psub(p1,p2) intrinsic
>>>> function, which subtracts two pointers and returns the difference. Its
>>>> semantic is as follows.
>>>> If p1 and p2 point to different objects, and neither of them is based
>>>> on a pointer casted from an integer, `llvm.psub(p1, p2)` returns poison.
>>>> For example,
>>>>
>>>> %p = alloca
>>>> %q = alloca
>>>> %i = llvm.psub(p, q) ; %i is poison
>>>>
>>>> This allows aggressive escape analysis on pointers. Given i =
>>>> llvm.psub(p1, p2), if neither of p1 and p2 is based on a pointer casted
>>>> from an integer, the llvm.psub call does not make p1 or p2 escape. (
>>>> https://reviews.llvm.org/D56601 )
>>>>
>>>> If either p1 or p2 is based on a pointer casted from integer, or p1 and
>>>> p2 point to a same object, it returns the result of subtraction (in bytes);
>>>> for example,
>>>>
>>>> %p = alloca
>>>> %q = inttoptr %x
>>>> %i = llvm.psub(p, q) ; %i is equivalent to (ptrtoint %p) - %x
>>>>
>>>> `null` is regarded as a pointer casted from an integer because
>>>> it is equivalent to `inttoptr 0`.
>>>>
>>>> Adding llvm.psub allows LLVM to utilize significant portion of
>>>> ptrtoints & reduce a portion of inttoptrs. After llvm.psub is used, when
>>>> SPECrate 2017 is compiled with -O3, # of inttoptr decreases to ~13,500
>>>> (59%) and # of ptrtoint decreases to ~14,300 (45%).
>>>>
>>>> To see the performance change, I ran SPECrate 2017 (thread # = 1) with
>>>> three versions of LLVM, which are r313797 (Sep 21, 2017), LLVM 6.0
>>>> official, and r348082 (Dec 2, 2018).
>>>> Running r313797 shows that 505.mcf_r has consistent 2.0% speedup over 3
>>>> different machines (which are i3-6100, i5-6600, i7-7700). For LLVM 6.0 and
>>>> r348082, there's neither consistent speedup nor slowdown, but the average
>>>> speedup is near 0. I believe there's still a room of improvement because
>>>> there are passes which are not aware of llvm.psub.
>>>>
>>>> Thank you for reading this, and any comment is welcome.
>>>>
>>>> Best Regards,
>>>> Juneyoung Lee
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>>
>> --
>>
>> Juneyoung Lee
>> Software Foundation Lab, Seoul National University
>>
>

-- 

Juneyoung Lee
Software Foundation Lab, Seoul National University
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