[llvm-dev] LoopVectorize fails to vectorize loops with induction variables with PtrToInt/IntToPtr conversions
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Mon Jun 19 18:26:29 PDT 2017
Hi, Adrien,
Thanks for reporting this. I recommend that you file a bug report at
https://bugs.llvm.org/
Whenever I see reports of missed optimization opportunities in the face
of ptrtoint/inttoptr, my first question is: why are these instructions
present in the first place? At the IR level, use of inttoptr is highly
discouraged. Our aliasing analysis, for example, does not look through
them, and so you'll generally see a lot of missed optimizations when
they're around.
In this case, inttoptr seems to be introduced by SROA. SROA should not
be introducing inttoptr, but rather should be using GEPs on i8* (at
least), to avoid introducing pointers that our AA can't analyze.
Beyond that, if we need to handle inttoptr/ptrtoint in SCEV, then maybe
there's a way to make it smarter about the expressions with which it can
deal. I'm actually not sure to what "aliasing rules" the comment you
quote below is referring. I can certainly understand not being able to
place "inbounds" on some generated GEPs, but otherwise this seems
non-obvious to me (i.e. either the expander can identify a base pointer
from which to generate the GEP, or it can't, in which case it needs to
generate a inttoptr).
Sanjoy, thoughts?
-Hal
On 06/17/2017 05:41 PM, Adrien Guinet via llvm-dev wrote:
> Hello all,
>
> There is a missing vectorization opportunity issue with clang 4.0 with
> the file attached.
>
> Indeed, when compiled with -O2, the "op_distance" function get
> vectorized, but not the "op" one.
>
> For information, this test case has been reduced from a file generated
> by the Pythran compiler (https://github.com/serge-sans-paille/pythran).
>
> If we take a look at the generated IR without vectorization (using the
> -fno-vectorize clang flag), we get:
>
>> $ clang -O2 -S -emit-llvm op_zip_iterator.cpp -std=c++11 -o - -fno-vectorize
>> ; Function Attrs: norecurse uwtable
>> define void @_Z11op_distancePi16add_zip_iteratorS0_(i32* nocapture, i32*, i32* nocapture readonly, i32*, i32* nocapture readnone) local_unnamed_addr #0 {
>> ; This one is vectorized!
>> %6 = ptrtoint i32* %1 to i64
>> %7 = ptrtoint i32* %3 to i64
>> %8 = sub i64 %7, %6
>> %9 = icmp sgt i64 %8, 0
>> br i1 %9, label %10, label %26
>>
>> ; <label>:10: ; preds = %5
>> %11 = lshr exact i64 %8, 2
>> br label %12
>>
>> ; <label>:12: ; preds = %12, %10
>> %13 = phi i64 [ %23, %12 ], [ %11, %10 ]
>> %14 = phi i32* [ %22, %12 ], [ %0, %10 ]
>> %15 = phi i32* [ %21, %12 ], [ %2, %10 ]
>> %16 = phi i32* [ %20, %12 ], [ %1, %10 ]
>> %17 = load i32, i32* %16, align 4, !tbaa !1
>> %18 = load i32, i32* %15, align 4, !tbaa !1
>> %19 = add nsw i32 %18, %17
>> store i32 %19, i32* %14, align 4, !tbaa !1
>> %20 = getelementptr inbounds i32, i32* %16, i64 1
>> %21 = getelementptr inbounds i32, i32* %15, i64 1
>> %22 = getelementptr inbounds i32, i32* %14, i64 1
>> %23 = add nsw i64 %13, -1
>> %24 = icmp sgt i64 %13, 1
>> br i1 %24, label %12, label %25
>>
>> ; <label>:25: ; preds = %12
>> br label %26
>>
>> ; <label>:26: ; preds = %25, %5
>> ret void
>> }
>>
>> ; Function Attrs: norecurse uwtable
>> define void @_Z2opPi16add_zip_iteratorS0_(i32* nocapture, i32*, i32* nocapture readonly, i32*, i32* nocapture readnone) local_unnamed_addr #0 {
>> ; This one isn't!
>> %6 = ptrtoint i32* %1 to i64
>> %7 = ptrtoint i32* %3 to i64
>> %8 = sub i64 %6, %7
>> %9 = icmp sgt i64 %8, 0
>> br i1 %9, label %10, label %28
>>
>> ; <label>:10: ; preds = %5
>> %11 = lshr exact i64 %8, 2
>> br label %12
>>
>> ; <label>:12: ; preds = %12, %10
>> %13 = phi i64 [ %25, %12 ], [ %11, %10 ]
>> %14 = phi i32* [ %24, %12 ], [ %0, %10 ]
>> %15 = phi i32* [ %23, %12 ], [ %2, %10 ]
>> %16 = phi i64 [ %22, %12 ], [ %6, %10 ]
>> %17 = inttoptr i64 %16 to i32*
>> %18 = load i32, i32* %17, align 4, !tbaa !1
>> %19 = load i32, i32* %15, align 4, !tbaa !1
>> %20 = add nsw i32 %19, %18
>> store i32 %20, i32* %14, align 4, !tbaa !1
>> %21 = getelementptr inbounds i32, i32* %17, i64 1
>> %22 = ptrtoint i32* %21 to i64
>> %23 = getelementptr inbounds i32, i32* %15, i64 1
>> %24 = getelementptr inbounds i32, i32* %14, i64 1
>> %25 = add nsw i64 %13, -1
>> %26 = icmp sgt i64 %13, 1
>> br i1 %26, label %12, label %27
>>
>> ; <label>:27: ; preds = %12
>> br label %28
>>
>> ; <label>:28: ; preds = %27, %5
>> ret void
>> }
> If we compile only the "op" function while activation the debug mode,
> here is the output:
>
>> $ clang -O2 -S -emit-llvm op_zip_iterator.cpp -std=c++11 -o - -fno-vectorize |~/dev/epona-llvm/build_debug_shared/bin/opt -debug -debug-only loop-vectorize -O2 -S
>>
>> LV: Checking a loop in "_Z2opPi16add_zip_iteratorS0_" from <stdin>
>> LV: Loop hints: force=? width=0 unroll=0
>> LV: Found a loop:
>> LV: Found an induction variable.
>> LV: Found an induction variable.
>> LV: Found an induction variable.
>> LV: Found an unidentified PHI. %16 = phi i64 [ %22, %12 ], [ %6, %10 ]
>> LV: Can't vectorize the instructions or CFG
>> LV: Not vectorizing: Cannot prove legality.
>> [...]
> The issue seems to be that the phi node "%16" can't be deduced as an
> induction variable. If we take a closer look, the cause seems to be in
> ScalarEvolution, in the createSCEV function
> (http://llvm.org/docs/doxygen/html/ScalarEvolution_8cpp_source.html#l04770)
> :
>
>> // It's tempting to handle inttoptr and ptrtoint as no-ops, however this can
>> // lead to pointer expressions which cannot safely be expanded to GEPs,
>> // because ScalarEvolution doesn't respect the GEP aliasing rules when
>> // simplifying integer expressions.
> Indeed, SCEV does not (legitimately) consider inttoptr/ptrtoint as
> no-op, and does not handle them. The thing is that, in our case, the GEP
> in %23 is thus not analyzed by SCEV, and the PHI %16 is thus not
> considered as an induction variable.
>
> To confirm this hypothesis, I created a small out-of-tree pass
> (https://github.com/aguinet/llvm-intptrcleanup) which registers before
> loop vectorization and does the following:
>
> * first, it search for phi nodes who have those properties:
> - every incoming value of the phi node is a ptrtoint instruction. The
> original pointer type of every ptrtoint instruction must be the same type T.
> - every user of this PHI node is an inttoptr instruction of the
> previous type T
> * for each of these PHI nodes, it creates a new PHI node which takes the
> original pointers as incoming values, and replace the uses of the
> inttoptr instructions that uses the original PHI node by the new one
> * it then removes the previous inttoptr instructions and the original
> PHI node
>
> The way I understand inttoptr and ptrtoint, this transformation should
> be valid (but I might have missed something!). Please note that this is
> a quick'n'dirty pass, which hasn't been heavily tested. Using this pass,
> the previous example is now vectorized correctly by the loop vectorizer.
> This can be seen by looking at the output of:
>
>> $ clang -Xclang -load -Xclang IntToPtrCleanup.so -O2 ./example/op_zip_operator.cpp -S -emit-llvm -o - -std=c++11
> The question that remains to me is how this should be correctly fixed:
>
> 1) Making SCEV support these no-op (in this case) inttoptr/ptrtoint
> conversions
> 2) insert the above transformation at some point in the optimization
> pipeline
> 3) clean the pass(es?) that somehow generated this case.
>
> I have to admit I'm not really sure which options is the best. 3) seems
> to be the way to go but might require some tedious work, and does not
> prevent the issue to come again in the future. 2) seems to be a quick
> patch that could be inserted in some "canonicalization" pass, let it be
> a valid transformation in the first place. I don't know SCEV enough to
> judge of the difficulty/faisability of 1).
>
> This mail is thus to discuss this issue and how to fix this properly :)
>
> Thanks everyone :)
>
>
>
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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