[llvm-dev] persuading licm to do the right thing
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Thu Dec 10 09:53:50 PST 2015
----- Original Message -----
> From: "Preston Briggs via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Mehdi Amini" <mehdi.amini at apple.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, December 9, 2015 4:25:38 PM
> Subject: Re: [llvm-dev] persuading licm to do the right thing
>
>
>
> I understand that GEPs do not access memory.
> They do a (possibly expensive) address calculation,
> perhaps adding a few values to a label and leaving the result in a
> register.
> Getting a label into a register is (to me) just like loading a 64-bit
> integer value
> into a register. It can happen in many places and it can cost a few
> instructions
> and several bytes. I'd like to see such things commoned and hoisted
> when possible.
>
>
> I agree with you that I may be fighting the spirit of the IR.
>
>
> I also agree with Daniel that the classic solution is to lower and
> re-optimize.
> Indeed, I tried it, sort of, by rewriting GEPs into a sequence of
> adds and multiplies,
> with bitcasts to gets the types right. This helps in some places, but
> the
> re-canonicalization defeats my efforts in others places.
>
>
> So now I feel like I'm going to have to write my own LICM and VN
> passes.
> I can do it and it won't take that long, but I had imagined that I
> could
> pick and choose from the tools already available to achieve the
> desired effect.
> Seems not, and I'm surprised.
>
At what point in the pipeline do you intend to run these tools? We do have some LICM, CSE, etc. capabilities late in the pipeline (at the SDAG and MI level), and we already so some GEP splitting in CodeGenPrep just before instruction selection. Once GEPs are fully decomposed during SDAG construction these later optimizations should do the things you describe. That having been said, they have known limitations in this area, but ideally we should all work together on improving that infrastructure.
-Hal
>
> Preston
>
>
>
>
> On Wed, Dec 9, 2015 at 1:10 PM, Mehdi Amini < mehdi.amini at apple.com >
> wrote:
>
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>
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>
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> On Dec 9, 2015, at 11:16 AM, Preston Briggs < briggs at reservoir.com >
> wrote:
>
>
> A GEP can represent a potentially large tree of instructions.
> Seems like all the sub-trees are hidden from optimization;
> that is, I never see licm or value numbering doing anything with
> them.
> If I rewrite the GEPs as lots of little adds and multiplies,
> then opt will do a better job (I speculate this happens during
> lowering).
>
>
> One of the computations that's hidden in the GEP in my example
> is the non-zero effort required to get the value of a label into a
> register.
> I'd like to expose that effort to the optimizer; instead, it seems
> hidden, papered over with the GEP.
>
>
> Your point about putting labels in global variables seems to work
> if I do it by hand. Kind of embarassing though, don't you think,
> introducing an indirection to achieve better code?
>
>
> Isn’t this indirection just how your target work?
> It seems to me that this indirection just achieve exactly what you
> want: a more risc-like and less implicit representation!
>
>
> As far as I can tell, accessing a “label” does not require a load at
> the IR level, (and indeed it does not on most targets). It is baked
> in everywhere in the code that operates on the IR. You will fight
> the model all the time if you try to circumvent that.
> For instance the bitcast of the global label I used in my first
> answer is a compile time constant expression for LLVM.
> On the same topic, integer literals are assumed to be inlined in the
> code, we don’t expose any load in the IR to operate on a literal
> value.
>
>
> Also, you are relying on an implicit load (only on your target) for
> global labels, which I believe is not valid IR. For LLVM IR, a GEP
> is nothing more than pointer arithmetic.
> Per LangRef:
>
>
> The ‘getelementptr‘ instruction is used to get the address of a
> subelement of an aggregate data structure. It performs address
> calculation only and does not access memory. The instruction can
> also be used to calculate a vector of such addresses.
>
> Note the *does not access memory* part (now, you may argue that the
> “load” you are emitting is not accessing memory exposed in the
> optimizer memory model and this does not apply).
>
>
> —
> Mehdi
>
>
>
>
>
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>
>
> Thanks,
> Preston
>
>
>
>
>
>
>
>
>
>
> On Wed, Dec 9, 2015 at 9:17 AM, Mehdi Amini < mehdi.amini at apple.com >
> wrote:
>
>
>
>
>
>
>
>
> On Dec 9, 2015, at 9:00 AM, Preston Briggs < briggs at reservoir.com >
> wrote:
>
>
> I suppose your view is reasonable, and perhaps common.
> My own "taste" has always preferred machine-independent code
> that is as simple as possible, so GEPs reduced to nothing more than
> an
> add, etc, i.e., quite risc-like. Then optimize it to reduce the total
> number
> of operations (as best we can), then raise the level during
> instruction
> selection, taking advantage of available instructions.
>
>
> I’m not sure I see something related to risc-like here, it seems to
> me that your problem is not GEP vs ADD but rather that you want to
> expose a mode where global addresses need to be loaded and can’t be
> referenced directly.
> (Unless I misunderstood the problem which is very possible as well)
>
>
> Maybe you could do this with a transformation that would put all the
> global variable addresses in a global array and reference them
> through the array. That’s the only workaround I could see.
>
>
> —
> Mehdi
>
>
>
>
>
>
>
>
>
>
>
>
>
> I guess my whole scheme of using opt in this context is probably
> wrong headed.
>
>
> Thanks
>
>
>
>
>
>
> On Wed, Dec 9, 2015 at 8:45 AM, Mehdi Amini < mehdi.amini at apple.com >
> wrote:
>
>
>
>
>
>
>
>
> On Dec 9, 2015, at 7:58 AM, Preston Briggs < briggs at reservoir.com >
> wrote:
>
>
> I'm trying to make the IR "better", in a machine-independent fashion,
> without having to do any lowering.
>
>
>
> The question is “would the IR be more canonical” with the
> representation you suggest? Why would the optimizer benefit from
> this representation instead of the current one in general?
> Right now this GEP reads as an offset from a constant global, which
> seems pretty optimal to me.
>
>
>
>
> My impression is that when you reach a point where the “better” is
> target specific, this is part of the lowering (I’m using lowering in
> the sense that you go away from the canonical representation the
> optimizer expects). I believe it is pretty common that targets need
> to do this kind of lowering.
>
>
> —
> Mehdi
>
>
>
>
>
>
>
>
>
>
>
> I've written code that rewrites GEPs as simple adds and multiplies,
> which helps a lot, but there's still some sort of re-canonicalization
> that's getting in my way. Is there perhaps a way to suppress it?
>
>
>
>
>
>
> Thanks,
> Preston
>
>
>
>
> On Wed, Dec 9, 2015 at 7:47 AM, Mehdi Amini < mehdi.amini at apple.com >
> wrote:
>
>
>
> I guess is has to be done as part of the lowering for such a target,
> either during CodegenPrepare or during something like MachineLICM.
>
>
> —
> Mehdi
>
>
>
>
>
>
>
>
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>
>
>
> On Dec 9, 2015, at 7:13 AM, Preston Briggs < briggs at reservoir.com >
> wrote:
>
>
> On some targets with limited addressing modes,
> getting that 64-bit relocatable but loop-invariant value into a
> register
> requires several instructions. I'd like those several instruction
> outside
> the loop, where they belong.
>
>
> Yes, my experience is that something (I assume instcombine)
> recanonicalizes.
>
>
> Thanks,
> Preston
>
>
>
>
> On Tue, Dec 8, 2015 at 11:21 PM, Mehdi Amini < mehdi.amini at apple.com
> > wrote:
>
>
>
> Hi Preston,
>
>
>
>
>
>
>
> On Dec 8, 2015, at 10:56 PM, Preston Briggs via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
>
>
> When I compile two different modules using
>
>
>
>
> clang -O -S -emit-llvm
>
>
> I get different .ll files, no surprise.
>
>
> The first looks like
>
>
>
>
>
> double *v;
>
>
>
>
> double zap(long n) {
>
> double sum = 0;
>
> for (long i = 0; i < n; i++)
>
> sum += v[i];
>
> return sum;
>
> }
>
>
> yielding
>
>
>
>
>
> @v = common global double* null, align 8
>
>
>
>
> ; Function Attrs: nounwind readonly uwtable
>
> define double @zap(i64 %n) #0 {
>
> entry:
>
> %cmp4 = icmp sgt i64 %n, 0
>
> br i1 %cmp4, label % for.body.lr.ph , label %for.end
>
>
>
>
> for.body.lr.ph : ; preds = %entry
>
> %0 = load double** @v, align 8, !tbaa !1
>
> br label %for.body
>
>
>
>
> for.body: ; preds = %for.body, % for.body.lr.ph
>
> %i.06 = phi i64 [ 0, % for.body.lr.ph ], [ %inc, %for.body ]
>
> %sum.05 = phi double [ 0.000000e+00, % for.body.lr.ph ], [ %add,
> %for.body ]
>
> %arrayidx = getelementptr inbounds double* %0, i64 %i.06
>
> %1 = load double* %arrayidx, align 8, !tbaa !5
>
> %add = fadd double %sum.05, %1
>
> %inc = add nsw i64 %i.06, 1
>
>
>
> %exitcond = icmp eq i64 %inc, %n
>
> br i1 %exitcond, label %for.end, label %for.body
>
>
>
>
> for.end: ; preds = %for.body, %entry
>
> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body
> ]
>
> ret double %sum.0.lcssa
>
> }
>
>
> and the second looks like
>
>
>
>
>
> double v[10000];
>
>
>
>
>
>
> double zap(long n) {
>
> double sum = 0;
>
> for (long i = 0; i < n; i++)
>
> sum += v[i];
>
> return sum;
>
> }
>
>
> yielding
>
>
>
>
>
> ; ModuleID = 'z.c'
>
> target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-f128:128:128-n8:16:32:64-S128"
>
> target triple = "x86_64-unknown-linux-gnu"
>
>
>
>
> @v = common global [10000 x double] zeroinitializer, align 16
>
>
>
>
> ; Function Attrs: nounwind readonly uwtable
>
> define double @zap(i64 %n) #0 {
>
> entry:
>
> %cmp4 = icmp sgt i64 %n, 0
>
> br i1 %cmp4, label %for.body, label %for.end
>
>
>
>
> for.body: ; preds = %entry, %for.body
>
> %i.06 = phi i64 [ %inc, %for.body ], [ 0, %entry ]
>
> %sum.05 = phi double [ %add, %for.body ], [ 0.000000e+00, %entry ]
>
> %arrayidx = getelementptr inbounds [10000 x double]* @v, i64 0, i64
> %i.06
>
> %0 = load double* %arrayidx, align 8, !tbaa !1
>
> %add = fadd double %sum.05, %0
>
> %inc = add nsw i64 %i.06, 1
>
> %exitcond = icmp eq i64 %inc, %n
>
> br i1 %exitcond, label %for.end, label %for.body
>
>
>
>
> for.end: ; preds = %for.body, %entry
>
> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body
> ]
>
> ret double %sum.0.lcssa
>
> }
>
>
>
>
> attributes #0 = { nounwind readonly uwtable
> "less-precise-fpmad"="false" "no-frame-pointer-elim"="false"
> "no-infs-fp-math"="false" "no-nans-fp-math"="false"
> "stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
> "use-soft-float"="false" }
>
>
>
>
> !llvm.ident = !{!0}
>
>
>
>
> !0 = metadata !{metadata !"Clang Front-End version 3.4.1
> (tags/RELEASE_34/final)"}
>
> !1 = metadata !{metadata !2, metadata !2, i64 0}
>
> !2 = metadata !{metadata !"double", metadata !3, i64 0}
>
> !3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
>
> !4 = metadata !{metadata !"Simple C/C++ TBAA"}
>
>
> (I included all the metadata and such for the 2nd case, on the off
> chance it matters.)
>
>
> Is there any way I can convince licm (or something) to rip open the
> GEP and hoist the reference to @v outside the loop, similar to the
> first example?
>
>
>
> I believe that in the second case, there is no need to load the
> address of v as it is constant. However you have a constant address
> to an array, which is represented by [10000 x double]* @v in the IR,
> which requires to use the two-level GEP.
>
>
> You “could” manage to represent it this way:
>
>
> define double @zap(i64 %n) #0 {
> entry:
> %cmp6 = icmp sgt i64 %n, 0
> %hoisted = bitcast [10000 x double]* @v to double*
> br i1 %cmp6, label %for.body.preheader, label %for.cond.cleanup
>
> for.body.preheader: ; preds = %entry
> br label %for.body
>
> for.cond.cleanup.loopexit: ; preds = %for.body
> %add.lcssa = phi double [ %add, %for.body ]
> br label %for.cond.cleanup
>
> for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry
> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add.lcssa,
> %for.cond.cleanup.loopexit ]
> ret double %sum.0.lcssa
>
> for.body: ; preds = %for.body.preheader, %for.body
> %i.08 = phi i64 [ %inc, %for.body ], [ 0, %for.body.preheader ]
> %sum.07 = phi double [ %add, %for.body ], [ 0.000000e+00,
> %for.body.preheader ]
> %arrayidx = getelementptr double, double* %hoisted, i64 %i.08
> %0 = load double, double* %arrayidx, align 8, !tbaa !2
> %add = fadd double %sum.07, %0
> %inc = add nuw nsw i64 %i.08, 1
> %exitcond = icmp eq i64 %inc, %n
> br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body
> }
>
>
>
>
> However instcombine will recanonicalize it like it was originally.
>
>
> Since it is a GEP that operate on a constant address, this shouldn’t
> matter, why would you want to split this?
>
>
> Best,
>
>
> —
> Mehdi
>
>
>
>
>
>
>
>
>
>
>
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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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