[llvm-dev] persuading licm to do the right thing
Daniel Berlin via llvm-dev
llvm-dev at lists.llvm.org
Wed Dec 9 11:48:41 PST 2015
On Wed, Dec 9, 2015 at 11:16 AM, Preston Briggs via llvm-dev <
llvm-dev at lists.llvm.org> 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.
>
Without arguing about the broader point of "we can't eliminate the
*lowered* operations these turn into", the higher level issue of not really
doing much with them are often just deficiencies in those passes.
There are plenty of those kinds of deficiencies
value numbering also doesn't really value number loads or stores, so would
never discover that a load and an add have the same value, etc.
Smart enough value numbering would understand the equivalence between
various forms of geps.
This won't address the issue of optimizing the lowered form of course.
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).
>
For some definition of better job. For example, this has a cost of making
things like AA significant more difficult
Thus it is likely you want the traditional answer, where the high level
form is optimized as well as it can, they are lowered, and then more
optimization is done.
> 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?
>
> 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
>>>>
>>>>
>>>>
>>>> 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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