[llvm-dev] persuading licm to do the right thing
Preston Briggs via llvm-dev
llvm-dev at lists.llvm.org
Wed Dec 9 11:16:25 PST 2015
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?
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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