[LLVMdev] RFC: Loop versioning for LICM
Adam Nemet
anemet at apple.com
Thu Feb 26 11:09:40 PST 2015
Hi Ashutosh,
Have you been following the recent Loop Access Analysis work? LAA was split out from the Loop Vectorizer that have been performing the kind of loop versioning that you describe. The main reason was to be able to share this functionality with other passes.
Loop Access Analysis is an analysis pass that computes basic memory dependence and the runtime checks. The versioning decision and then performing the transformation are left to the transform passes using this analysis.
If we decide that a stand-alone memcheck-based loop-versioning is desired we should probably use this analysis and possibly extend it instead of duplicating the code.
Adam
> On Feb 26, 2015, at 2:31 AM, Nema, Ashutosh <Ashutosh.Nema at amd.com> wrote:
>
> I like to propose a new loop multi versioning optimization for LICM.
> For now I kept this for LICM only, but it can be used in multiple places.
> The main motivation is to allow optimizations stuck because of memory
> alias dependencies. Most of the time when alias analysis is unsure about
> memory access and it says may-alias. This un surety from alias analysis restrict
> some of the memory based optimizations to proceed further.
> We observed some cases with LICM, where things are beyond aliasing.
> In cases where alias analysis is unsure we like to use loop versioning as an alternative.
>
> Loop Versioning will creates version of the loop with aggressive alias and the other
> with conservative (default) alias. Aggressive alias version of loop will have all the
> memory access marked as no-alias. These two version of loop will be preceded by a
> memory runtime check. This runtime check consists of bound checks for all unique memory
> accessed in loop, and it ensures aliasing of memory. Based on this check result at runtime
> any of the loops gets executed, if memory is non aliased then aggressive aliasing loop
> gets executed, else when memory is aliased then non aggressive aliased version gets executed.
>
> By setting no-alias to memory accessed in aggressive alias version of loop, enable other
> optimization to continue further.
>
> Following are the top level steps:
>
> 1) Perform loop do versioning feasibility check.
> 2) If loop is a candidate for versioning then create a memory bound check, by considering
> all the memory access in loop body.
> 3) Clone original loop and set all memory access as no-alias in new loop.
> 4) Set original loop & versioned loop as a branch target of runtime check result.
> 5) Call LICM on aggressive alias versioned of loop(For now LICM is scheduled later and not directly
> called from LoopVersioning pass).
>
> Consider following test:
>
> 1 int foo(int * var1, int * var2, int * var3, unsigned itr) {
> 2 unsigned i = 0, j = 0;
> 3 for(; i < itr; i++) {
> 4 for(; j < itr; j++) {
> 5 var1[j] = itr + i;
> 6 var3[i] = var1[j] + var3[i];
> 7 }
> 8 }
> 9 }
>
> At line #6 store to var3 can be moved out by LICM(promoteLoopAccessesToScalars)
> but because of alias analysis un surety about memory access it unable to move it out.
>
> After Loop versioning IR:
>
> <Versioned Loop>
> for.body3.loopVersion: ; preds = %for.body3.loopVersion.preheader, %for.body3.loopVersion
> %indvars.iv.loopVersion = phi i64 [ %indvars.iv.next.loopVersion, %for.body3.loopVersion ], [ %2, %for.body3.loopVersion.preheader ]
> %arrayidx.loopVersion = getelementptr inbounds i32* %var1, i64 %indvars.iv.loopVersion
> store i32 %add, i32* %arrayidx.loopVersion, align 4, !tbaa !1, !alias.scope !11, !noalias !11
> %indvars.iv.next.loopVersion = add nuw nsw i64 %indvars.iv.loopVersion, 1
> %lftr.wideiv.loopVersion = trunc i64 %indvars.iv.loopVersion to i32
> %exitcond.loopVersion = icmp eq i32 %lftr.wideiv.loopVersion, %0
> br i1 %exitcond.loopVersion, label %for.inc11.loopexit38, label %for.body3.loopVersion
>
> <Original Loop>
> for.body3: ; preds = %for.body3.lr.ph, %for.body3
> %indvars.iv = phi i64 [ %indvars.iv.next, %for.body3 ], [ %2, %for.body3.lr.ph ]
> %arrayidx = getelementptr inbounds i32* %var1, i64 %indvars.iv
> store i32 %add, i32* %arrayidx, align 4, !tbaa !1
> %8 = load i32* %arrayidx7, align 4, !tbaa !1
> %add8 = add nsw i32 %8, %add
> store i32 %add8, i32* %arrayidx7, align 4, !tbaa !1
> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
> %lftr.wideiv = trunc i64 %indvars.iv to i32
> %exitcond = icmp eq i32 %lftr.wideiv, %0
> br i1 %exitcond, label %for.inc11, label %for.body3
>
> In versioned loop difference is visible, 1 store has moved out.
>
> Following are some high level details about current implementation:
>
> - LoopVersioning
> LoopVersioning is main class which holds multi versioning functionality.
>
> - LoopVersioning :: isVersioningBeneficial
> Its member to ‘LoopVersioning’
> Does feasibility check for loop versioning.
> a) Checks layout of loop.
> b) Instruction level check.
> c) memory checks.
>
> - LoopVersioning :: versionizeLoop
> a) Clone original loo
> b) Create a runtime memory check.
> c) Add both loops under runtime check results target.
>
> - RuntimeMemoryCheck
> This class take cares runtime memory check.
>
> - RuntimeMemoryCheck ::createRuntimeCheck
> It creates runtime memory check.
>
> In this patch used maximum loop nest threshold as 2, and maximum number
> of pointers in runtime memory check as 5.
>
> Later I like to make this as a utility so others can use it.
>
> Requesting to go through patch for detailed approach.
> Patch available at http://reviews.llvm.org/D7900 <http://reviews.llvm.org/D7900>
>
> Suggestions are comments are welcome.
>
> Regards,
> Ashutosh
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