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LoopVersioning for LICM

Authored by ashutosh.nema on Feb 26 2015, 2:23 AM.



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 [, %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 = 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

%indvars.iv = phi i64 [, %for.body3 ], [ %2, ]
%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 = 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.
Suggestions are comments are welcome.

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ashutosh.nema retitled this revision from to LoopVersioning for LICM.
ashutosh.nema updated this object.
ashutosh.nema edited the test plan for this revision. (Show Details)
ashutosh.nema set the repository for this revision to rL LLVM.
ashutosh.nema abandoned this revision.Feb 12 2016, 3:46 AM