[PATCH] Add getUnrollingPreferences to TTI
Hal Finkel
hfinkel at anl.gov
Wed Aug 28 22:40:47 PDT 2013
----- Original Message -----
> On Wed, Aug 28, 2013 at 03:07:31PM -0500, Hal Finkel wrote:
> > Nadav, et al.,
> >
> > The attached patch adds the following interface to TTI:
> >
> > /// Parameters that control the generic loop unrolling
> > transformation.
> > struct UnrollingPreferences {
> > unsigned Threshold; ///< The cost threshold for the unrolled
> > loop.
> > unsigned OptSizeThreshold; ///< The cost threshold for the
> > unrolled loop
> > ///< when optimizing for size.
> > bool Partial; ///< Allow partial loop unrolling.
> > bool Runtime; ///< Perform runtime unrolling.
> > };
> >
> > /// \brief Get target-customized preferences for the generic loop
> > unrolling
> > /// transformation. Returns true if the UnrollingPreferences struct
> > has been
> > /// initialized.
> > virtual bool getUnrollingPreferences(UnrollingPreferences &UP)
> > const;
> >
> > I'd like to use this in the PowerPC backend when targeting the A2
> > core. For this target, using more aggressive unrolling helps a lot
> > because it is in-order with a deep pipeline. As a result,
> > unrolling is important for hiding instruction latency and branch
> > overhead (especially when combined with the -enable-aa-sched-mi
> > functionality).
> >
> > I discussed this briefly with Chandler on IRC, and he expressed the
> > opinion that changing the unrolling factor does not really change
> > the canonical form (and TTI is already used to calculate the loop
> > body costs), and so this seems like an appropriate use of TTI.
> >
> > Please review.
> >
> > Thanks again,
> > Hal
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
>
> > diff --git a/include/llvm/Analysis/TargetTransformInfo.h
> > b/include/llvm/Analysis/TargetTransformInfo.h
> > index 06810a7..908612c 100644
> > --- a/include/llvm/Analysis/TargetTransformInfo.h
> > +++ b/include/llvm/Analysis/TargetTransformInfo.h
> > @@ -191,6 +191,20 @@ public:
> > /// incurs significant execution cost.
> > virtual bool isLoweredToCall(const Function *F) const;
> >
> > + /// Parameters that control the generic loop unrolling
> > transformation.
> > + struct UnrollingPreferences {
> > + unsigned Threshold; ///< The cost threshold for the unrolled
> > loop.
> > + unsigned OptSizeThreshold; ///< The cost threshold for the
> > unrolled loop
> > + ///< when optimizing for size.
> > + bool Partial; ///< Allow partial loop unrolling.
> > + bool Runtime; ///< Perform runtime unrolling.
> > + };
> > +
> > + /// \brief Get target-customized preferences for the generic
> > loop unrolling
> > + /// transformation. Returns true if the UnrollingPreferences
> > struct has been
> > + /// initialized.
> > + virtual bool getUnrollingPreferences(UnrollingPreferences &UP)
> > const;
> > +
>
> Would it be possible to pass a Loop object to this function? It
> would
> be useful for the R600 target at least to be able to inspect the loop
> to
> decide what threshold to use.
>
> For example, it would probably be best for R600 not to unroll loops
> with
> barrier intrinsics, since there are several restrictions on the
> control
> flow constructs in which barriers can be used. On the other hand,
> with
> loops that index an array in the private address space, R600 may want
> to use a higher than normal threshold if it makes it possible for
> SROA
> to move the entire array to registers.
I don't see why not ;) -- I've attached a revised patch with this feature (and also with the ability to override the default unrolling count). How's this?
-Hal
>
> -Tom
>
>
> > /// @}
> >
> > /// \name Scalar Target Information
> > diff --git a/lib/Analysis/TargetTransformInfo.cpp
> > b/lib/Analysis/TargetTransformInfo.cpp
> > index 0a215aa..8a443cc 100644
> > --- a/lib/Analysis/TargetTransformInfo.cpp
> > +++ b/lib/Analysis/TargetTransformInfo.cpp
> > @@ -96,6 +96,11 @@ bool TargetTransformInfo::isLoweredToCall(const
> > Function *F) const {
> > return PrevTTI->isLoweredToCall(F);
> > }
> >
> > +bool TargetTransformInfo::getUnrollingPreferences(
> > + UnrollingPreferences &UP) const {
> > + return PrevTTI->getUnrollingPreferences(UP);
> > +}
> > +
> > bool TargetTransformInfo::isLegalAddImmediate(int64_t Imm) const {
> > return PrevTTI->isLegalAddImmediate(Imm);
> > }
> > @@ -461,6 +466,10 @@ struct NoTTI : ImmutablePass,
> > TargetTransformInfo {
> > return true;
> > }
> >
> > + virtual bool getUnrollingPreferences(UnrollingPreferences &)
> > const {
> > + return false;
> > + }
> > +
> > bool isLegalAddImmediate(int64_t Imm) const {
> > return false;
> > }
> > diff --git a/lib/CodeGen/BasicTargetTransformInfo.cpp
> > b/lib/CodeGen/BasicTargetTransformInfo.cpp
> > index d5340e6..e1380b7 100644
> > --- a/lib/CodeGen/BasicTargetTransformInfo.cpp
> > +++ b/lib/CodeGen/BasicTargetTransformInfo.cpp
> > @@ -84,6 +84,7 @@ public:
> > virtual unsigned getJumpBufSize() const;
> > virtual bool shouldBuildLookupTables() const;
> > virtual bool haveFastSqrt(Type *Ty) const;
> > + virtual bool getUnrollingPreferences(UnrollingPreferences &UP)
> > const;
> >
> > /// @}
> >
> > @@ -189,6 +190,10 @@ bool BasicTTI::haveFastSqrt(Type *Ty) const {
> > return TLI->isTypeLegal(VT) &&
> > TLI->isOperationLegalOrCustom(ISD::FSQRT, VT);
> > }
> >
> > +bool BasicTTI::getUnrollingPreferences(UnrollingPreferences &)
> > const {
> > + return false;
> > +}
> > +
> > //===----------------------------------------------------------------------===//
> > //
> > // Calls used by the vectorizers.
> > diff --git a/lib/Transforms/Scalar/LoopUnrollPass.cpp
> > b/lib/Transforms/Scalar/LoopUnrollPass.cpp
> > index 80d060b..f8ff275 100644
> > --- a/lib/Transforms/Scalar/LoopUnrollPass.cpp
> > +++ b/lib/Transforms/Scalar/LoopUnrollPass.cpp
> > @@ -55,6 +55,8 @@ namespace {
> > CurrentAllowPartial = (P == -1) ? UnrollAllowPartial :
> > (bool)P;
> >
> > UserThreshold = (T != -1) ||
> > (UnrollThreshold.getNumOccurrences() > 0);
> > + UserAllowPartial = (P != -1) ||
> > + (UnrollAllowPartial.getNumOccurrences() >
> > 0);
> >
> > initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
> > }
> > @@ -76,6 +78,7 @@ namespace {
> > unsigned CurrentThreshold;
> > bool CurrentAllowPartial;
> > bool UserThreshold; // CurrentThreshold is
> > user-specified.
> > + bool UserAllowPartial; // CurrentAllowPartial is
> > user-specified.
> >
> > bool runOnLoop(Loop *L, LPPassManager &LPM);
> >
> > @@ -145,16 +148,20 @@ bool LoopUnroll::runOnLoop(Loop *L,
> > LPPassManager &LPM) {
> > << "] Loop %" << Header->getName() << "\n");
> > (void)Header;
> >
> > + TargetTransformInfo::UnrollingPreferences UP;
> > + bool HasUP = TTI.getUnrollingPreferences(UP);
> > +
> > // Determine the current unrolling threshold. While this is
> > normally set
> > // from UnrollThreshold, it is overridden to a smaller value if
> > the current
> > // function is marked as optimize-for-size, and the unroll
> > threshold was
> > // not user specified.
> > - unsigned Threshold = CurrentThreshold;
> > + unsigned Threshold = (HasUP && !UserThreshold) ? UP.Threshold :
> > +
> > CurrentThreshold;
> > if (!UserThreshold &&
> > Header->getParent()->getAttributes().
> > hasAttribute(AttributeSet::FunctionIndex,
> > Attribute::OptimizeForSize))
> > - Threshold = OptSizeUnrollThreshold;
> > + Threshold = HasUP ? UP.OptSizeThreshold :
> > OptSizeUnrollThreshold;
> >
> > // Find trip count and trip multiple if count is not available
> > unsigned TripCount = 0;
> > @@ -184,6 +191,9 @@ bool LoopUnroll::runOnLoop(Loop *L,
> > LPPassManager &LPM) {
> > Count = TripCount;
> > }
> >
> > + bool Runtime = (HasUP && UnrollRuntime.getNumOccurrences() == 0)
> > ?
> > + UP.Runtime : UnrollRuntime;
> > +
> > // Enforce the threshold.
> > if (Threshold != NoThreshold) {
> > unsigned NumInlineCandidates;
> > @@ -204,7 +214,9 @@ bool LoopUnroll::runOnLoop(Loop *L,
> > LPPassManager &LPM) {
> > if (TripCount != 1 && Size > Threshold) {
> > DEBUG(dbgs() << " Too large to fully unroll with count: "
> > << Count
> > << " because size: " << Size << ">" << Threshold <<
> > "\n");
> > - if (!CurrentAllowPartial && !(UnrollRuntime && TripCount ==
> > 0)) {
> > + bool AllowPartial = (HasUP && !UserAllowPartial) ?
> > UP.Partial :
> > +
> > CurrentAllowPartial;
> > + if (!AllowPartial && !(Runtime && TripCount == 0)) {
> > DEBUG(dbgs() << " will not try to unroll partially
> > because "
> > << "-unroll-allow-partial not given\n");
> > return false;
> > @@ -215,7 +227,7 @@ bool LoopUnroll::runOnLoop(Loop *L,
> > LPPassManager &LPM) {
> > while (Count != 0 && TripCount%Count != 0)
> > Count--;
> > }
> > - else if (UnrollRuntime) {
> > + else if (Runtime) {
> > // Reduce unroll count to be a lower power-of-two value
> > while (Count != 0 && Size > Threshold) {
> > Count >>= 1;
> > @@ -231,7 +243,7 @@ bool LoopUnroll::runOnLoop(Loop *L,
> > LPPassManager &LPM) {
> > }
> >
> > // Unroll the loop.
> > - if (!UnrollLoop(L, Count, TripCount, UnrollRuntime,
> > TripMultiple, LI, &LPM))
> > + if (!UnrollLoop(L, Count, TripCount, Runtime, TripMultiple, LI,
> > &LPM))
> > return false;
> >
> > return true;
>
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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