[llvm] r210721 - Teach LoopUnrollPass to respect loop unrolling hints in metadata.

Eli Bendersky eliben at google.com
Thu Jun 12 11:47:50 PDT 2014


On Thu, Jun 12, 2014 at 11:44 AM, Eric Christopher <echristo at gmail.com>
wrote:

> On Thu, Jun 12, 2014 at 11:15 AM, Eli Bendersky <eliben at google.com> wrote:
> >
> >
> >
> > On Thu, Jun 12, 2014 at 2:43 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> >>
> >> ----- Original Message -----
> >> > From: "Eli Bendersky" <eliben at google.com>
> >> > To: llvm-commits at cs.uiuc.edu
> >> > Sent: Wednesday, June 11, 2014 6:15:35 PM
> >> > Subject: [llvm] r210721 - Teach LoopUnrollPass to respect loop
> unrolling
> >> > hints        in metadata.
> >> >
> >> > Author: eliben
> >> > Date: Wed Jun 11 18:15:35 2014
> >> > New Revision: 210721
> >> >
> >> > URL: http://llvm.org/viewvc/llvm-project?rev=210721&view=rev
> >> > Log:
> >> > Teach LoopUnrollPass to respect loop unrolling hints in metadata.
> >> >
> >> > See http://reviews.llvm.org/D4090 for more details.
> >> >
> >> > The Clang change that produces this metadata was committed in r210667
> >> >
> >> > Patch by Mark Heffernan.
> >> >
> >> >
> >> > Added:
> >> >     llvm/trunk/test/Transforms/LoopUnroll/unroll-pragmas.ll
> >> > Modified:
> >> >     llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
> >> >
> >> > Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
> >> > URL:
> >> >
> >> >
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp?rev=210721&r1=210720&r2=210721&view=diff
> >> >
> >> >
> ==============================================================================
> >> > --- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp (original)
> >> > +++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp Wed Jun 11
> >> > 18:15:35 2014
> >> > @@ -20,6 +20,7 @@
> >> >  #include "llvm/IR/DataLayout.h"
> >> >  #include "llvm/IR/Dominators.h"
> >> >  #include "llvm/IR/IntrinsicInst.h"
> >> > +#include "llvm/IR/Metadata.h"
> >> >  #include "llvm/Support/CommandLine.h"
> >> >  #include "llvm/Support/Debug.h"
> >> >  #include "llvm/Support/raw_ostream.h"
> >> > @@ -36,7 +37,8 @@ UnrollThreshold("unroll-threshold", cl::
> >> >
> >> >  static cl::opt<unsigned>
> >> >  UnrollCount("unroll-count", cl::init(0), cl::Hidden,
> >> > -  cl::desc("Use this unroll count for all loops, for testing
> >> > purposes"));
> >> > +  cl::desc("Use this unroll count for all loops including those with
> >> > "
> >> > +           "unroll_count pragma values, for testing purposes"));
> >> >
> >> >  static cl::opt<bool>
> >> >  UnrollAllowPartial("unroll-allow-partial", cl::init(false),
> >> >  cl::Hidden,
> >> > @@ -47,6 +49,13 @@ static cl::opt<bool>
> >> >  UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false),
> >> >  cl::Hidden,
> >> >    cl::desc("Unroll loops with run-time trip counts"));
> >> >
> >> > +// Maximum allowed unroll count for a loop being fully unrolled
> >> > +// because of a pragma unroll(enable) statement (ie, metadata
> >> > +// "llvm.loopunroll.enable" is true).  This prevents unexpected
> >> > +// behavior like crashing when using this pragma on high trip count
> >> > +// loops.
> >> > +static const unsigned PragmaFullUnrollCountLimit = 1024;
> >>
> >> Please make this a cl::opt! Also, it needs to be a limit on the final
> loop
> >> size, not on the unroll count itself (see the PartialThreshold variable
> near
> >> the top of runOnLoop).
> >>
> >> > +
> >> >  namespace {
> >> >    class LoopUnroll : public LoopPass {
> >> >    public:
> >> > @@ -151,6 +160,63 @@ static unsigned ApproximateLoopSize(cons
> >> >    return LoopSize;
> >> >  }
> >> >
> >> > +// Returns the value associated with the given metadata node name
> >> > (for
> >> > +// example, "llvm.loopunroll.count").  If no such named metadata
> >> > node
> >> > +// exists, then nullptr is returned.
> >> > +static const ConstantInt *GetUnrollMetadataValue(const Loop *L,
> >> > +                                                 StringRef Name) {
> >> > +  MDNode *LoopID = L->getLoopID();
> >> > +  if (!LoopID) return nullptr;
> >> > +
> >> > +  // First operand should refer to the loop id itself.
> >> > +  assert(LoopID->getNumOperands() > 0 && "requires at least one
> >> > operand");
> >> > +  assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
> >> > +
> >> > +  for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
> >> > +    const MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
> >> > +    if (!MD) continue;
> >> > +
> >> > +    const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
> >> > +    if (!S) continue;
> >> > +
> >> > +    if (Name.equals(S->getString())) {
> >> > +      assert(MD->getNumOperands() == 2 &&
> >> > +             "Unroll hint metadata should have two operands.");
> >> > +      return cast<ConstantInt>(MD->getOperand(1));
> >> > +    }
> >> > +  }
> >> > +  return nullptr;
> >> > +}
> >> > +
> >> > +// Returns true if the loop has an unroll(enable) pragma.
> >> > +static bool HasUnrollEnablePragma(const Loop *L) {
> >> > +  const ConstantInt *EnableValue =
> >> > +      GetUnrollMetadataValue(L, "llvm.loopunroll.enable");
> >> > +  return (EnableValue && EnableValue->getZExtValue());
> >> > +  return false;
> >> > +}
> >> > +
> >> > +// Returns true if the loop has an unroll(disable) pragma.
> >> > +static bool HasUnrollDisablePragma(const Loop *L) {
> >> > +  const ConstantInt *EnableValue =
> >> > +      GetUnrollMetadataValue(L, "llvm.loopunroll.enable");
> >> > +  return (EnableValue && !EnableValue->getZExtValue());
> >> > +  return false;
> >> > +}
> >> > +
> >> > +// Check for unroll_count(N) pragma.  If found, return true and set
> >> > +// Count to the integer parameter of the pragma.
> >> > +static bool HasUnrollCountPragma(const Loop *L, int &Count) {
> >> > +  const ConstantInt *CountValue =
> >> > +      GetUnrollMetadataValue(L, "llvm.loopunroll.count");
> >> > +  if (CountValue) {
> >> > +    Count = CountValue->getZExtValue();
> >> > +    assert(Count >= 1 && "Unroll count must be positive.");
> >> > +    return true;
> >> > +  }
> >> > +  return false;
> >> > +}
> >> > +
> >> >  bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
> >> >    if (skipOptnoneFunction(L))
> >> >      return false;
> >> > @@ -202,12 +268,49 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPa
> >> >      TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
> >> >    }
> >> >
> >> > -  bool Runtime = UserRuntime ? CurrentRuntime : UP.Runtime;
> >> > +  // User-specified count (either as a command-line option or
> >> > +  // constructor parameter) has highest precedence.
> >> > +  unsigned Count = UserCount ? CurrentCount : 0;
> >> > +
> >> > +  // If there is no user-specified count, unroll pragmas have the
> >> > next
> >> > +  // highest precendence.
> >> > +  if (Count == 0) {
> >> > +    if (HasUnrollDisablePragma(L)) {
> >> > +      // Loop has unroll(disable) pragma.
> >> > +      return false;
> >> > +    }
> >> >
> >> > -  // Use a default unroll-count if the user doesn't specify a value
> >> > -  // and the trip count is a run-time value.  The default is
> >> > different
> >> > -  // for run-time or compile-time trip count loops.
> >> > -  unsigned Count = UserCount ? CurrentCount : UP.Count;
> >> > +    int PragmaCount;
> >> > +    if (HasUnrollCountPragma(L, PragmaCount)) {
> >> > +      if (PragmaCount == 1) {
> >> > +        // Nothing to do.
> >> > +        return false;
> >> > +      }
> >> > +      Count = PragmaCount;
> >> > +      Threshold = NoThreshold;
> >> > +    } else if (HasUnrollEnablePragma(L)) {
> >> > +      // Loop has unroll(enable) pragma without a unroll_count
> >> > pragma,
> >> > +      // so unroll loop fully if possible.
> >> > +      if (TripCount == 0) {
> >> > +        DEBUG(dbgs() << "  Loop has unroll(enable) pragma but loop
> >> > cannot be "
> >> > +                        "fully unrolled because trip count is
> >> > unknown.\n");
> >>
> >> This should not say "fully".
> >>
> >> > +        // Continue with standard heuristic unrolling.
> >> > +      } else if (TripCount > PragmaFullUnrollCountLimit) {
> >> > +        DEBUG(dbgs() << "  Loop has unroll(enable) pragma but loop
> >> > cannot be "
> >> > +                        "fully unrolled because loop count is
> >> > greater than "
> >> > +                     << PragmaFullUnrollCountLimit);
> >> > +        // Continue with standard heuristic unrolling.
> >>
> >>
> >> This should also not say "fully". In addition, this is misleading
> because
> >> we might unroll this anyway (just not to the extent requested by the
> user.
> >>
> >> Also, I think that we should be using emitOptimizationRemark here (like
> >> LoopVectorize does) instead of debug messages.
> >>
> >> Thanks again,
> >> Hal
> >>
> >
> > Thanks Hal. We'll address the comments once the patch is revised and gets
> > back in.
> >
>
> Well, since it's out you might as well address them before it goes back in.


That's what I meant, of course :) Sorry if I wasn't clear.

Eli
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