[llvm] r210721 - Teach LoopUnrollPass to respect loop unrolling hints in metadata.
Eli Bendersky
eliben at google.com
Thu Jun 12 11:15:20 PDT 2014
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.
Eli
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