[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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