Author: Michael Berg
Date: 2021-12-07T14:31:40-08:00
New Revision: ea88dfda6f90b33f1e1bd7231bf249ac3b73a6f8
URL:
https://github.com/llvm/llvm-project/commit/ea88dfda6f90b33f1e1bd7231bf249ac3b73a6f8
DIFF:
https://github.com/llvm/llvm-project/commit/ea88dfda6f90b33f1e1bd7231bf249ac3b73a6f8.diff
LOG: [RISCV] Add target specific loop unrolling and peeling preferences
Both these preference helper functions have initial support with
this change. The loop unrolling preferences are set with initial
settings to control thresholds, size and attributes of loops to
unroll with some tuning done. The peeling preferences may need
some tuning as well as the initial support looks much like what
other architectures utilize.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D113798
Added:
llvm/test/Transforms/LoopUnroll/RISCV/unroll.ll
Modified:
llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
Removed:
diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
index 56f0952fafc9b..54458d1ce6d56 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
@@ -162,3 +162,82 @@ InstructionCost RISCVTTIImpl::getGatherScatterOpCost(
getMemoryOpCost(Opcode, VTy->getElementType(), Alignment, 0, CostKind,
I);
return NumLoads * MemOpCost;
}
+
+void RISCVTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
+ TTI::UnrollingPreferences &UP,
+ OptimizationRemarkEmitter *ORE) {
+ // TODO: More tuning on benchmarks and metrics with changes as needed
+ // would apply to all settings below to enable performance.
+
+ // Enable Upper bound unrolling universally, not dependant upon the
conditions
+ // below.
+ UP.UpperBound = true;
+
+ // Disable loop unrolling for Oz and Os.
+ UP.OptSizeThreshold = 0;
+ UP.PartialOptSizeThreshold = 0;
+ if (L->getHeader()->getParent()->hasOptSize())
+return;
+
+ SmallVector ExitingBlocks;
+ L->getExitingBlocks(ExitingBlocks);
+ LLVM_DEBUG(dbgs() << "Loop has:\n"
+<< "Blocks: " << L->getNumBlocks() << "\n"
+<< "Exit blocks: " << ExitingBlocks.size() << "\n");
+
+ // Only allow another exit other than the latch. This acts as an early exit
+ // as it mirrors the profitability calculation of the runtime unroller.
+ if (ExitingBlocks.size() > 2)
+return;
+
+ // Limit the CFG of the loop body for targets with a branch predictor.
+ // Allowing 4 blocks permits if-then-else diamonds in the body.
+ if (L->getNumBlocks() > 4)
+return;
+
+ // Don't unroll vectorized loops, including the remainder loop
+ if (getBooleanLoopAttribute(L, "llvm.loop.isvectorized"))
+return;
+
+ // Scan the loop: don't unroll loops with calls as this could prevent
+ // inlining.
+ InstructionCost Cost = 0;
+ for (auto *BB : L->getBlocks()) {
+for (auto &I : *BB) {
+ // Initial setting - Don't unroll loops containing vectorized
+ // instructions.
+ if (I.getType()->isVectorTy())
+return;
+
+ if (isa(I) || isa(I)) {
+if (const Function *F = cast(I).getCalledFunction()) {
+ if (!isLoweredToCall(F))
+continue;
+}
+return;
+ }
+
+ SmallVector Operands(I.operand_values());
+ Cost +=
+ getUserCost(&I, Operands, TargetTransformInfo::TCK_SizeAndLatency);
+}
+ }
+
+ LLVM_DEBUG(dbgs() << "Cost of loop: " << Cost << "\n");
+
+ UP.Partial = true;
+ UP.Runtime = true;
+ UP.UnrollRemainder = true;
+ UP.UnrollAndJam = true;
+ UP.UnrollAndJamInnerLoopThreshold = 60;
+
+ // Force unrolling small loops can be very useful because of the branch
+ // taken cost of the backedge.
+ if (Cost < 12)
+UP.Force = true;
+}
+
+void RISCVTTIImpl::getPeelingPreferences(Loop *L, ScalarEvolution &SE,
+ TTI::PeelingPreferences &PP) {
+ BaseT::getPeelingPreferences(L, SE, PP);
+}
diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
index 675681616d6e4..016db9d4c26dc 100644
--- a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h
@@ -73,6 +73,13 @@ class RISCVTTIImpl : public BasicTTIImplBase {
llvm_unreachable("Unsupported register kind");
}
+ void getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
+ TTI::UnrollingPreferences &UP,
+ OptimizationRemarkEmitter *ORE);
+
+ void getPeelingPreferences(Loop *L, ScalarEvolution &SE,
+ TTI::PeelingPreferences &PP);
+
unsigned getMinVectorRegisterBitWidth() const {
r
