[RISCV] Add RISCV-specific TargetTransformInfo

Summary:
LLVM Allows Targets to provide information that guides optimisations
made to LLVM IR. This is done with callbacks on a TargetTransformInfo object.

This patch adds a TargetTransformInfo class for RISC-V. This will allow us to
implement RISC-V specific callbacks as they become necessary.

This commit also adds the getIntImmCost callbacks, and tests them with a simple
constant hoisting test. Our immediate costs are on the conservative side, for
the moment, but we prevent hoisting in most circumstances anyway.

Previous review was on D63007

Reviewers: asb, luismarques

Reviewed By: asb

Subscribers: ributzka, MaskRay, llvm-commits, Jim, benna, psnobl, jocewei, PkmX, rkruppe, the_o, brucehoult, MartinMosbeck, rogfer01, edward-jones, zzheng, jrtc27, shiva0217, kito-cheng, niosHD, sabuasal, apazos, simoncook, johnrusso, rbar, hiraditya, mgorny

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63433

llvm-svn: 364046

1 files changed, 90 insertions, 0 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
new file mode 100644
index 00000000000..41a2b5c1f20
--- /dev/null
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
@@ -0,0 +1,90 @@
+//===-- RISCVTargetTransformInfo.cpp - RISC-V specific TTI ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "RISCVTargetTransformInfo.h"
+#include "Utils/RISCVMatInt.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/CodeGen/BasicTTIImpl.h"
+#include "llvm/CodeGen/TargetLowering.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "riscvtti"
+
+int RISCVTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty) {
+  assert(Ty->isIntegerTy() &&
+         "getIntImmCost can only estimate cost of materialising integers");
+
+  // We have a Zero register, so 0 is always free.
+  if (Imm == 0)
+    return TTI::TCC_Free;
+
+  // Otherwise, we check how many instructions it will take to materialise.
+  const DataLayout &DL = getDataLayout();
+  return RISCVMatInt::getIntMatCost(Imm, DL.getTypeSizeInBits(Ty),
+                                    getST()->is64Bit());
+}
+
+int RISCVTTIImpl::getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm,
+                                Type *Ty) {
+  assert(Ty->isIntegerTy() &&
+         "getIntImmCost can only estimate cost of materialising integers");
+
+  // We have a Zero register, so 0 is always free.
+  if (Imm == 0)
+    return TTI::TCC_Free;
+
+  // Some instructions in RISC-V can take a 12-bit immediate. Some of these are
+  // commutative, in others the immediate comes from a specific argument index.
+  bool Takes12BitImm = false;
+  unsigned ImmArgIdx = ~0U;
+
+  switch (Opcode) {
+  case Instruction::GetElementPtr:
+    // Never hoist any arguments to a GetElementPtr. CodeGenPrepare will
+    // split up large offsets in GEP into better parts than ConstantHoisting
+    // can.
+    return TTI::TCC_Free;
+  case Instruction::Add:
+  case Instruction::And:
+  case Instruction::Or:
+  case Instruction::Xor:
+  case Instruction::Mul:
+    Takes12BitImm = true;
+    break;
+  case Instruction::Sub:
+  case Instruction::Shl:
+  case Instruction::LShr:
+  case Instruction::AShr:
+    Takes12BitImm = true;
+    ImmArgIdx = 1;
+    break;
+  default:
+    break;
+  }
+
+  if (Takes12BitImm) {
+    // Check immediate is the correct argument...
+    if (Instruction::isCommutative(Opcode) || Idx == ImmArgIdx) {
+      // ... and fits into the 12-bit immediate.
+      if (getTLI()->isLegalAddImmediate(Imm.getSExtValue()))
+        return TTI::TCC_Free;
+    }
+
+    // Otherwise, use the full materialisation cost.
+    return getIntImmCost(Imm, Ty);
+  }
+
+  // By default, prevent hoisting.
+  return TTI::TCC_Free;
+}
+
+int RISCVTTIImpl::getIntImmCost(Intrinsic::ID IID, unsigned Idx,
+                                const APInt &Imm, Type *Ty) {
+  // Prevent hoisting in unknown cases.
+  return TTI::TCC_Free;
+}