[llvm] [GlobalIsel] Visit ICmp (PR #105991)

[Pierre van Houtryve via llvm-commits]

Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>,
Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>,
Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>,
Thorsten =?utf-8?q?Schütt?= <schuett at gmail.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/105991 at github.com>


================
@@ -0,0 +1,292 @@
+//===- CombinerHelperCompares.cpp------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements CombinerHelper for G_ICMP
+//
+//===----------------------------------------------------------------------===//
+#include "llvm/CodeGen/GlobalISel/CombinerHelper.h"
+#include "llvm/CodeGen/GlobalISel/GenericMachineInstrs.h"
+#include "llvm/CodeGen/GlobalISel/LegalizerHelper.h"
+#include "llvm/CodeGen/GlobalISel/LegalizerInfo.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
+#include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/CodeGen/LowLevelTypeUtils.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetOpcodes.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/ErrorHandling.h"
+#include <cstdlib>
+
+#define DEBUG_TYPE "gi-combiner"
+
+using namespace llvm;
+
+bool CombinerHelper::constantFoldICmp(const GICmp &ICmp,
+                                      const GIConstant &LHSCst,
+                                      const GIConstant &RHSCst,
+                                      BuildFnTy &MatchInfo) {
+  if (LHSCst.getKind() != GIConstantKind::Scalar)
+    return false;
+
+  Register Dst = ICmp.getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+
+  if (!isConstantLegalOrBeforeLegalizer(DstTy))
+    return false;
+
+  CmpInst::Predicate Pred = ICmp.getCond();
+  APInt LHS = LHSCst.getScalarValue();
+  APInt RHS = RHSCst.getScalarValue();
+
+  bool Result;
+
+  switch (Pred) {
+  case CmpInst::Predicate::ICMP_EQ:
+    Result = LHS.eq(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_NE:
+    Result = LHS.ne(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_UGT:
+    Result = LHS.ugt(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_UGE:
+    Result = LHS.uge(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_ULT:
+    Result = LHS.ult(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_ULE:
+    Result = LHS.ule(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_SGT:
+    Result = LHS.sgt(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_SGE:
+    Result = LHS.sge(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_SLT:
+    Result = LHS.slt(RHS);
+    break;
+  case CmpInst::Predicate::ICMP_SLE:
+    Result = LHS.sle(RHS);
+    break;
+  default:
+    llvm_unreachable("Unexpected predicate");
+  }
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    if (Result)
+      B.buildConstant(Dst, getICmpTrueVal(getTargetLowering(),
+                                          /*IsVector=*/DstTy.isVector(),
+                                          /*IsFP=*/false));
+    else
+      B.buildConstant(Dst, 0);
+  };
+
+  return true;
+}
+
+bool CombinerHelper::visitICmp(const MachineInstr &MI, BuildFnTy &MatchInfo) {
+  const GICmp *Cmp = cast<GICmp>(&MI);
+
+  Register Dst = Cmp->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+  Register LHS = Cmp->getLHSReg();
+  Register RHS = Cmp->getRHSReg();
+
+  CmpInst::Predicate Pred = Cmp->getCond();
+  assert(CmpInst::isIntPredicate(Pred) && "Not an integer compare!");
+  if (auto CLHS = GIConstant::getConstant(LHS, MRI)) {
+    if (auto CRHS = GIConstant::getConstant(RHS, MRI))
+      return constantFoldICmp(*Cmp, *CLHS, *CRHS, MatchInfo);
+
+    // If we have a constant, make sure it is on the RHS.
+    std::swap(LHS, RHS);
+    Pred = CmpInst::getSwappedPredicate(Pred);
+
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildICmp(Pred, Dst, LHS, RHS); };
+    return true;
+  }
+
+  [[maybe_unused]] MachineInstr *MILHS = MRI.getVRegDef(LHS);
+  MachineInstr *MIRHS = MRI.getVRegDef(RHS);
+
+  // For EQ and NE, we can always pick a value for the undef to make the
+  // predicate pass or fail, so we can return undef.
+  // Matches behavior in llvm::ConstantFoldCompareInstruction.
+  if (isa<GImplicitDef>(MIRHS) && ICmpInst::isEquality(Pred) &&
+      isLegalOrBeforeLegalizer({TargetOpcode::G_IMPLICIT_DEF, {DstTy}})) {
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildUndef(Dst); };
+    return true;
+  }
+
+  // icmp X, X -> true/false
+  // icmp X, undef -> true/false because undef could be X.
+  if ((LHS == RHS || isa<GImplicitDef>(MIRHS)) &&
+      isConstantLegalOrBeforeLegalizer(DstTy)) {
+    MatchInfo = [=](MachineIRBuilder &B) {
+      if (CmpInst::isTrueWhenEqual(Pred))
+        B.buildConstant(Dst, getICmpTrueVal(getTargetLowering(),
+                                            /*IsVector=*/DstTy.isVector(),
+                                            /*IsFP=*/false));
+      else
+        B.buildConstant(Dst, 0);
+    };
+    return true;
+  }
+
+  return false;
+}
+
+bool CombinerHelper::matchSextOfICmp(const MachineInstr &MI,
+                                     BuildFnTy &MatchInfo) {
+  const GICmp *Cmp = cast<GICmp>(&MI);
+
+  Register Dst = Cmp->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+  Register LHS = Cmp->getLHSReg();
+  Register RHS = Cmp->getRHSReg();
+  CmpInst::Predicate Pred = Cmp->getCond();
+
+  GSext *SL = cast<GSext>(MRI.getVRegDef(LHS));
+  GSext *SR = cast<GSext>(MRI.getVRegDef(RHS));
+
+  LLT SLTy = MRI.getType(SL->getSrcReg());
+  LLT SRTy = MRI.getType(SR->getSrcReg());
+
+  // Turn icmp (sext X), (sext Y) into a compare of X and Y if they have the
+  // same type.
+  if (SLTy != SRTy)
+    return false;
+
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_ICMP, {DstTy, SLTy}}))
+    return false;
+
+  // Compare X and Y. Note that the predicate does not change.
+  MatchInfo = [=](MachineIRBuilder &B) {
+    B.buildICmp(Pred, Dst, SL->getSrcReg(), SR->getSrcReg());
+  };
+  return true;
+}
+
+bool CombinerHelper::matchZextOfICmp(const MachineInstr &MI,
+                                     BuildFnTy &MatchInfo) {
+  const GICmp *Cmp = cast<GICmp>(&MI);
+
+  Register Dst = Cmp->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+  Register LHS = Cmp->getLHSReg();
+  Register RHS = Cmp->getRHSReg();
+  CmpInst::Predicate Pred = Cmp->getCond();
+
+  GZext *ZL = cast<GZext>(MRI.getVRegDef(LHS));
+  GZext *ZR = cast<GZext>(MRI.getVRegDef(RHS));
+
+  LLT ZLTy = MRI.getType(ZL->getSrcReg());
+  LLT ZRTy = MRI.getType(ZR->getSrcReg());
+
+  // Turn icmp (zext X), (zext Y) into a compare of X and Y if they have
+  // the same type.
+  if (ZLTy != ZRTy)
+    return false;
+
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_ICMP, {DstTy, ZLTy}}))
+    return false;
+
+  // Compare X and Y. Note that signed predicates become unsigned.
+  MatchInfo = [=](MachineIRBuilder &B) {
+    B.buildICmp(ICmpInst::getUnsignedPredicate(Pred), Dst, ZL->getSrcReg(),
+                ZR->getSrcReg());
+  };
+  return true;
+}
----------------
Pierre-vh wrote:

> I don't like to have two MIR patterns going to the same helper function.

This happens often though, and IMO it's completely fine.  I also recommend to template this


https://github.com/llvm/llvm-project/pull/105991