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

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


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+//===- 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/IR/Instructions.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 = ICmpInst::compare(LHS, RHS, Pred);
+
+  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;
+  }
+
+  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;
+  }
----------------
aemerson wrote:

By unrelated I mean that there's no need for it to be tied to the combine above and can be split out. You can also then name the combines more descriptively instead of visitICmp. I'm not saying that these are wrong to do, it's a matter of organization.

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