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

[Thorsten Schütt via llvm-commits]

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@@ -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);
----------------
tschuett wrote:

Yes:
https://github.com/llvm/llvm-project/blob/52ae891036e3ab1f668eb103c46ca57257901c6b/llvm/include/llvm/Target/GlobalISel/Combine.td#L481

Otherwise:
```
def icmp_of_zero : GICombineRule<
  (defs root:$root, build_fn_matchinfo:$matchinfo),
  (match (G_CONSTANT $zero, 0),
         (G_ICMP $root, $pred, $lhs, $zero):$cmp,
         [{ return Helper.matchCmpOfZero(*${cmp}, ${matchinfo}); }]),
  (apply [{ Helper.applyBuildFn(*${cmp}, ${matchinfo}); }])>;
```
would hit less often, see my first comment above.

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