[GlobalISel] Reduce KnownBits usage in matcher combines (#92381)

Two icmp/and combines forced computation of KnownBits on all operands
everytime. We can avoid computing KnownBits on the LHS by exploiting a
couple of properties:
- Constants are always on the RHS for those instructions. If we have no
KnownBits on the RHS, we can bail out early and avoid computing LHS
knownbits.
- For icmp uge/ult 0, we don't need to know the KBs of the LHS to infer
the result

This allows to save some KnownBits calls, which are very expensive,
without affecting codegen.

Author: Pierre-vh

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -3220,8 +3220,15 @@ bool CombinerHelper::matchRedundantAnd(MachineInstr &MI,
   Register AndDst = MI.getOperand(0).getReg();
   Register LHS = MI.getOperand(1).getReg();
   Register RHS = MI.getOperand(2).getReg();
-  KnownBits LHSBits = KB->getKnownBits(LHS);
+
+  // Check the RHS (maybe a constant) first, and if we have no KnownBits there,
+  // we can't do anything. If we do, then it depends on whether we have
+  // KnownBits on the LHS.
   KnownBits RHSBits = KB->getKnownBits(RHS);
+  if (RHSBits.isUnknown())
+    return false;
+
+  KnownBits LHSBits = KB->getKnownBits(LHS);
 
   // Check that x & Mask == x.
   // x & 1 == x, always
@@ -3260,6 +3267,7 @@ bool CombinerHelper::matchRedundantOr(MachineInstr &MI, Register &Replacement) {
   Register OrDst = MI.getOperand(0).getReg();
   Register LHS = MI.getOperand(1).getReg();
   Register RHS = MI.getOperand(2).getReg();
+
   KnownBits LHSBits = KB->getKnownBits(LHS);
   KnownBits RHSBits = KB->getKnownBits(RHS);
 
@@ -4253,43 +4261,67 @@ bool CombinerHelper::matchICmpToTrueFalseKnownBits(MachineInstr &MI,
                                                    int64_t &MatchInfo) {
   assert(MI.getOpcode() == TargetOpcode::G_ICMP);
   auto Pred = static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate());
-  auto KnownLHS = KB->getKnownBits(MI.getOperand(2).getReg());
+
+  // We want to avoid calling KnownBits on the LHS if possible, as this combine
+  // has no filter and runs on every G_ICMP instruction. We can avoid calling
+  // KnownBits on the LHS in two cases:
+  //
+  //  - The RHS is unknown: Constants are always on RHS. If the RHS is unknown
+  //  we cannot do any transforms so we can safely bail out early.
+  //  - The RHS is zero: we don't need to know the LHS to do unsigned <0 and
+  //  >=0.
   auto KnownRHS = KB->getKnownBits(MI.getOperand(3).getReg());
+  if (KnownRHS.isUnknown())
+    return false;
+
   std::optional<bool> KnownVal;
-  switch (Pred) {
-  default:
-    llvm_unreachable("Unexpected G_ICMP predicate?");
-  case CmpInst::ICMP_EQ:
-    KnownVal = KnownBits::eq(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_NE:
-    KnownVal = KnownBits::ne(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_SGE:
-    KnownVal = KnownBits::sge(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_SGT:
-    KnownVal = KnownBits::sgt(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_SLE:
-    KnownVal = KnownBits::sle(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_SLT:
-    KnownVal = KnownBits::slt(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_UGE:
-    KnownVal = KnownBits::uge(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_UGT:
-    KnownVal = KnownBits::ugt(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_ULE:
-    KnownVal = KnownBits::ule(KnownLHS, KnownRHS);
-    break;
-  case CmpInst::ICMP_ULT:
-    KnownVal = KnownBits::ult(KnownLHS, KnownRHS);
-    break;
+  if (KnownRHS.isZero()) {
+    // ? uge 0 -> always true
+    // ? ult 0 -> always false
+    if (Pred == CmpInst::ICMP_UGE)
+      KnownVal = true;
+    else if (Pred == CmpInst::ICMP_ULT)
+      KnownVal = false;
   }
+
+  if (!KnownVal) {
+    auto KnownLHS = KB->getKnownBits(MI.getOperand(2).getReg());
+    switch (Pred) {
+    default:
+      llvm_unreachable("Unexpected G_ICMP predicate?");
+    case CmpInst::ICMP_EQ:
+      KnownVal = KnownBits::eq(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_NE:
+      KnownVal = KnownBits::ne(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_SGE:
+      KnownVal = KnownBits::sge(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_SGT:
+      KnownVal = KnownBits::sgt(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_SLE:
+      KnownVal = KnownBits::sle(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_SLT:
+      KnownVal = KnownBits::slt(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_UGE:
+      KnownVal = KnownBits::uge(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_UGT:
+      KnownVal = KnownBits::ugt(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_ULE:
+      KnownVal = KnownBits::ule(KnownLHS, KnownRHS);
+      break;
+    case CmpInst::ICMP_ULT:
+      KnownVal = KnownBits::ult(KnownLHS, KnownRHS);
+      break;
+    }
+  }
+
   if (!KnownVal)
     return false;
   MatchInfo =