[GlobalISel] Reduce KnownBits usage in matcher combines

[Pierre-vh]

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.

[llvmbot]

@llvm/pr-subscribers-llvm-globalisel

Author: Pierre van Houtryve (Pierre-vh)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/92381.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp (+67-35)

diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index 9999776b98260..809f13abcadd1 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/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 =