[llvm] [GISel] Fold shifts to constant result. (PR #123510)

[Matt Arsenault via llvm-commits]

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@@ -6590,12 +6590,57 @@ bool CombinerHelper::matchRedundantBinOpInEquality(MachineInstr &MI,
   return CmpInst::isEquality(Pred) && Y.isValid();
 }
 
-bool CombinerHelper::matchShiftsTooBig(MachineInstr &MI) const {
+/// Return the minimum useless shift amount that results in complete loss of the
+/// source value. Return std::nullopt when it cannot determine a value.
+static std::optional<unsigned>
+getMinUselessShift(KnownBits ValueKB, unsigned Opcode,
+                   std::optional<int64_t> &Result) {
+  assert(Opcode == TargetOpcode::G_SHL || Opcode == TargetOpcode::G_LSHR ||
+         Opcode == TargetOpcode::G_ASHR && "Expect G_SHL, G_LSHR or G_ASHR.");
+  auto SignificantBits = 0;
+  switch (Opcode) {
+  case TargetOpcode::G_SHL:
+    SignificantBits = ValueKB.countMinTrailingZeros();
+    Result = 0;
+    break;
+  case TargetOpcode::G_LSHR:
+    Result = 0;
+    SignificantBits = ValueKB.countMinLeadingZeros();
+    break;
+  case TargetOpcode::G_ASHR:
+    if (ValueKB.isNonNegative()) {
+      SignificantBits = ValueKB.countMinLeadingZeros();
+      Result = 0;
+    } else if (ValueKB.isNegative()) {
+      SignificantBits = ValueKB.countMinLeadingOnes();
+      Result = -1;
+    } else {
+      // Cannot determine shift result.
+      Result = std::nullopt;
+    }
+    break;
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arsenm wrote:

This should be using computeKnownSignBits instead of computeKnownBits, it can be slightly smarter 

https://github.com/llvm/llvm-project/pull/123510