[llvm] [GlobalISel] Use named constant for impossible repair cost (PR #180490)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-globalisel

Author: Eliz Habiboullah (elizhabiboullah)

<details>
<summary>Changes</summary>

replace magic value `std::numeric_limits<unsigned>::max()` with a named constant `ImpossibleRepairCost` to improve readability

---
Full diff: https://github.com/llvm/llvm-project/pull/180490.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp (+9-4) 


``````````diff
diff --git a/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp b/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
index 5db631be32acd..4dfd5179a4e56 100644
--- a/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/RegBankSelect.cpp
@@ -51,6 +51,12 @@
 
 using namespace llvm;
 
+/// Cost value representing an impossible or invalid repairing.
+/// This matches the value returned by RegisterBankInfo::copyCost() and
+/// RegisterBankInfo::getBreakDownCost() when the cost cannot be computed.
+static constexpr unsigned ImpossibleRepairCost =
+    std::numeric_limits<unsigned>::max();
+
 static cl::opt<RegBankSelect::Mode> RegBankSelectMode(
     cl::desc("Mode of the RegBankSelect pass"), cl::Hidden, cl::Optional,
     cl::values(clEnumValN(RegBankSelect::Mode::Fast, "regbankselect-fast",
@@ -278,12 +284,11 @@ uint64_t RegBankSelect::getRepairCost(
     // repairing placement.
     unsigned Cost = RBI->copyCost(*DesiredRegBank, *CurRegBank,
                                   RBI->getSizeInBits(MO.getReg(), *MRI, *TRI));
-    // TODO: use a dedicated constant for ImpossibleCost.
-    if (Cost != std::numeric_limits<unsigned>::max())
+    if (Cost != ImpossibleRepairCost)
       return Cost;
     // Return the legalization cost of that repairing.
   }
-  return std::numeric_limits<unsigned>::max();
+  return ImpossibleRepairCost;
 }
 
 const RegisterBankInfo::InstructionMapping &RegBankSelect::findBestMapping(
@@ -535,7 +540,7 @@ RegBankSelect::MappingCost RegBankSelect::computeMapping(
     uint64_t RepairCost = getRepairCost(MO, ValMapping);
 
     // This is an impossible to repair cost.
-    if (RepairCost == std::numeric_limits<unsigned>::max())
+    if (RepairCost == ImpossibleRepairCost)
       return MappingCost::ImpossibleCost();
 
     // Bias used for splitting: 5%.

``````````

</details>


https://github.com/llvm/llvm-project/pull/180490