[llvm] [CodeGen] Allow negative frame indicies in Register class. (PR #164459)

[Mikhail Gudim via llvm-commits]

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@@ -35,19 +36,23 @@ class Register {
   // DenseMapInfo<unsigned> uses -1u and -2u.
   static_assert(std::numeric_limits<decltype(Reg)>::max() >= 0xFFFFFFFF,
                 "Reg isn't large enough to hold full range.");
-  static constexpr unsigned FirstStackSlot = 1u << 30;
-  static_assert(FirstStackSlot >= MCRegister::LastPhysicalReg);
+  static constexpr unsigned MaxFrameIndexBitwidth = 30;
+  static constexpr unsigned StackSlotZero = 1u << MaxFrameIndexBitwidth;
+  static constexpr const unsigned StackSlotMask = StackSlotZero - 1;
+  static_assert(StackSlotZero >= MCRegister::LastPhysicalReg);
   static constexpr unsigned VirtualRegFlag = 1u << 31;
 
   /// Return true if this is a stack slot.
   constexpr bool isStack() const {
-    return Register::FirstStackSlot <= Reg && Reg < Register::VirtualRegFlag;
+    return Register::StackSlotZero <= Reg && Reg < Register::VirtualRegFlag;
   }
 
   /// Convert a non-negative frame index to a stack slot register value.
   static Register index2StackSlot(int FI) {
-    assert(FI >= 0 && "Cannot hold a negative frame index.");
-    return Register(FI + Register::FirstStackSlot);
+    unsigned FIMasked = FI & Register::StackSlotMask;
+    assert(isUInt<MaxFrameIndexBitwidth>(FIMasked) &&
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mgudim wrote:

Oh right, thanks!

https://github.com/llvm/llvm-project/pull/164459