[CodeGen] Allow negative frame indicies in Register class.

llvm/include/llvm/CodeGen/Register.h

@@ -10,6 +10,7 @@
 #define LLVM_CODEGEN_REGISTER_H
 
 #include "llvm/MC/MCRegister.h"
+#include "llvm/Support/MathExtras.h"
 #include <cassert>
 
 namespace llvm {
@@ -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);
+    assert(isInt<MaxFrameIndexBitwidth>(FI) &&
+           "Frame index must be at most 30 bits.");
+    unsigned FIMasked = FI & Register::StackSlotMask;
+    return Register(FIMasked | Register::StackSlotZero);
   }
 
   /// Return true if the specified register number is in
@@ -87,7 +92,7 @@ class Register {
   /// Compute the frame index from a register value representing a stack slot.
   int stackSlotIndex() const {
     assert(isStack() && "Not a stack slot");
-    return static_cast<int>(Reg - Register::FirstStackSlot);
+    return SignExtend32<MaxFrameIndexBitwidth>(Reg & Register::StackSlotMask);
   }
 
   constexpr operator unsigned() const { return Reg; }

llvm/lib/CodeGen/ReachingDefAnalysis.cpp

@@ -193,7 +193,6 @@ void ReachingDefInfo::processDefs(MachineInstr *MI) {
   for (auto &MO : MI->operands()) {
     if (MO.isFI()) {
       int FrameIndex = MO.getIndex();
-      assert(FrameIndex >= 0 && "Can't handle negative frame indicies yet!");
       if (!isFIDef(*MI, FrameIndex, TII))
         continue;
       MBBFrameObjsReachingDefs[{MBBNumber, FrameIndex}].push_back(CurInstr);
@@ -302,8 +301,6 @@ void ReachingDefInfo::print(raw_ostream &OS) {
         Register Reg;
         if (MO.isFI()) {
           int FrameIndex = MO.getIndex();
-          assert(FrameIndex >= 0 &&
-                 "Can't handle negative frame indicies yet!");
           Reg = Register::index2StackSlot(FrameIndex);
         } else if (MO.isReg()) {
           if (MO.isDef())

llvm/unittests/CodeGen/CMakeLists.txt

@@ -39,6 +39,7 @@ add_llvm_unittest(CodeGenTests
   MachineOperandTest.cpp
   MIR2VecTest.cpp
   RegAllocScoreTest.cpp
+  RegisterTest.cpp
   PassManagerTest.cpp
   ScalableVectorMVTsTest.cpp
   SchedBoundary.cpp

llvm/unittests/CodeGen/RegisterTest.cpp

@@ -0,0 +1,36 @@
+//===- RegisterTest.cpp -----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Register.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+TEST(RegisterTest, Idx2StackSlot) {
+  EXPECT_EQ(Register::index2StackSlot(0), Register::StackSlotZero);
+  EXPECT_EQ(Register::index2StackSlot(1), Register::StackSlotZero | 1);
+  EXPECT_EQ(Register::index2StackSlot(-1),
+            Register::StackSlotZero | Register::StackSlotMask);
+  int MaxPowOf2 = 1 << (Register::MaxFrameIndexBitwidth - 1);
+  // check that we do not crash on the highest possible value of frame index.
+  EXPECT_NO_FATAL_FAILURE(Register::index2StackSlot(MaxPowOf2 - 1));
+  // check that we do not crash on the lowest possible value of frame index.
+  EXPECT_NO_FATAL_FAILURE(Register::index2StackSlot(-MaxPowOf2));
+}
+
+TEST(RegisterTest, StackSlotIndex) {
+  int MaxPowOf2 = 1 << (Register::MaxFrameIndexBitwidth - 1);
+  std::vector<int> FIs = {0, 1 - 1, MaxPowOf2 - 1, -MaxPowOf2};
+
+  for (int FI : FIs) {
+    Register Reg = Register::index2StackSlot(FI);
+    EXPECT_EQ(Reg.stackSlotIndex(), FI);
+  }
+}
+} // end namespace