[Support][SelectionDAG][GlobalISel] Hoist PostShift adjustment for IsAdd into UnsignedDivideUsingMagic.

[craig.topper]

Summary

Instead of doing the adjustment in 3 different places in the code
base, do it inside UnsignedDivideUsingMagic::get.

[lebedev.ri]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

4965–4973

Probably want similar assertion for PreShift?

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

6059–6068

Probably want similar assertion for PreShift?

6063

The name PostShift be propagated into UnsignedDivisionByConstantInfo struct?

[craig.topper]

Address review comments

[lebedev.ri]

LG

llvm/include/llvm/Support/DivisionByConstantInfo.h

 
 /// Magic data for optimising unsigned division by a constant.
 struct UnsignedDivisionByConstantInfo {
   static UnsignedDivisionByConstantInfo
   get(const APInt &D, unsigned LeadingZeros = 0,
       bool AllowEvenDivisorOptimization = true);
   APInt Magic;          ///< magic number
   bool IsAdd;           ///< add indicator
-  unsigned ShiftAmount; ///< shift amount
+  unsigned PostShift;   ///< post-shift amount
   unsigned PreShift;    ///< pre-shift amount
 };
 
 } // namespace llvm
 
 #endif

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

auto BuildUDIVPattern = [&](const Constant *C) {
     // at the end.
     // TODO: Use undef values for divisor of 1.
     if (!Divisor.isOneValue()) {
       UnsignedDivisionByConstantInfo magics =
           UnsignedDivisionByConstantInfo::get(Divisor);
 
       Magic = std::move(magics.Magic);
 
-      if (!magics.IsAdd) {
-        assert(magics.ShiftAmount < Divisor.getBitWidth() &&
-               "We shouldn't generate an undefined shift!");
-        PostShift = magics.ShiftAmount;
-        PreShift = magics.PreShift;
-      } else {
-        assert(magics.PreShift == 0 && "Unexpected pre-shift");
-        PostShift = magics.ShiftAmount - 1;
-        SelNPQ = true;
-      }
+      assert(magics.PreShift < Divisor.getBitWidth() &&
+      assert(magics.PostShift < Divisor.getBitWidth() &&
+             "We shouldn't generate an undefined shift!");
+      assert((!magics.IsAdd || magics.PreShift == 0) && "Unexpected pre-shift");
+      PostShift = magics.PostShift;
+      SelNPQ = magics.IsAdd;
     }
 
     PreShifts.push_back(
         MIB.buildConstant(ScalarShiftAmtTy, PreShift).getReg(0));
     MagicFactors.push_back(MIB.buildConstant(ScalarTy, Magic).getReg(0));
     NPQFactors.push_back(
         MIB.buildConstant(ScalarTy,
                           SelNPQ ? APInt::getOneBitSet(EltBits, EltBits - 1)
                                  : APInt::getZero(EltBits))

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

auto BuildUDIVPattern = [&](ConstantSDNode *C) {
     // at the end.
     // TODO: Use undef values for divisor of 1.
     if (!Divisor.isOne()) {
       UnsignedDivisionByConstantInfo magics =
           UnsignedDivisionByConstantInfo::get(Divisor, LeadingZeros);
 
       Magic = std::move(magics.Magic);
 
-      if (!magics.IsAdd) {
-        assert(magics.ShiftAmount < Divisor.getBitWidth() &&
-               "We shouldn't generate an undefined shift!");
-        PostShift = magics.ShiftAmount;
-        PreShift = magics.PreShift;
-      } else {
-        assert(magics.PreShift == 0 && "Unexpected pre-shift");
-        PostShift = magics.ShiftAmount - 1;
-        SelNPQ = true;
-      }
+      assert(magics.PreShift < Divisor.getBitWidth() &&
+      assert(magics.PostShift < Divisor.getBitWidth() &&
+             "We shouldn't generate an undefined shift!");
+      assert((!magics.IsAdd || magics.PreShift == 0) &&
+             "Unexpected pre-shift");
+      PostShift = magics.PostShift;
+      SelNPQ = magics.IsAdd;
     }
 
     PreShifts.push_back(DAG.getConstant(PreShift, dl, ShSVT));
     MagicFactors.push_back(DAG.getConstant(Magic, dl, SVT));
     NPQFactors.push_back(
         DAG.getConstant(SelNPQ ? APInt::getOneBitSet(EltBits, EltBits - 1)
                                : APInt::getZero(EltBits),
                         dl, SVT));
     PostShifts.push_back(DAG.getConstant(PostShift, dl, ShSVT));

llvm/lib/Support/DivisionByConstantInfo.cpp

Retval =
         UnsignedDivisionByConstantInfo::get(ShiftedD, LeadingZeros + PreShift);
     assert(Retval.IsAdd == 0 && Retval.PreShift == 0);
     Retval.PreShift = PreShift;
     return Retval;
   }
 
   Retval.Magic = std::move(Q2);             // resulting magic number
   ++Retval.Magic;
-  Retval.ShiftAmount = P - D.getBitWidth(); // resulting shift
+  Retval.PostShift = P - D.getBitWidth(); // resulting shift
+  // Reduce shift amount for IsAdd.
+  if (Retval.IsAdd) {
+    assert(Retval.PostShift > 0 && "Unexpected shift");
+    Retval.PostShift -= 1;
+  }
   Retval.PreShift = 0;
   return Retval;
 }

llvm/unittests/Support/DivisionByConstantTest.cpp

if (LZOptimization) {
     LeadingZeros = std::min(LeadingZeros, Divisor.countLeadingZeros());
     if (LeadingZeros > 0) {
       Magics = UnsignedDivisionByConstantInfo::get(
           Divisor, LeadingZeros, AllowEvenDivisorOptimization);
       assert(!Magics.IsAdd && "Should use cheap fixup now");
     }
   }
 
-  unsigned PreShift = 0;
-  unsigned PostShift = 0;
-  bool UseNPQ = false;
-  if (!Magics.IsAdd) {
-    assert(Magics.ShiftAmount < Divisor.getBitWidth() &&
-           "We shouldn't generate an undefined shift!");
-    PreShift = Magics.PreShift;
-    PostShift = Magics.ShiftAmount;
-    UseNPQ = false;
-  } else {
-    assert(Magics.PreShift == 0 && "Unexpected pre-shift");
-    PostShift = Magics.ShiftAmount - 1;
-    assert(PostShift < Divisor.getBitWidth() &&
-           "We shouldn't generate an undefined shift!");
-    UseNPQ = true;
-  }
+  assert(Magics.PreShift < Divisor.getBitWidth() &&
+  assert(Magics.PostShift < Divisor.getBitWidth() &&
+  assert((!Magics.IsAdd || Magics.PreShift == 0) && "Unexpected pre-shift");
+  unsigned PreShift = Magics.PreShift;
+  unsigned PostShift = Magics.PostShift;
+  bool UseNPQ = Magics.IsAdd;
 
   APInt NPQFactor =
       UseNPQ ? APInt::getSignedMinValue(Bits) : APInt::getZero(Bits);
 
   APInt Q = Numerator.lshr(PreShift);
 
   // Multiply the numerator by the magic value.
   Q = MULHU(Q, Magics.Magic);