[libc-commits] [libc] [InstCombine] Refactor matchFunnelShift to allow more pattern (NFC) (PR #68474)

[via libc-commits]

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@@ -2732,100 +2732,114 @@ static Instruction *matchFunnelShift(Instruction &Or, InstCombinerImpl &IC) {
   // rotate matching code under visitSelect and visitTrunc?
   unsigned Width = Or.getType()->getScalarSizeInBits();
 
-  // First, find an or'd pair of opposite shifts:
-  // or (lshr ShVal0, ShAmt0), (shl ShVal1, ShAmt1)
-  BinaryOperator *Or0, *Or1;
-  if (!match(Or.getOperand(0), m_BinOp(Or0)) ||
-      !match(Or.getOperand(1), m_BinOp(Or1)))
-    return nullptr;
-
-  Value *ShVal0, *ShVal1, *ShAmt0, *ShAmt1;
-  if (!match(Or0, m_OneUse(m_LogicalShift(m_Value(ShVal0), m_Value(ShAmt0)))) ||
-      !match(Or1, m_OneUse(m_LogicalShift(m_Value(ShVal1), m_Value(ShAmt1)))) ||
-      Or0->getOpcode() == Or1->getOpcode())
+  Instruction *Or0, *Or1;
+  if (!match(Or.getOperand(0), m_Instruction(Or0)) ||
+      !match(Or.getOperand(1), m_Instruction(Or1)))
     return nullptr;
 
-  // Canonicalize to or(shl(ShVal0, ShAmt0), lshr(ShVal1, ShAmt1)).
-  if (Or0->getOpcode() == BinaryOperator::LShr) {
-    std::swap(Or0, Or1);
-    std::swap(ShVal0, ShVal1);
-    std::swap(ShAmt0, ShAmt1);
-  }
-  assert(Or0->getOpcode() == BinaryOperator::Shl &&
-         Or1->getOpcode() == BinaryOperator::LShr &&
-         "Illegal or(shift,shift) pair");
-
-  // Match the shift amount operands for a funnel shift pattern. This always
-  // matches a subtraction on the R operand.
-  auto matchShiftAmount = [&](Value *L, Value *R, unsigned Width) -> Value * {
-    // Check for constant shift amounts that sum to the bitwidth.
-    const APInt *LI, *RI;
-    if (match(L, m_APIntAllowUndef(LI)) && match(R, m_APIntAllowUndef(RI)))
-      if (LI->ult(Width) && RI->ult(Width) && (*LI + *RI) == Width)
-        return ConstantInt::get(L->getType(), *LI);
-
-    Constant *LC, *RC;
-    if (match(L, m_Constant(LC)) && match(R, m_Constant(RC)) &&
-        match(L, m_SpecificInt_ICMP(ICmpInst::ICMP_ULT, APInt(Width, Width))) &&
-        match(R, m_SpecificInt_ICMP(ICmpInst::ICMP_ULT, APInt(Width, Width))) &&
-        match(ConstantExpr::getAdd(LC, RC), m_SpecificIntAllowUndef(Width)))
-      return ConstantExpr::mergeUndefsWith(LC, RC);
-
-    // (shl ShVal, X) | (lshr ShVal, (Width - x)) iff X < Width.
-    // We limit this to X < Width in case the backend re-expands the intrinsic,
-    // and has to reintroduce a shift modulo operation (InstCombine might remove
-    // it after this fold). This still doesn't guarantee that the final codegen
-    // will match this original pattern.
-    if (match(R, m_OneUse(m_Sub(m_SpecificInt(Width), m_Specific(L))))) {
-      KnownBits KnownL = IC.computeKnownBits(L, /*Depth*/ 0, &Or);
-      return KnownL.getMaxValue().ult(Width) ? L : nullptr;
-    }
+  bool IsFshl = true; // Sub on LSHR.
+  SmallVector<Value *, 3> FShiftArgs;
 
-    // For non-constant cases, the following patterns currently only work for
-    // rotation patterns.
-    // TODO: Add general funnel-shift compatible patterns.
-    if (ShVal0 != ShVal1)
+  // First, find an or'd pair of opposite shifts:
+  // or (lshr ShVal0, ShAmt0), (shl ShVal1, ShAmt1)
+  if (isa<BinaryOperator>(Or0) && isa<BinaryOperator>(Or1)) {
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goldsteinn wrote:

This code seems new? Not that its wrong or anything.

But either way don't understand why it can't be an early return. If we don't enter the `if` `FShiftArgs` will be null and we will return anyways.

https://github.com/llvm/llvm-project/pull/68474