[PATCH] D149471: [InstCombine] Add A - B + B << 1 --> A + B

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp:1396
+  if (match(LHS, m_Sub(m_Value(A), m_Value(B))) &&
+      match(RHS, m_Shl(m_Value(B), m_One())) &&
+      LHS->hasOneUse() && RHS->hasOneUse())
----------------
This can be make dramatically more generic:
https://alive2.llvm.org/ce/z/6aKx_e
https://alive2.llvm.org/ce/z/GqhncR

Something along the following lines:
```
  auto ScaleConst = [&](std::optional<unsigned> Opc,
                        const APInt *C) -> std::optional<APInt> {
    if (!Opc)
      return APInt::getZero(BitWidth);
    if (Opc == Instruction::Mul)
      return *C;
    if (Opc == Instruction::Shl)
      return APInt::getOne(BitWidth).shl(*C);
    return std::nullopt;
  };
// A - B case
// Todo: A + B case.
if(match(m_Sub(m_Value(A), m_Value(B1))) &&
   match(m_c_Add(m_Specific(A), m_Value(B2))) {
    std::optional<APInt> C1, C2;
    std::optional<unsigned> Opc1, Opc2;
    Value * Mul1, *Mul2;
    if (match(B1, m_c_BinOp(Opc1, m_Value(D), m_APInt(MatchC1))) &&
        match(B2, m_c_BinOp(Opc2, m_Specific(D), m_APInt(MatchC2)))) {
      C1 = ScaleConst(Opc1, MatchC1);
      C2 = ScaleConst(Opc2, MatchC2);
    } else if (match(B1, m_c_BinOp(Opc1, m_Specific(B2), m_APInt(MatchC1)))) {
      C1 = ScaleConst(Opc1, MatchC1);
      C2 = ScaleConst(Opc2, MatchC2);
      D = B2;
    } else if (match(B2, m_c_BinOp(Opc2, m_Specific(B1), m_APInt(MatchC2)))) {
      C1 = ScaleConst(Opc1, MatchC1);
      C2 = ScaleConst(Opc2, MatchC2);
      D = B1;
    }

    // Result if Add(A, (C2 - C1) * D
   }
```


================
Comment at: llvm/test/Transforms/InstCombine/add.ll:2945
 }
 
+define i32 @add_shl_sub(i32 %a, i32 %b) {
----------------
Can you split the creation of new tests to a new patch (w.o the new instcombe logic)  so we can see the dif generated.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D149471/new/

https://reviews.llvm.org/D149471