[llvm] [InstCombine] Fold selection between less than zero and one (PR #69961)

[via llvm-commits]

XChy wrote:

> @XChy What is wrong with this approach?
> 
> ```
> CmpInst::Predicate Pred;
>   Value *A;
>   ConstantInt *C1, *C2;
>   const APInt *C3;
>   if (match(CondVal, m_Cmp(Pred, m_Value(A), m_ConstantInt(C1))) &&
>       match(TrueVal, m_LShr(m_Specific(A), m_ConstantInt(C2))) &&
>       match(FalseVal, m_APInt(C3)) && C3->isOne() &&
>       C2->getValue() == C2->getValue().getBitWidth() - 1) {
>     auto *Cond = Builder.CreateICmp(Pred, A, C1);
>     auto *LArm = Builder.CreateICmpSLE(A, ConstantInt::getNullValue(A->getType()));
>     Constant *RArm = ConstantInt::get(SelType, *C3);
>     return SelectInst::Create(Cond, LArm, RArm);
>   }
> ```

m_Cmp -> m_ICmp
m_ConstantInt -> m_SpecificInt(getScalarSizeInBits() - 1)
m_APInt(C3) -> m_One()
Builder.CreateICmpSLE -> Builder.CreateICmpSLT
RArm  -> C3

You may need to refer to https://reviews.llvm.org/D154791 and https://github.com/llvm/llvm-project/pull/68244.

https://github.com/llvm/llvm-project/pull/69961