[PATCH] Patch for PR19753 InstCombine: constant comparison involving "ashr exact" not optimized well

[Rafael Espíndola]

I committed it as r209903.

On 30 May 2014 11:25, suyog <suyog.sarda at samsung.com> wrote:
> Hi Rafael,
>
> Thanks for the review!
>
> I have made changes as suggested by you :
> * FIXME/TODO about the lhsr
> * The comment variable names update in the comment.
> * A comment about the inexact div and log being handled in instsimplify
>
> If this looks good to you, can you please commit this on my behalf.
>
> Thanks once again for reviews and help.
>
> http://reviews.llvm.org/D3959
>
> Files:
>   lib/Transforms/InstCombine/InstCombineCompares.cpp
>   test/Transforms/InstCombine/icmp.ll
>
> Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
> ===================================================================
> --- lib/Transforms/InstCombine/InstCombineCompares.cpp
> +++ lib/Transforms/InstCombine/InstCombineCompares.cpp
> @@ -2439,6 +2439,23 @@
>        return new ICmpInst(I.getPredicate(), A, B);
>      }
>
> +    // PR19753:
> +    // (icmp (ashr exact const2, A), const1) -> icmp A, Log2(const2/const1)
> +    // Cases where const1 doesn't divide const2 exactly or Quotient is not
> +    // exact of log2 are handled by SimplifyICmpInst call above where we
> +    // return false.
> +    // TODO: Handle this for lshr exact with udiv.
> +    {
> +      ConstantInt *CI2;
> +      if (match(Op0, m_AShr(m_ConstantInt(CI2), m_Value(A))) &&
> +          (cast<BinaryOperator>(Op0)->isExact())) {
> +        APInt Quotient = CI2->getValue().sdiv(CI->getValue());
> +        unsigned shift = Quotient.logBase2();
> +        return new ICmpInst(I.getPredicate(), A,
> +                            ConstantInt::get(A->getType(), shift));
> +      }
> +    }
> +
>      // If we have an icmp le or icmp ge instruction, turn it into the
>      // appropriate icmp lt or icmp gt instruction.  This allows us to rely on
>      // them being folded in the code below.  The SimplifyICmpInst code has
> Index: test/Transforms/InstCombine/icmp.ll
> ===================================================================
> --- test/Transforms/InstCombine/icmp.ll
> +++ test/Transforms/InstCombine/icmp.ll
> @@ -1365,3 +1365,11 @@
>    %2 = icmp slt i32 %1, -10
>    ret i1 %2
>  }
> +
> +; CHECK-LABEL: @exact_ashr_eq_false
> +; CHECK-NEXT: icmp eq i32 %a, 1
> +define i1 @exact_ashr_eq_false(i32 %a) {
> +  %shr = ashr exact i32 -30, %a
> +  %cmp = icmp eq i32 %shr, -15
> +  ret i1 %cmp
> +}