[PATCH] [InstCombineCompares] Added shl optimization for the instruction - icmp eq/ne (shl Const2, A), Const1

[Sean Silva]

On Fri, Oct 17, 2014 at 3:00 AM, Ankur Garg <ankur29.garg at samsung.com>
wrote:

> Hi majnemer, dexonsmith, suyog,
>
> Hi,
> The following patch implements the optimization for the instructions of
> the type:
> icmp eq/ne (shl Const2, A), Const1
>
> Such instructions can be converted to:
> icmp eq/ne A, (TrailingZeros(Const1) - TrailingZeros(Const2))
>

What?

(1 << A) == 0b1100
is equivalent to
A == 2
?

Notice that the produced expression in the proposed optimization is
invariant under the following transformation of the constants: changing any
bit above the least significant 1. However, the recognized expression is
not invariant under that change.

-- Sean Silva


>
> This handles only the equality operators for now. Other operators need to
> be handled.
>
> This patch is related to the http://reviews.llvm.org/D4068. The D4068
> implements this optimization for ashr/lshr operators. The following patch
> implements the optimization for shl operator.
>
> Please help in reviewing it.
>
> Thanks.
> Ankur
>
> http://reviews.llvm.org/D5839
>
> Files:
>   lib/Transforms/InstCombine/InstCombine.h
>   lib/Transforms/InstCombine/InstCombineCompares.cpp
>   test/Transforms/InstCombine/icmp.ll
>
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