[llvm-dev] About CodeGen quality

[陳韋任 via llvm-dev]

Here is the complete IR I am dealing with, if that helps discussion.

-------------------------------------------------------------

    %struct.A = type { %struct.Z*, %struct.Z*, %struct.Z*, %struct.Z*,
%struct.Z* }

    %struct.Z = type { %union.X, [180 x %union.Y] }

    %union.X = type { %struct.anon }

    %struct.anon = type { i64 }

    %union.Y = type { %struct.anon.0 }

    %struct.anon.0 = type { i32 }

    %struct.D = type { i64 }


    ; Function Attrs: norecurse nounwind

    define void @func(%struct.A* noalias nocapture readonly %a, %struct.D*
noalias nocapture readonly %d) local_unnamed_addr #0 {

    entry:

      %a2 = getelementptr inbounds %struct.A, %struct.A* %a, i32 0, i32 1

      %0 = load %struct.Z*, %struct.Z** %a2, align 4, !tbaa !1

      %1 = getelementptr inbounds %struct.D, %struct.D* %d, i32 0, i32 0

      %bf.load = load i64, i64* %1, align 4

      %bf.lshr = lshr i64 %bf.load, 8

      %2 = trunc i64 %bf.lshr to i32

      %bf.cast = and i32 %2, 255

      %3 = getelementptr inbounds %struct.Z, %struct.Z* %0, i32 0, i32 1,
i32 %bf.cast, i32 0, i32 0

      %bf.load1 = load i32, i32* %3, align 4

      %bf.clear2 = and i32 %bf.load1, 65535

      store i32 %bf.clear2, i32* %3, align 4

      ret void

    }

-------------------------------------------------------------


Regards,

chenwj

2017-06-16 14:13 GMT+08:00 Ehsan Amiri via llvm-dev <llvm-dev at lists.llvm.org
>:

>
> Forgot to reply to all
>
> Hi Eli
>
>
>     struct S {
>       unsigned int a : 8;
>       unsigned int b : 8;
>       unsigned int c : 8;
>       unsigned int d : 8;
>
>       unsigned int e;
>     }
>
> We want to read S->b for example. The size of struct S is 64 bits, and
> seems LLVM treats it as i64.
> Below is the IR corresponding to S->b, IIRC.
>
>     %0 = load i64, *i64 ptr, align 4;
>     %1 = %0 lshr 8;
>     %2 = %1 and 255;
>
>
> This looks fine.
>
>
> Why can't we expect InstCombine to simplify this to an 8 bit load,
> assuming each of %0 and %1 has only one use ?
>
> Thanks
> Ehsan
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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>
>


-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj
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