[llvm-dev] Optimization Problem

[Doerfert, Johannes via llvm-dev]

Hi Peter,

In general, int2ptr/ptr2int open up a variety of issues. If you can,
avoid them in the first place and use GEPs, e.g., do your arithmetic on
char *.

I dug up a few posts that might help:
  http://llvm.1065342.n5.nabble.com/GEP-vs-IntToPtr-PtrToInt-td44587.html
  http://llvm.1065342.n5.nabble.com/Canonicalization-of-ptrtoint-inttoptr-and-getelementptr-td72015.html
  https://reviews.llvm.org/D37419#860016

There are also good talks (and papers) about the topic:
  https://www.youtube.com/watch?v=CPokKMqAVdY
  https://www.youtube.com/watch?v=r0XVS4Atl3U

Enabling more transformations in their presence, e.g.,
  https://reviews.llvm.org/D9152
often caused problems:
  https://github.com/llvm-mirror/llvm/commit/185966c6502046ad33f3fb64d70b4ef3c7e8faa3

I know I didn't actually answer your questions, but I hope this might
still help.

Cheers,
  Johannes

On 04/27, Garba Peter via llvm-dev wrote:
> Hi,
> 
> I try to understand why llvm can't optimize this small piece of code.
> My expectation is that the IR will be optimized to a " ret i64 2101545.
> But somehow the alias analysis fails when the pointer is casted to an integer value,
> so the dead code eliminator can't optimize it away.
> 
> Could someone explain to me why this is failing and if there is a solution for that ?
> 
> I can trick the alias analysis by setting %5 to a concrete integer so the code can be optimized further,
> but I'm still wondering why llvm can't do it.
> 
> Thanks,
> Peter
> 
> define dso_local i64 @func(i64, i64, i64) local_unnamed_addr #0 {
> .split:
>   %3 = alloca [100 x i64], align 16
>   %4 = getelementptr inbounds [100 x i64], [100 x i64]* %3, i64 0, i64 2
>   %5 = ptrtoint i64* %4 to i64
>   %6 = add i64 %5, -8
>   %7 = inttoptr i64 %6 to i64*
>   store i64 0, i64* %7, align 8
>   %8 = add i64 %5, -12
>   %9 = inttoptr i64 %8 to i32*
>   store i32 0, i32* %9, align 4
>   %10 = add i64 %5, -16
>   %11 = inttoptr i64 %10 to i32*
>   store i32 0, i32* %11, align 16
>   ret i64 2101545
> }
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-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
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