[llvm-dev] instcombine produces strange i32*

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 14 13:35:34 PDT 2016

> On Sep 14, 2016, at 12:17 PM, Frank Winter via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> I am getting wrong code from one of the passes, namely instcombine. I
> see it in my JIT application when calling the pass directly through a
> pass manager.
> It was not possible for me to reproduce this behaviour with 'opt'
> however I could isolate the IR and write a minimal 'opt' myself and
> reproduce it.
> As you can see the input IR (f.ll) operates most of the time with
> floating point numbers and strangly the instcombine pass replaces some
> float* with i32*. This is wrong and completely messes up the code and
> gives strange restults and segfaults.
> I attach the input IR, program and output:
> f.ll (input IR)
> main.cc (reads f.ll and applies the passes and prints out the
> resulting IR)
> f_main.ll (output of main)
> You can see in f_main.ll strange bitcasts and loads to i32*.
>  %31 = getelementptr float, float* %arg3, i64 %13
>  %32 = bitcast float* %31 to i32*
>  %33 = load i32, i32* %32, align 4

It is not clear to me what is “wrong” here?
 %33 is only stored after, using an i32 instead of a float is OK.

> I tried to mimic this with 'opt' with the following command
> opt -basicaa -licm -gvn -mem2reg -loop-vectorize -early-cse
> -instcombine -S f.ll

First your IR does not have a triple/datalayout.
Second you should add  -targetlibinfo -tti at the beginning of the opt arguments list.
And finally, it’ll help in general to get the IR right before instcombine so that you can reproduce with opt -instcombine alone.

For example by modifying your program to comment out the run of InstCombine.


> This doesn't reproduce the behaviour.
> I am seeing this in LLVM 3.8 and 3.9. This is a longer-standing issue
> and probably not fixed in trunk right now.
> Any idea?
> Frank
> <f.ll><f_main.ll><main.cc>_______________________________________________
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