[LLVMdev] optimizer problem, possibly involving instcombine

[Duncan Sands]

Hi Jeff,

> Continuing to enjoy LLVM. I'm seeing something strange with the simple function
> "f(a,b) = (a+b == b+a)", which before optimization passes has the following LLVM
> code:
>
> define i1 @f(i64, i64) {
> top:
>    %a = alloca i64, !dbg !5697
>    %b = alloca i64, !dbg !5697
>    store i64 %0, i64* %a, !dbg !5697
>    store i64 %1, i64* %b, !dbg !5697
>    %2 = load i64* %a, !dbg !5704
>    %3 = load i64* %b, !dbg !5704
>    %4 = add i64 %2, %3, !dbg !5704
>    %5 = load i64* %b, !dbg !5704
>    %6 = load i64* %a, !dbg !5704
>    %7 = add i64 %5, %6, !dbg !5704
>    %8 = icmp eq i64 %4, %7, !dbg !5704
>    ret i1 %8, !dbg !5704
> }
>
> After our set of optimization passes, I get this strange result:
>
> define i1 @f(i64, i64) {
> top:
>    %2 = icmp eq i64 %1, %0, !dbg !5697
>    ret i1 %2, !dbg !5697
> }

as this is wrong, it is a bug: it shouldn't matter in what order you run passes,
the result should always be correct.  I suggest you open a bug report including
complete instructions on how to reproduce.  The best thing would be if you
attach a complete LLVM IR testcase, and an "opt" command line that reproduces,
along the lines of "opt testcase.ll -S -o - -instcombine -sroa -xyzpass ..."

Best wishes, Duncan.

>
> If I switch the order of the arguments in the second addition, so the function
> is "f(a,b) = (a+b == a+b)", then the code is optimized to
>
> define i1 @f(i64, i64) {
> top:
>    ret i1 true, !dbg !5832
> }
>
> which is correct.
>
> This is LLVM 3.1. You can see the full list of passes I'm running here:
> https://github.com/JuliaLang/julia/blob/master/src/codegen.cpp#L2564
>
> The linked code currently contains a workaround I found, which was to disable
> the first two instcombine passes. With that, the original function also reduces
> to returning a constant true.
>
> Is this a known bug, or am I just mis-ordering the optimization passes somehow?
>
> Many thanks,
>
> -Jeff
>
>
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