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Re: Irreducible loops in generated code


On Wed, Aug 18, 2010 at 11:53 AM, Alain Ketterlin
<alain.ketterlin@gmail.com> wrote:
>
> I'm working on decompiling x86-64 binary programs, using branches to rebuild
> a control-flow graph and looking for loops. I've found a significant number
> of irreducible loops in gcc-produced code (irreducible loops are loops with
> more than one entry point), especially in -O3 optimized binaries, even when
> the source code is "well" structured. My experiments are done mainly on the
> SPEC CPU-2006 benchmarks.
>
> My question is: where do these irreducible loops come from? Which
> optimization pass leads to irreducible regions? Thanks in advance for any
> pointer.

Questions right back at you: What compiler version are you using? Got
a specific exampe (test case)?

In older releases of GCC4, jump threading sometimes resulted in
irreducible regions, but more recent GCCs carefully try to avoid them.

Ciao!
Steven


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