Irreducible loops in generated code

Irreducible loops in generated code

[Alain Ketterlin]

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.

-- Alain.

Re: Irreducible loops in generated code

[Steven Bosscher]

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

Re: Irreducible loops in generated code

[Alain Ketterlin]

On 18/08/2010 12:37, Steven Bosscher wrote:
> On Wed, Aug 18, 2010 at 11:53 AM, Alain Ketterlin
> <alain.ketterlin@gmail.com>  wrote:

>> 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)?

Sure, sorry, forgot that: it's gcc 4.4.3 (the one that comes with Ubuntu 
Lucid). My test cases are SPEC programs. I haven't been able to extract 
a simple example, it usually happens on large functions with heavily 
nested loops.

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

Excellent news :-) OK, I'll try the new version and report back what I 
find. Thanks for the help.

-- Alain.

Re: Irreducible loops in generated code

[Zdenek Dvorak]

Hi,

> > 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.

I am not sure that is actually true.  Afaik, we only avoid this on gimple,
while rtl jump threading does not care,

Zdenek