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.