On 05/12/16 16:50, Hannes Frederic Sowa wrote: > On 05.12.2016 17:40, Edward Cree wrote: >> I may be completely mistaken here, but can't the verifier unroll the loop >> 'for >> verification' without it actually being unrolled in the program? >> I.e., any "proof that the loop terminates" should translate into "rewrite of >> the directed graph to make it a DAG, possibly duplicating a lot of insns", >> and >> you feed the rewritten graph to the verifier, while using the original loopy >> version as the actual program to store and later execute. >> Then the verifier happily checks things like array indices being valid, >> without >> having to know about the bounded loops. > That is what is already happening. E.g. __builtin_memset is expanded up > to 128 rounds (which is a lot) but at some point llvm doesn't do enoug > unrolling of that. > > The BPF target configures that in > http://llvm.org/docs/doxygen/html/BPFISelLowering_8cpp_source.html on > line 166-169. I think you're talking about the _compiler_ unrolling loops before it submits the program to the kernel. I'm talking about having the _verifier_ unroll them, so that we can execute the original (non-unrolled) version. Or am I misunderstanding?
-Ed
