[LLVMdev] Traditional Dataflow Algorithm

[Andrew Trick]

On Apr 16, 2013, at 10:27 AM, John Criswell <criswell at illinois.edu> wrote:
> On 4/16/13 11:30 AM, dag at cray.com wrote:
>> Is there a traditional dataflow algorithm buried in LLVM somewhere?  I
>> need to be able to do some very late transformations (after regalloc)
>> and we aren't in SSA anymore.  I will need a dataflow analysis to ensure
>> correctness.
>> 
>> At one point I thought I remembered seeing a generic fixed-point
>> dataflow analysis engine but now I can't find it.  Does it still exist
>> or did it ever exist?  If it's not there, would folks be interested in
>> such a thing if I wrote one?
> 
> I might have use for a machine instruction level dataflow analysis for a project that I'm working on.

Generic MI level data flow across virtual registers could make sense. Although the generic part is trivial anyway, so implementing it as some kind of template seems silly until we have multiple clients.

David was asking for a post-register-rewrite analysis, which I think is a totally different problem and best implemented separately by each target.  The target knows best which register units are overwritten or preserved by an operation. We try to fake it with undef/implicit-use/implicit-def, but it isn't reliable. Reaching defs just can't be reliably rediscovered in a generic way after we drop explicit subregister information.

-Andy