[LLVMdev] Target data question

[Dan Gohman]

On Oct 21, 2009, at 9:31 AM, Kenneth Uildriks wrote:

> On Wed, Oct 21, 2009 at 10:59 AM, Kenneth Uildriks <kennethuil at gmail.com> wrote:
>> If the TargetData pass isn't registered in the global registry,
>> getPassInfo() returns null.
>> 
>> Now when you add a TargetData pass, it winds up in ImmutablePasses.
>> Any search through ImmutablePasses assumes that getPassInfo() for
>> every member returns something other than null.  So findAnalysisPass
>> for *any* analysis pass can crash the system if the TargetData pass is
>> lurking in the list without being registered.
>> 
>> Since we want to be able to run opt without a TargetData pass, this
>> will never do.  If TargetData is registered globally, any
>> findAnalysisPass call will find it if there isn't another TargetData
>> pass in the PassManager.  Should TargetData now not be considered an
>> ImmutablePass?  Should findAnalysisPass include a null check on the
>> getPassInfo of ImmutablePasses?
>> 
> 
> Never mind, I got confused.  Registering a pass doesn't mean that
> getAnalysisIfAvailable will return it; it still has to be in the pass
> manager's collection.  It just means that PassInfo will be available
> for it if it's there.
> 
> I think...

Yes, that's what's intended.

> 
> Anyway, my present plan of attack is to have a "-defaulttarget" option
> with "none", "host", or a target string.  If -defaulttarget is not
> specified, the behavior of "opt" will be the same as it is presently.
> The defaulttarget will be overridden by the Module's target data if it
> has some.  "none" means that no TargetData pass will be added unless
> the Module supplies target data.  "host" uses the running host's
> TargetData as the default.
> 
> What do y'all think?

I think it's more intuitive to have command-line information override
Module information. That's how llc works, for example.

Also, is the argument to -defaulttarget a triple, an architecture name,
or a targetdata string? If it's a triple, it'd be nice to be consistent
with llc and call it -mtriple=. For an architecture name, -march=.
If it's a targetdata string, perhaps -targetdata= would be a good name.

(As an aside, I wouldn't object to having llc's options renamed to
remove the leading 'm', as that seems to have been intended to follow
GCC's targeting options, and they aren't the same.)

Dan