[PATCH] Allow for the use of StringRef command line options instead of cl::opt<std::string>

[David Blaikie]

I've been following this thread only tangentially for the last few weeks.
Personally, I would like to live in a world where callers of
ParseCommandLineOptions handle the lifetime of those strings to ensure they
remain live for the lifetime of the code that needs the command line
options.

Can you point me to a particular situation where a caller is unable to do
this that ends up necessitating the GetPersistentClArgStr you're referring
to?


On Tue, Oct 1, 2013 at 2:33 PM, Puyan Lotfi <plotfi at apple.com> wrote:

> David, Chris:
>
> It seems that in certain cases it not very easy for callers of
> ParseCommandLineOptions to get to the global static instances of a given
> string and that is being parsed (when the strings come from some
> dynamically populated string table rather than main’s argv that is passed,
> as is the case with the driver code in clang).
>
> I’ve found that in one case (in clang), a uniquing table fixes the problem
> of handing strings given to the ParseCommandLineOptions function; something
> like this:
>
> static const char* GetPersistentClArgStr(const StringRef &key) {
>   static StringMap<const char*> PersistentClArgString;
>   StringMap<const char*>::const_iterator I =
> PersistentClArgString.find(key);
>   if (I == PersistentClArgString.end()) {
>     const char* persistVal = strdup(key.data());
>     PersistentClArgString[key] = persistVal;
>     return persistVal;
>   }
>   return I->second;
> }
>
> I used this in one particular case in clang, but it now seems like it
> would be useful to integrate into the CommandLine library and optionally
> have it turned on with an optional parameter to Parse*CommandLine*Options.
> For example, the Parse*Environment*Options (which calls
> ParseCommandLineOptions) function will fail when used with a
> cl::opt<StringRef> rather than std::string. It actually looks like
> ParseEnvironmentOptions has a StrDup class it uses to hold on to argument
> string duplicates but it seems to then go and delete them. afterward.
>
> I am very eager for suggestions on this.
>
> Thanks
>
> -Puyan
>
> On Aug 15, 2013, at 10:57 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
> On Thu, Aug 15, 2013 at 4:56 PM, Puyan Lotfi <plotfi at apple.com> wrote:
>
>
> This sounds like a workable compromise.
> To avoid leaking how about a CommandLine.cpp handled string pool that
> allocates copies on demand (specified maybe by an optional flag to
> ParseCommandLineOptions/ParseEnvironmentOptions) but also has the option of
> flushing/deallocating the pool?
>
>
> I'm with Chris - I think it should be simpler to have
> ParseCommandLineOptions/CommandLine in general not be concerned with
> task of owning the strings. That should be the responsibility of the
> caller.
>
>
> -Puyan
>
> On Aug 15, 2013, at 1:47 PM, Chris Lattner <clattner at apple.com> wrote:
>
>
> On Aug 14, 2013, at 4:06 PM, Puyan Lotfi <plotfi at apple.com> wrote:
>
> Oh wow. I see that.
>
> I am not completely sure how the char* arrays would get handled this way.
> I did not make any clang modifications in this patch (although since clang
> is using the cl library it is affected) but I noticed that the use of cl::
> in the llvm source tree made use of global cl::opt variables that specify
> the opt type.
> I don’t see that in the files handling the -mllvm options; I’ll have to
> look at the CommandLine.cpp code a little more closely to be sure in this
> case that the char*’s aren’t being implicitly put into StringRefs that
> might have their data pointers deallocated after
> cl::ParseCommandLineOptions() is called.
>
>
> After thinking about it for a bit, how about we go with this approach:
>
> 1. Document ParseCommandLineOptions/ParseEnvironmentOptions to only work
> with strings that live forever in CommandLine.h
> 2. Change clang to leak (or store globally) the strings that are affected.
>
> I don't think it is worth designing a context system or anything like that
> to handle this case.
>
> -Chris
>
>
>
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