[PATCH] Allow multiple modules with the same name to coexist in the module cache

[Richard Smith]

On Fri, Mar 28, 2014 at 3:27 PM, Ben Langmuir <blangmuir at apple.com> wrote:

> Hi Richard,
>
>
> On Mar 28, 2014, at 2:45 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Fri, Mar 28, 2014 at 1:12 PM, Ben Langmuir <blangmuir at apple.com> wrote:
>
>> This patch allows multiple modules that have the same name to coexist in
>> the module cache.  To differentiate between two modules with the same name,
>> we will consider the path the module map file that they are defined by*
>> part of the 'key' for looking up the precompiled module (pcm file).
>>  Specifically, this patch renames the precompiled module (pcm) files from
>>
>> cache-path/<module hash>/Foo.pcm
>>
>> to
>>
>> cache-path/<module hash>/Foo-<hash of module map path>.pcm
>>
>
> From a high level, I don't really see why we need a second hash here.
> Shouldn't the -I options be included in the <module hash>? If I build the
> same module with different -I flags, that should resolve to different .pcm
> files, regardless of whether it makes the module name resolve to a
> different module.map file.
>
>
> If we include the -I options in the module hash, we will explode the
> number of module compilations needed.  The following should all be able to
> share a module 'A'.
>

I really don't think they should, if that second -I path is used in any way
when building module 'A'.

clang -fmodules -I /path/to/A -I /path/to/B some_file.c
> clang -fmodules -I /path/to/A -I /path/to/C some_file2.c
> clang -fmodules -I /path/to/A -I /path/to/D some_file3.c
>

I would think a better solution here would be to not have that second -I
path in header search when building module A (and thus not include it in
the hash for a principled reason).

Ultimately, each module should have its own header search path -- the model
of one set of include paths for the entire TU (with that include path
potentially causing some libraries to find the wrong files) is a broken,
antiquated legacy of the non-modules world. In the short term, we should
probably drop all header search paths that are before the path in which the
module map was found.

Are you trying to cope with the case where the -I path finds multiple
> module.map files defining the same module (where it's basically chance
> which one will get built and used)? I don't really feel like this is the
> right solution to that problem either -- we should remove the 'luck' aspect
> and use some sane mechanism to determine which module.map files are loaded,
> and in what order.
>
>
> Nope - that is not being addressed.
>
>
> Is this addressing some other case?
>
> See above.
>
>
>
>> In addition, I've taught the ASTReader to re-resolve the names of
>> imported modules during module loading so that if the header search context
>> changes between when a module was originally built and when it is loaded we
>> can rebuild it if necessary.  For example, if module A imports module B
>>
>> first time:
>> clang -I /path/to/A -I /path/to/B ...
>>
>> second time:
>> clang -I /path/to/A -I /different/path/to/B ...
>>
>> will now rebuild A as expected.
>>
>>
>> * in the case of inferred modules, we use the module map file that
>> *allowed* the inference, not the __inferred_module.map file, since the
>> inferred file path is the same for every inferred module.
>
>
>
> Review comments on the patch itself:
>
>  +  /// the inferrence (e.g. contained 'module *') rather than the virtual
>
> Typo 'inference', 'Module *'.
>
> +  /// For an explanaition of \p ModuleMap, see Module::ModuleMap.
>
> Typo 'explanation'.
>
> +  // safe becuase the FileManager is shared between the compiler
> instances.
>
> Typo 'because'
>
>
> Thanks for catching the embarrassing quantity of typos :)
>
>
> +  // the inferred module. If this->ModuleMap is nullptr, then we are using
> +  // -emit-module directly, and we cannot have an inferred module.
>
> I don't understand what this comment is trying to say. If we're using
> -emit-module, then we were given a module map on the command-line; should
> that not be referred to by this->ModuleMap? (Also, why 'this->'?) How can a
> top-level module be inferred? Is that a framework-specific thing?
>
>
> Hmm, I don't recall why I didn't just pass in the InputFile as the module
> map.  I'll do that.
>
> Yes, AFAIK only framework modules can be inferred at the top-level.
>
>
> +    StringRef ModuleMap = this->ModuleMap ? this->ModuleMap->getName() :
> InFile;
>
> Please pick a different variable name rather than shadowing a member of
> '*this' here.
>
>
> Will do.
>
>
> +    // Construct the name <ModuleName>-<hash of ModuleMapPath>.pcm which
> should
> +    // be globally unique to this particular module.
> +    llvm::APInt Code(64, llvm::hash_value(ModuleMapPath));
> +    SmallString<128> HashStr;
> +    Code.toStringUnsigned(HashStr);
>
> Use base 36, like the module hash.
>
>
> I could have sworn I did... must have got lost along the way.  Will do.
>
>
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