[cfe-dev] Changing module maps

[Sean Silva]

On Thu, May 21, 2015 at 2:23 PM, <blangmuir at apple.com> wrote:

>
> On May 20, 2015, at 6:14 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On Wed, May 20, 2015 at 5:59 PM, Thompson, John <
> John_Thompson at playstation.sony.com> wrote:
>
>>  Hi,
>>
>>
>>
>> I’ve experienced a few situations where I’ve been changing a module map
>> during development, or passing module maps to others to try, who then get
>> mysterious warnings or errors, such as:
>>
>>
>>
>> test.cpp:2:2: warning: missing submodule
>> '_Builtin_intrinsics.intel.x86intrin'
>>
>>       [-Wincomplete-umbrella]
>>
>> #include <x86intrin.h>
>>
>> ^       ~
>>
>> 1 warning generated.
>>
>>
>>
>> Sometimes clang detects it, and issues a relevant warning.  Other times
>> it does not.  (Perhaps due to a time tag being earlier?)  The recovery is
>> to delete the module cache manually.
>>
>>
>>
>> Is there something we could do to better detect a changed module map, and
>> perhaps even better, also automatically delete the prior module cache to
>> avoid potential problems?  Checksum/CRC the module map?  Use size and file
>> time?
>>
>
> We do these things, but the checks are turned off by default for system
> modules (to avoid stat'ing all the system headers on every compile). I
> think there's a flag to turn on the checks even for system modules,
> -fmodules-validate-system-headers I think?
>
>
> Maybe we should flip the default, and make the unsafe behaviour opt-in?
> IDEs and build systems can also use
> -fmodules-validate-once-per-build-session to avoid unnecessary re-checking
> without making it hard to edit system module maps.
>

Do we know why it was put in with its current default in the first place,
and how much it actually speeds things up?

-- Sean Silva


>
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