[cfe-dev] Why clang needs to fork into itself?

Ted Kremenek kremenek at apple.com
Tue Jan 28 12:35:36 PST 2014

On Jan 28, 2014, at 12:15 PM, Manuel Klimek <klimek at google.com> wrote:

> On Tue, Jan 28, 2014 at 9:11 PM, Ted Kremenek <kremenek at apple.com> wrote:
> On Jan 28, 2014, at 12:08 PM, Manuel Klimek <klimek at google.com> wrote:
>> On Tue, Jan 28, 2014 at 2:38 AM, Richard Smith <metafoo at gmail.com> wrote:
>> On Mon Jan 27 2014 at 4:51:40 PM, Yuri <yuri at rawbw.com> wrote:
>> On 01/27/2014 16:37, Jean-Daniel Dupas wrote:
>> > If you want to debug/profile clang, you can invoke it directly with the -cc1 flag, and passing the right arguments.
>> >
>> > To get the full command line used to invoke the real compilation process, you can use the -### argument:
>> >
>> > clang -### -c -emit-llvm c.cpp
>> >
>> > For the record, in the early days, the clang driver was a separate binary that used to invoke the compiler (which was called ccc IIRC).
>> > Some time ago, the driver and the compiler were merged into a single clang binary, but it continue to work the same way it used to do. That explains why it executes itself.
>> I see.
>> So I wrote up my proposal to make this opt-in:
>> http://llvm.org/bugs/show_bug.cgi?id=18638
>> I don't think the reasons why we spawn another binary have really been captured in this thread. The biggest reason is that the clang driver accepts multiple files to compile:
>>    clang foo.c bar.c baz.c -o thing
>> ... and runs one compile process for each source file (and in this case, one link process for the binary). Crash recovery is just a nice side-effect of having a separate driver and frontend. The main benefit is that we get a consistent execution model regardless of the number of files passed to the driver.
>> But nowadays with modules we also have in-process compilation steps of dependent modules without going through the whole driver enchilada, so is this becoming an obsolete argument?
> Clang forking itself remains quite useful for getting test cases for crashers.  If the forked clang crashes the parent clang process tries to generate a preprocessed source to serve as a test case.  We have found this to be invaluable for users to file useful test cases.
> Will that still work if the crash comes from a dependent module?

I’m not 100% certain it will cover all cases, but I have seen in the case of modules that the generated preprocessed file contains @import lines for importing modules.
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