[cfe-dev] adding FileSkipped() to PPCallbacks

Zhanyong Wan (λx.x x) wan at google.com
Mon Apr 19 12:38:51 PDT 2010

Please take a look at the new patch, which doesn't call createFileID()
for a skipped file.  Thanks!

On Fri, Apr 16, 2010 at 4:41 PM, Chris Lattner <clattner at apple.com> wrote:
> On Apr 16, 2010, at 2:38 PM, Zhanyong Wan (λx.x x) wrote:
>> On Fri, Apr 16, 2010 at 2:14 PM, Chris Lattner <clattner at apple.com> wrote:
>>> On Apr 16, 2010, at 2:09 PM, Zhanyong Wan (λx.x x) wrote:
>>>> Thanks for the review, Chris.  Please see the new patch.  I kept the
>>>> signature of FileSkipped() unchanged to be consistent with
>>>> FileChanged().
>>> Keeping the interfaces consistent is nice, but is it really worth it?  This will punish other PPCallbacks that don't care about this, like -MM mode and friends.  SourceMgr.createFileID isn't hyper-expensive, but it is good to avoid work that no other client at all needs.  Sinking this work into the implementation that cares makes sense to me.
>> If createFileID() is called here, the file being skipped has already
>> been processed earlier, which is a much more expensive operation.  So
>> do we really want to sacrifice the cleanness of the API for it?
> I'm pushing back because the preprocessor is a particularly performance sensitive part of the compiler, and this adds no value for any current clients.  I think the cost should be sunk into your client, even if it makes its implementation a little less elegant.
> -Chris

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