[ccache] direct mode

Ian Norton inorton at gmail.com
Fri Oct 18 04:05:02 MDT 2013

Ok. I should have tried it before hand.  ccache *doesnt't* notice the
addition of the new header and still gives me a .o file from the first


On 18 October 2013 10:54, Ian Norton <inorton at gmail.com> wrote:
> Hello All,
> I have a question about direct mode, it follows on from an old thread
> I've seen in the archives:
> http://www.mail-archive.com/ccache@lists.samba.org/msg00150.html
> I'll quote inline and follow on.
> Joel Rosdahl wrote:
>> tridge wrote:
>> > Also, does the hashtable used for included_files preserve the
>> > ordering? (the order of includes is also vital). Or do you rely on the
>> > hash of the file that does the #include changing for that?
>> The hashtable is unordered, and yes, I rely on the hash of the input
>> file to keep track of the ordering, and also of course on the include
>> file hashes. For a given manifest, the source file (and therefore the
>> order of the first level of include files) is known since the manifest
>> is looked up given the hash of the input file (and some more
>> information), and all other levels of include files are taken
>> care of using the same kind of reasoning. In other words, if the
>> include file order changes in some file, then the hash of that file
>> changes too, which leads to a cache miss. Which include files the
>> preprocessor reads is of course also a function of compiler options
>> like -I, but that is handled by also hashing those options when
>> computing the hash in direct mode. Do you see any potential problem
>> here?
> I realise I'm probably missing something, but how does direct mode
> handle the case where
> the command line args have not changed, and nor have the source file
> or previously used headers *but* a header file has been added to a
> folder on one of the -I paths? eg:
> hello.c:
> #include "test.h"
> inc1/test.h:
> void hello(void);
> gcc -I inc2 -I inc1 -c hello.c
> later, someone makes a new file:
> inc2/test.h:
> int hello(void);
> The same command line and original inputs would result in a different file.
> How does direct mode cover this case ( all our common input data has
> not changed )
> Many Thanks, ccache is fantastic btw!
> Ian

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