Re: #import and PCH

[Geoffrey Keating <geoffk at apple dot com>]

On 22/01/2004, at 3:27 PM, Neil Booth wrote:
> Geoffrey Keating wrote:-
> > This fixes #import and #pragma once support when using PCH files.
>
> Am I right that we now MD5 every header, penalizing non-users
> of #import?

No; my patch does not change behaviour when PCH is not in use, and if a 
user never uses #import or #pragma once, headers are not MD5ed except 
while building a PCH.  Even if the user does use #import, a header is 
usually not checksummed unless it would actually match, although I am 
not sure what the exact behaviour is.

When I originally wrote this code, the overhead of doing the MD5s was 
too small to be measurable relative to the cost of building a PCH, but 
YMMV.

It is in any case necessary for correctness, and I don't believe there 
is a less-costly solution that meets all requirements.