RFR (tedious) 8216022: Use #pragma once

[Andrew Haley]

On 1/3/19 7:48 AM, Kim Barrett wrote:

> I think we shouldn't make this change without considering the impact
> of the following bug:
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58770
> GCC very slow compiling with #pragma once
> 
> According to comment 3, using "#pragma once" introduces N^2 behavior
> on the number of included files, because the duplicate check uses a
> list rather than a hashtable.
> 
> At the very least, a performance comparison should be made to find out
> what the impact of that bug is.

Thank you for a very detailed analysis. I agree with you that this
shouldn't be changed, at least for now. I've been following the
discussion in the GCC lists and others for years, and the overall
feeling I get is that #pragma once isn't quite what we need for header
file inclusion. Instead, GCC and other compilers have a very high
performance implementation of include guards.

Nevertheless, I am still a GCC maintainer and I'll fix the bug in
#pragma once if it's really going to be useful. But based on some of
the known issues with #pragma once, I don't believe that it will
be. #pragma once is a nice idea, but I think that's all it is.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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