It seems gcc prior to 5 suffered from preprocessor bug. We have to
provide workarounds.
See 591138012
----
Here is something interesting. According to C99, strictly speaking,
the gcc behaviour was in fact legal(!) and everything else were
wrong. This was not a bug of gcc, rather a bug of the ISO C Standard.
This defect was reported to the committee as DR#412, and fixed
accordingly. All tested compilers now conform C17's updated
preprocessor description.
See http://www.open-std.org/JTC1/SC22/WG14/www/docs/dr_412.htm
This reverts commit 443389effc.
This reverts commit d94960f22e.
Inclusion of header files must be explicit. Every file shall directly
include what is necessary.
https://github.com/include-what-you-use/include-what-you-use says:
> When every file includes what it uses, then it is possible to edit any
> file and remove unused headers, without fear of accidentally breaking
> the upwards dependencies of that file. It also becomes easy to
> automatically track and update dependencies in the source code.
Though we don't use iwyu itself, the principle quoted above is a good
thing that we can agree.
Now that include guards were added to every and all of the headers
inside of our project this changeset does not increase compile time, at
least on my machine.
According to MSVC manual (*1), cl.exe can skip including a header file
when that:
- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.
GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).
Sun C lacked #pragma once for a looong time. Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.
This changeset modifies header files so that each of them include
strictly one #ifndef...#endif. I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]
*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html
Without this patch, 20k files are opened (openat syscall) because
of duplicate includes. This patch reduced it to 3k and build time
was reduced compile time of range.o from 15sec -> 3sec on my machine.
[Bug #16772]
