jyknight added inline comments.
================
Comment at: lib/Driver/ToolChains/Linux.cpp:755
+ !Version.isOlderThan(4, 8, 0)) {
+ // For gcc >= 4.8.x, clang will preinclude <stdc-predef.h>
+ // -ffreestanding suppresses this behavior.
----------------
jyknight wrote:
> I don't see why it makes any sense to condition this based on a GCC
> installation existing and being >= 4.8, since this header comes from libc,
> and is necessary for standards compliance even if there's absolutely no GCC
> installed or involved.
>
> Additionally, something like this -- a way for the libc to be involved in
> setting up predefined macros -- seems probably necessary for *any* libc, if
> they want to be strictly standards-compliant. Some of the
> required-to-be-predefined macros like `__STDC_ISO_10646__` can only really be
> provided by the libc, since the appropriate value is dependent on the locale
> implementation in the libc, and not on the compiler at all.
>
> It's possible that glibc on Linux is the only one who's cared to implement it
> thus far (and, only when you're using GCC, at that, hence this change...).
> That seems likely, really, since mostly the impacted macros are nearly
> useless, so who cares... :) However, if others want to be conforming, I
> expect they _ought_ to be using this mechanism, as well...
>
> Thus, perhaps it ought to be an unconditional behavior of clang to include
> the file if it exists, unless `-ffreestanding` is passed.
This is still checking GCCInstallation.isValid(), OPT_nostdinc, and only
attempting to load the header on Linux OSes.
I don't think it shouldn't be conditioned by the former two, and I think it
should attempt to include stdc-predef.h if it exists on *all* OSes, not just
Linux (for the reasons described in the above comment).
https://reviews.llvm.org/D34158
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