[cfe-commits] [PATCH] Fixing include order for Windows

[Chandler Carruth]

On Sun, Feb 19, 2012 at 5:35 PM, Aaron Ballman <aaron at aaronballman.com>wrote:

> On Sun, Feb 19, 2012 at 5:18 PM, Chandler Carruth <chandlerc at google.com>
> wrote:
> > On Sun, Feb 19, 2012 at 3:09 PM, Aaron Ballman <aaron at aaronballman.com>
> > wrote:
> >>
> >> On Sun, Feb 19, 2012 at 4:45 PM, Eli Friedman <eli.friedman at gmail.com>
> >> wrote:
> >> > On Sun, Feb 19, 2012 at 10:28 AM, Aaron Ballman <
> aaron at aaronballman.com>
> >> > wrote:
> >> >> On Windows, the default search paths for includes would put the
> >> >> CMake-found headers (<build dir>\lib\clang\3.1\include) before the OS
> >> >> headers.  For the most part, this is fine.  However, there are some
> >> >> header files that share names with the system headers, which are
> >> >> incorrect.  For instance, immintrin.h is completely different from
> the
> >> >> SDK include.  This causes miscompilations in surprising places
> because
> >> >> a system header will attempt to include one of these duplicates, and
> >> >> find the clang header instead of the other system header.
> >> >>
> >> >> A simple test case of: #include <map> when building from MSVC
> >> >> demonstrates the issue.
> >> >>
> >> >> A user can work around this behavior by specifying -nobuiltininc on
> >> >> the command line, but that seems rather obtuse for such normal use
> >> >> cases.
> >> >>
> >> >> I've fixed this by changing the order that the headers are included
> >> >> from the toolchain.  The SDK headers get added *before* the clang
> >> >> headers, and this resolves the issue.
> >> >>
> >> >> This patch does cause one regression test to fail
> >> >> (Modules\compiler_builtins.m).  This is because the platform SDK
> >> >> intrinsics are differently structured from the ones that come with
> >> >> clang.  The OS APIs expect the SDK intrinsics, but our tests seem to
> >> >> require our own intrinsics.  I'm open to suggestions.
> >> >
> >> > If on Windows, we specifically need the OS version of immintrin.h, we
> >> > should modify our version to forward to it; completely ignoring the
> >> > builtin headers is a bad idea (for example,
> >> > http://llvm.org/bugs/show_bug.cgi?id=11918 is a miscompile caused by
> >> > pulling in the wrong version of stdarg.h).
> >>
> >> The patch doesn't completely ignore the builtin headers though -- it
> >> simply prefers the OS headers over the builtin headers.
> >
> >
> > That doesn't work in the general case though -- look at all the places in
> > our builtin headers where we specifically forward *to* the system
> headers.
> > If the builtin doesn't come first, lots of things go wrong here.
> >
> > We should make our builtins always come first, and always be correct when
> > they come first.
> >
> >> That being
> >> said, I am not against forwarding our version.  What would be a good
> >> approach to it?
> >
> >
> > Look at the other builtin headers that use '__has_include_next' and
> > 'include_next' to fold together a Clang builtin header and a system
> builtin
> > header.
> >
> > I suspect you will have to essentially tack on the system 'immintrin.h'
> > header to the end of the builtin, as we do expect #include <immintrin.h>
> to
> > pull in the Clang builtin bits... Essentially, this doesn't seem like it
> > should be either-or, it should be both.
>
> So how should I handle the duplicate declarations?  For instance:
>
> // mmintrin.h from MSVC
> typedef union __declspec(intrin_type) _CRT_ALIGN(8) __m64
> {
>    unsigned __int64    m64_u64;
>    float               m64_f32[2];
>    __int8              m64_i8[8];
>    __int16             m64_i16[4];
>    __int32             m64_i32[2];
>    __int64             m64_i64;
>    unsigned __int8     m64_u8[8];
>    unsigned __int16    m64_u16[4];
>    unsigned __int32    m64_u32[2];
> } __m64;
>
> void  _m_empty(void);
> #define _mm_empty         _m_empty
>
> // mmintrin.h from clang
> typedef long long __m64 __attribute__((__vector_size__(8)));
>
> static __inline__ void __attribute__((__always_inline__, __nodebug__))
> _mm_empty(void)
> {
>    __builtin_ia32_emms();
> }
> #define _m_empty _mm_empty
>
> It seems to me that trying to keep *both* headers is going to be
> rather challenging given that both make the same typedefs in
> incompatible ways, the declarations/macros are opposite one another,
> etc.


Hold on -- the MS one is trying to provide the same intel intrinsics? Now I
think that the original search order and header file is correct -- we don't
want the MS version of this header as it is likely to use MS extensions
Clang doesn't support (or for which Clang has better models). Maybe what we
really need is to understand what the clang immintrin.h *lacks* in order
for MS code to use it?
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