Silencing warnings from official headers

[derek-gerstmann]

The HalideRuntime.h and HalideBuffer.h header files can cause warnings to be emitted when compiled with -pedantic which can be a problem if a project is compiled with -Werror.

These are the pragmas I needed to add around the header files to avoid any warnings or errors:

#if defined(__GNUC__) || defined(__clang__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wunused-parameter"
    #pragma GCC diagnostic ignored "-Wmissing-field-initializers"
#elif defined(_MSC_VER)
    #pragma warning(push)
    #pragma warning(disable : 4100) // unused formal parameter"
    #pragma warning(disable : 4700) // uninitialized local variable
    #pragma warning(disable : 4701) // potentially uninitialized local variable
    #pragma warning(disable : 4127) // conditional expression is constant
    #pragma warning(disable : 4702) // unreachable code
#endif

#include "HalideRuntime.h"
#include "HalideBuffer.h"

#if defined(__GNUC__) || defined(__clang__)
    #pragma GCC diagnostic pop
#elif defined(_MSC_VER)
    #pragma warning(pop)
#endif

Likewise, the halide_image_io.h caused similar problems under msvc. These are the pragmas I needed to silence those warnings:

#if defined(_MSC_VER)
    #pragma warning(push)
    #pragma warning(disable : 4100) // unreferenced formal parameter
    #pragma warning(disable : 4244) // conversion from 'XXX' to 'XXX', possible loss of data
    #pragma warning(disable : 4267) // 'initializing': conversion from 'XXX' to 'XXX', possible loss of data)
    #pragma warning(disable : 4456) // declaration of 'XXX' hides previous local declaration 
    #pragma warning(disable : 4458) // declaration of 'XXX' hides class member
    #pragma warning(disable : 4459) // declaration of 'XXX' hides global declaration
    #pragma warning(disable : 4611) // interaction between '_setjmp' and C++ object destruction is non-portable
    #pragma warning(disable : 4996) // function or variable may be unsafe. 
#endif

#include "halide_image_io.h"

#if defined(_MSC_VER)
    #pragma warning(pop)
#endif

[alexreinking]

The HalideRuntime.h and HalideBuffer.h header files can cause warnings to be emitted when compiled with -pedantic which can be a problem if a project is compiled with -Werror.

I think the best resolution is twofold:

1. Do our best to fix the warnings upstream
2. Instruct downstreams to mark Halide's headers as external. On GCC/clang, this is -isystem. MSVC has /external:I. CMake handles this automatically already when using find_package (the targets are marked as IMPORTED, which marks their include paths as SYSTEM).

[derek-gerstmann]

2. Instruct downstreams to mark Halide's headers as external. On GCC/clang, this is -isystem. MSVC has /external:I. CMake handles this automatically already when using find_package (the targets are marked as IMPORTED, which marks their include paths as SYSTEM).

Using -isystem is problematic in that ccache and some toolchains (like xcode) completely ignore all files in system include paths when computing hashes for changes. So, updating to a new version of Halide can potentially cause strange build failures. This has happened several times for downstream projects.

[alexreinking]

I thought that ccache behavior was opt in via adding system_headers to CCACHE_SLOPPINESS? That feels very much "use at your own peril".

[alexreinking]

Looking at the docs now... seems there is a way to limit the system header slop more explicitly/surgically: ignore_headers_in_manifest

https://ccache.dev/manual/4.11.3.html#config_ignore_headers_in_manifest