[PATCH] D29206: [Support/ExecutionEngine] Use const void* for storing pointers-to-global.

[David L. Jones via Phabricator via llvm-commits]

dlj created this revision.
Herald added a reviewer: deadalnix.

For global variables (and functions), const void* is a reasonable default in
many cases. In cases where writing to a value is necessary (and known to be
safe, such as global initialization in ExecutionEngine), const_cast<> gets back
the mutable value.

A reproducible case where this matters is building against musl-libc. Under
musl, the type of stdin, stdout, and stderr is 'FILE *const'.

From lib/Support/DynamicLibrary.cpp:

  // On linux we have a weird situation. The stderr/out/in symbols are both
  // macros and global variables because of standards requirements. So, we
  // boldly use the EXPLICIT_SYMBOL macro without checking for a #define first.
  EXPLICIT_SYMBOL(stderr);
  EXPLICIT_SYMBOL(stdout);
  EXPLICIT_SYMBOL(stdin);

In this case, EXPLICIT_SYMBOL takes the address of variables of type 'extern
FILE *const', which can be converted to 'const void*', but not to 'void*'. So,
in other words, a global 'T *' is stored by pointer (a 'T **') cast to 'void*',
but a global 'T *const' stored by pointer is a 'T *const *', which can be
converted to 'const void *' (but not 'void *').

The ExecutionEngine changes might be separable, but it seems slightly better to
keep consistent const-correctness. That means pushing const_cast down to where
it is known to be safe, as opposed of casting away const from variables early,
when we know it is not generally safe to do so.


https://reviews.llvm.org/D29206

Files:
  include/llvm-c/ExecutionEngine.h
  include/llvm-c/Support.h
  include/llvm/ExecutionEngine/ExecutionEngine.h
  include/llvm/ExecutionEngine/GenericValue.h
  include/llvm/Support/DynamicLibrary.h
  lib/ExecutionEngine/ExecutionEngine.cpp
  lib/ExecutionEngine/ExecutionEngineBindings.cpp
  lib/ExecutionEngine/Interpreter/Execution.cpp
  lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp
  lib/ExecutionEngine/MCJIT/MCJIT.cpp
  lib/ExecutionEngine/MCJIT/MCJIT.h
  lib/Support/DynamicLibrary.cpp
  unittests/ExecutionEngine/MCJIT/MCJITObjectCacheTest.cpp
  unittests/ExecutionEngine/MCJIT/MCJITTest.cpp

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