[COFF] Support MinGW automatic dllimport of data

[COFF] Support MinGW automatic dllimport of data

Normally, in order to reference exported data symbols from a different
DLL, the declarations need to have the dllimport attribute, in order to
use the __imp_<var> symbol (which contains an address to the actual
variable) instead of the variable itself directly. This isn't an issue
in the same way for functions, since any reference to the function without
the dllimport attribute will end up as a reference to a thunk which loads
the actual target function from the import address table (IAT).

GNU ld, in MinGW environments, supports automatically importing data
symbols from DLLs, even if the references didn't have the appropriate
dllimport attribute. Since the PE/COFF format doesn't support the kind
of relocations that this would require, the MinGW's CRT startup code
has an custom framework of their own for manually fixing the missing
relocations once module is loaded and the target addresses in the IAT
are known.

For this to work, the linker (originall in GNU ld) creates a list of
remaining references needing fixup, which the runtime processes on
startup before handing over control to user code.

While this feature is rather controversial, it's one of the main features
allowing unix style libraries to be used on windows without any extra
porting effort.

Some sort of automatic fixing of data imports is also necessary for the
itanium C++ ABI on windows (as clang implements it right now) for importing
vtable pointers in certain cases, see D43184 for some discussion on that.

The runtime pseudo relocation handler supports 8/16/32/64 bit addresses,
either PC relative references (like IMAGE_REL_*_REL32*) or absolute
references (IMAGE_REL_AMD64_ADDR32, IMAGE_REL_AMD64_ADDR32,
IMAGE_REL_I386_DIR32). On linking, the relocation is handled as a
relocation against the corresponding IAT slot. For the absolute references,
a normal base relocation is created, to update the embedded address
in case the image is loaded at a different address.

The list of runtime pseudo relocations contains the RVA of the
imported symbol (the IAT slot), the RVA of the location the relocation
should be applied to, and a size of the memory location. When the
relocations are fixed at runtime, the difference between the actual
IAT slot value and the IAT slot address is added to the reference,
doing the right thing for both absolute and relative references.

With this patch alone, things work fine for i386 binaries, and mostly
for x86_64 binaries, with feature parity with GNU ld. Despite this,
there are a few gotchas:
- References to data from within code works fine on both x86 architectures,
  since their relocations consist of plain 32 or 64 bit absolute/relative
  references. On ARM and AArch64, references to data doesn't consist of
  a plain 32 or 64 bit embedded address or offset in the code. On ARMNT,
  it's usually a MOVW+MOVT instruction pair represented by a
  IMAGE_REL_ARM_MOV32T relocation, each instruction containing 16 bit of
  the target address), on AArch64, it's usually an ADRP+ADD/LDR/STR
  instruction pair with an even more complex encoding, storing a PC
  relative address (with a range of +/- 4 GB). This could theoretically
  be remedied by extending the runtime pseudo relocation handler with new
  relocation types, to support these instruction encodings. This isn't an
  issue for GCC/GNU ld since they don't support windows on ARMNT/AArch64.
- For x86_64, if references in code are encoded as 32 bit PC relative
  offsets, the runtime relocation will fail if the target turns out to be
  out of range for a 32 bit offset.
- Fixing up the relocations at runtime requires making sections writable
  if necessary, with the VirtualProtect function. In Windows Store/UWP apps,
  this function is forbidden.

These limitations are addressed by a few later patches in lld and
llvm.

Differential Revision: https://reviews.llvm.org/D50917

llvm-svn: 340726