This avoids the requirement of putting the code image in a
section/segment which is both writable and executable, which is good
for security and avoids trouble with systems like iOS which disallow
such things.
The implementation relies on relative addressing such that the offset
of the desired address is fixed as a compile-time constant relative to
the start of the memory area of interest (e.g. the code image, heap
image, or thunk table). At runtime, the base pointer to the memory
area is retrieved from the thread structure and added to the offset to
compute the final address. Using the thread pointer allows us to
generate read-only, position-independent code while avoiding the use
of IP-relative addressing, which is not available on all
architectures.
This fixes a number of bugs concerning cross-architecture bootimage
builds involving diffent endianesses. There will be more work to do
before it works.
This monster commit is the first step towards supporting
cross-architecture bootimage builds. The challenge is to build a heap
and code image for the target platform where the word size and
endianess may differ from those of the build architecture. That means
the memory layout of objects may differ due to alignment and size
differences, so we can't just copy objects into the heap image
unchanged; we must copy field by field, resizing values, reversing
endianess and shifting offsets as necessary.
This commit also removes POD (plain old data) type support from the
type generator because it added a lot of complication and little
value.
