I know it's been a couple years since the bug was closed, but I found it
today when wishing for a similar thing (and wondering if anyone had
taken up Knuth on his idea).
FWIW I think you are significantly overstating what it would take to do
this. I think this is comparable in scope to the -malign-double flag,
which likewise changes the ABI slightly, but doesn't require a new ABI
document, a new GNU triple, or changes to binutils.
GCC already knows how to compile for x86-64, the only difference would
be that it would only allocate 32 bits for pointers in structs, and it
would load/store those pointers using DWORD PTR instead of QWORD PTR.
Besides that, the ABI would remain unchanged (for example, there would
be no impact on calling conventions).
It's true that you'd need to get the kernel to allocate the stack and
vdso within 4GB, but since the kernel already does this for
compatibility mode binaries (ie. 32-bit binaries run on a 64-bit OS), I
can't imagine it would be too difficult to accomplish.
The only big problem I can see is compatibility with kernel/glibc
structures that have pointers in them, like "struct iovec"
struct iovec {
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
To use structures like this you'd need to compile a separate glibc that
uses 32-bit pointers. I'm not sure how -malign-double gets around this
problem; maybe there just aren't any glibc/OS structs that have doubles
in them. But I don't think too many of the structs that are exchanged
between user code and glibc/kernel have pointers in them, so perhaps
this problem wouldn't be too wide-spread.
Anyway, I'm not commenting on whether Ubuntu is the right place to make
this feature request, I'm just commenting on the feasibility of doing
this. I think it's a lot easier than you're making it sound.
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https://bugs.launchpad.net/bugs/185263
Title:
-mlong32 is an unknown option
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