Re: An unusual x86_64 code model

[Jed Davis <jedidiah at vmware dot com>]

Second attempt: I now have a modified GCC 4.4.3 which recognizes
-mcmodel=smallhigh; in CM_SMALLHIGH, pic_32bit_operand acts as it does
for PIC (to get lea instead of movabs), and legitimate_address_p accepts
SYMBOLIC_CONSTs with no indexing (for anything with a memory constraint).

Beyond that, the defaults (i.e., what happens if the code model isn't
any of the previously defined ones) happen to be more or less what I
want.  In particular, the operand printer chooses %rip-relative mode
over plain disp32 for any symbolic displacement in all the smallish
modes; this is commented as if it were just a space optimization
(avoiding the SIB byte), but in fact it's necessary for -fPIC to work,
so I think I can depend on it.

One thing I'm not so sure about is accepting any SYMBOLIC_CONST as a
legitimate address.  That allows, for example, a symbol address cast
to uintptr_t and added to (6ULL << 32), which will never fit.  On the
other hand, -fPIC allows offsets of up to +/- 16Mib for some unexplained
reason, meaning that I can break it by pushing the code+data size almost
to 2GiB with a large .bss and evaluating (uintptr_t)&_end+0xffffff.

I think I could try to fix that by interrogating the SYMBOL_REF_DECL for
the object's size, but given that -fPIC doesn't go that far, it's not
clear that I need to.

Thoughts?

Also, it may actually work now.  I've successfully bootstrapped with
BOOT_CFLAGS='-mcmodel=smallhigh -O2 -g', after which the only _32 or
_32S relocations in the gcc/ subdirectory of the objdir were either
.debug references that I assume are safe, or in crtstuff that's part of
the libgcc build and not affected by BOOT_CFLAGS.  (It also successfully
builds ESXi with kernel coverage enabled, but that's less informative
for people on this list.)

Once our lawyers approve, I can also send the actual diff; by my count,
it makes nontrivial changes to a whole seven lines of code.

--Jed