Known bugs/issues in nativeGen, 000202 (JRS)
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All these bugs are for x86; I don't know about sparc/alpha.

-- absC -> stix translation for GET_TAG and in fact anything
   to do with the packed-halfword layout info itbl field is
   pretty dubious.  I think I have it fixed for big and little
   endian 32-bit, but it won't work at all on a 64 bit platform.

-- Most of the x86 insn selector code in MachCode.lhs needs to
   be checked against the Rules of the Game recorded in that file.
   I think there are a lot of subtle violations.

-- When selecting spill regs, don't use %eax if there is a CALL insn
   (perhaps excluding calls to newCAF, since it doesn't return a
    result).

-- Keep track of the stack offset so that correct spill code can
   be generated even if %esp moves.  At the moment %esp doesn't
   move, so the problem doesn't exist, but there is a different
   problem: ccalls put args in memory below %esp and only move
   %esp immediately prior to the call.  This is dangerous because
   (1) writing below %esp can cause a segmentation fault (as deemed
   by the OS), and (2) if a signal should be handled on that stack
   during argument construction, the args will get silently trashed.

-- nofib/real/hidden gets slightly different FP answers from the
   via-C route; possibly due to exp/log not being done in-line.

-- Possibly implement GLDZ and GLD1 as analogues of FLDZ and FLD1
   (x86), to reduce number of constants emitted in f-p code.