gnu1987/gcc/PROBLEMS

3. When find_reloads is used to count number of spills needed
it does not take into account the fact that a reload may
turn out to be a dummy.

I'm not sure this really happens any more.  Doesn't it find
all the dummies on both passes?

10.     movl a3@,a0
	movl a3@(16),a1
	clrb a0@(a1:l)
is generated and may be worse than
	movl a3@,a0
	addl a3@(16),a0
	clrb a0@
If ordering of operands is improved, many more
such cases will be generated from typical array accesses.

23. (memory >> 24) and (memory >> 24) == CONST  optimizations
ought to be done machine independently.

38. Hack expand_mult so that if there is no same-modes multiply
it will use a widening multiply and then truncate rather than
calling the library.

39. Hack expanding of division to notice cases for
long -> short division.

40. Represent divide insns as (DIV:SI ...) followed by
a separate lowpart extract.  Represent remainder insns as DIV:SI
followed by a separate highpart extract.  Then cse can work on
the DIV:SI part.  Problem is, this may not be desirable on machines
where computing the quotient alone does not necessarily give
a remainder--such as the 68020 for long operands.

42. In subst in combine.c at line 704 or so, a reg that really
wants an areg gets a dreg.  It is i*4, for indexing.  Why?

45. What about reloading DFmode values?  Need a block of 2 regs
as one spill.  But the 68000 and 68020 have no problems because
they don't ever need to put a DF into ordinary registers and a
DF needs only one FP register.

48. Often see  tstl foo; jeq ...; movl foo,reg
and it would be better perhaps to do  movl foo,dreg; jeq ...
In some cases this could make things worse through the
additional constraint on register usage (reg must be available
before the jump and must be a dreg).  Also, this is a saving
only if movl sets the cc's, which is a machine-dependent condition
hard to test for.
-optforcemem may solve this problem.

49. Scheme for making cse work on operations done by library calls:
generate insns for them using a new LIBCALL rtl type, then converting
that during or after cse to actual push and call instructions.
(SET result (LIBCALL:SI (MULT:SI foo bar) "mulsi3"))
No additional temporaries are needed to turn this into
push push call pop store.

52. Reloading can look at how reload_contents got set up.
If it was copied from a register, just reload from that register.
Otherwise, perhaps can change the previous insn to move the
data via the reload reg, thus avoiding one memory ref.

53. Know that certain library routines do not clobber memory.

63. Potential problem in cc_status.value2, if it ever activates itself
after a two-address subtraction (which currently cannot happen).
It is supposed to compare the current value of the destination
but eliminating it would use the results of the subtraction, equivalent
to comparing the previous value of the destination.

65. Should loops that neither start nor end with a break
be rearranged to end with the last break?

68. At line 767 of flow.c, we have
  (set (reg foo) (ashift 1 (reg bar)))
Local-alloc puts foo and bar both in register 0 since no reason not to.
Later, the shift must be done in another register and the result copied to 0.
It would not need to copy if this insn were 2op'd before local-alloc,
which is possible since one can be sure in advance that this insn will
need a reload.

69. Define the floating point converting arithmetic instructions
for the 68881.

74. Combine loop opt with cse opt in one pass.  Do cse on each loop,
then loop opt on that loop, and go from innermost loops outward.
Make loop invariants available for cse at end of loop.

85. pea can force a value to be reloaded into an areg
which can make it worse than separate adding and pushing.
This can only happen for adding something within addql range
and it only loses if the qty becomes dead at that point
so it can be added to with no copying.

93. If a pseudo doesn't get a hard reg everywhere,
can it get one during a loop?

95. Can simplify shift of result of a bfextu.  See testunsfld.c.
Likewise and of result of a bfextu.  See hyph.c.

96. Can do SImode bitfield insns without reloading, but must
alter the operands in special ways.

98. Reloading an operand of a compare insn
prevented final from noticing that the compare was redundant.
See hyph.c line 108.

99. final could check loop-entry branches to see if they
screw up deletion of a test instruction.  If they do,
can put another test instruction before the branch and
make it conditional and redirect it.

101. In cse, stores at addresses that contain SYMBOL_REFS
cannot alias if the symbols are different!
But the symbols may not be explicitly present--they may
be in address registers.

106. Aliasing may be impossible if data types of refs differ
and data type of containing objects also differ.
(But check this wrt unions.)

108. Can speed up flow analysis by making a table saying which
register is set and which registers are used by each instruction that
only sets one register and only uses two.  This way avoid the tree
walk for such instructions (most instructions).

109. It is desirable to avoid converting INDEX to SImode if a
narrower mode suffices, as HImode does on the 68000.
How can this be done?

110. Possible special combination pattern:
If the two operands to a comparison die there and both come from insns
that are identical except for replacing one operand with the other,
throw away those insns.  Ok if insns being discarded are known 1 to 1.
An andl #1 after a seq is 1 to 1, but how should compiler know that?