386BSD 0.0 development

[unix-history] / usr / src / usr.bin / gcc / cc1 / 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?

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?

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.

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.

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?

112. Can convert float to unsigned int by subtracting a constant,
converting to signed int, and changing the sign bit.

117. Any number of slow zero-extensions in one loop, that have
their clr insns moved out of the loop, can share one register
if their original life spans are disjoint.
But it may be hard to be sure of this since
the life span data that regscan produces may be hard to interpret
validly or may be incorrect after cse.

118. In cse, when a bfext insn refers to a register, if the field
corresponds to a halfword or a byte and the register is equivalent
to a memory location, it would be possible to detect this and
replace it with a simple memory reference.

121. Insns that store two values cannot be moved out of loops.
The code in scan_loop doesn't even try to deal with them.

122. When insn-output.c turns a bit-test into a sign-test,
it should see whether the cc is already set up with that sign.

123. When a conditional expression is used as a function arg, it would
be faster (and in some cases shorter) to push each alternative rather
than compute in a register and push that.  This would require
being able to specify "push this" as a target for expand_expr.

124. On the 386, bad code results from foo (bar ()) when bar
returns a double, because the pseudo used fails to get preferenced
into an fp reg because of the distinction between regs 8 and 9.