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Annotation of GNUtools/cc/PROBLEMS, revision 1.1

1.1     ! root        1: 3. When find_reloads is used to count number of spills needed
        !             2: it does not take into account the fact that a reload may
        !             3: turn out to be a dummy.
        !             4: 
        !             5: I'm not sure this really happens any more.  Doesn't it find
        !             6: all the dummies on both passes?
        !             7: 
        !             8: 10.     movl a3@,a0
        !             9:        movl a3@(16),a1
        !            10:        clrb a0@(a1:l)
        !            11: is generated and may be worse than
        !            12:        movl a3@,a0
        !            13:        addl a3@(16),a0
        !            14:        clrb a0@
        !            15: If ordering of operands is improved, many more
        !            16: such cases will be generated from typical array accesses.
        !            17: 
        !            18: 38. Hack expand_mult so that if there is no same-modes multiply
        !            19: it will use a widening multiply and then truncate rather than
        !            20: calling the library.
        !            21: 
        !            22: 39. Hack expanding of division to notice cases for
        !            23: long -> short division.
        !            24: 
        !            25: 40. Represent divide insns as (DIV:SI ...) followed by
        !            26: a separate lowpart extract.  Represent remainder insns as DIV:SI
        !            27: followed by a separate highpart extract.  Then cse can work on
        !            28: the DIV:SI part.  Problem is, this may not be desirable on machines
        !            29: where computing the quotient alone does not necessarily give
        !            30: a remainder--such as the 68020 for long operands.
        !            31: 
        !            32: 52. Reloading can look at how reload_contents got set up.
        !            33: If it was copied from a register, just reload from that register.
        !            34: Otherwise, perhaps can change the previous insn to move the
        !            35: data via the reload reg, thus avoiding one memory ref.
        !            36: 
        !            37: 63. Potential problem in cc_status.value2, if it ever activates itself
        !            38: after a two-address subtraction (which currently cannot happen).
        !            39: It is supposed to compare the current value of the destination
        !            40: but eliminating it would use the results of the subtraction, equivalent
        !            41: to comparing the previous value of the destination.
        !            42: 
        !            43: 65. Should loops that neither start nor end with a break
        !            44: be rearranged to end with the last break?
        !            45: 
        !            46: 69. Define the floating point converting arithmetic instructions
        !            47: for the 68881.
        !            48: 
        !            49: 74. Combine loop opt with cse opt in one pass.  Do cse on each loop,
        !            50: then loop opt on that loop, and go from innermost loops outward.
        !            51: Make loop invariants available for cse at end of loop.
        !            52: 
        !            53: 85. pea can force a value to be reloaded into an areg
        !            54: which can make it worse than separate adding and pushing.
        !            55: This can only happen for adding something within addql range
        !            56: and it only loses if the qty becomes dead at that point
        !            57: so it can be added to with no copying.
        !            58: 
        !            59: 93. If a pseudo doesn't get a hard reg everywhere,
        !            60: can it get one during a loop?
        !            61: 
        !            62: 96. Can do SImode bitfield insns without reloading, but must
        !            63: alter the operands in special ways.
        !            64: 
        !            65: 99. final could check loop-entry branches to see if they
        !            66: screw up deletion of a test instruction.  If they do,
        !            67: can put another test instruction before the branch and
        !            68: make it conditional and redirect it.
        !            69: 
        !            70: 106. Aliasing may be impossible if data types of refs differ
        !            71: and data type of containing objects also differ.
        !            72: (But check this wrt unions.)
        !            73: 
        !            74: 108. Can speed up flow analysis by making a table saying which
        !            75: register is set and which registers are used by each instruction that
        !            76: only sets one register and only uses two.  This way avoid the tree
        !            77: walk for such instructions (most instructions).
        !            78: 
        !            79: 109. It is desirable to avoid converting INDEX to SImode if a
        !            80: narrower mode suffices, as HImode does on the 68000.
        !            81: How can this be done?
        !            82: 
        !            83: 110. Possible special combination pattern:
        !            84: If the two operands to a comparison die there and both come from insns
        !            85: that are identical except for replacing one operand with the other,
        !            86: throw away those insns.  Ok if insns being discarded are known 1 to 1.
        !            87: An andl #1 after a seq is 1 to 1, but how should compiler know that?
        !            88: 
        !            89: 112. Can convert float to unsigned int by subtracting a constant,
        !            90: converting to signed int, and changing the sign bit.
        !            91: 
        !            92: 117. Any number of slow zero-extensions in one loop, that have
        !            93: their clr insns moved out of the loop, can share one register
        !            94: if their original life spans are disjoint.
        !            95: But it may be hard to be sure of this since
        !            96: the life span data that regscan produces may be hard to interpret
        !            97: validly or may be incorrect after cse.
        !            98: 
        !            99: 118. In cse, when a bfext insn refers to a register, if the field
        !           100: corresponds to a halfword or a byte and the register is equivalent
        !           101: to a memory location, it would be possible to detect this and
        !           102: replace it with a simple memory reference.
        !           103: 
        !           104: 121. Insns that store two values cannot be moved out of loops.
        !           105: The code in scan_loop doesn't even try to deal with them.
        !           106: 
        !           107: 122. When insn-output.c turns a bit-test into a sign-test,
        !           108: it should see whether the cc is already set up with that sign.
        !           109: 
        !           110: 123. When a conditional expression is used as a function arg, it would
        !           111: be faster (and in some cases shorter) to push each alternative rather
        !           112: than compute in a register and push that.  This would require
        !           113: being able to specify "push this" as a target for expand_expr.
        !           114: 
        !           115: 124. On the 386, bad code results from foo (bar ()) when bar
        !           116: returns a double, because the pseudo used fails to get preferenced
        !           117: into an fp reg because of the distinction between regs 8 and 9.

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