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