gnu1987/gnu1988/gcc-1.21/PROJECTS

1. Better optimization.

* More cse

The techniques for doing full global cse are described in the
red dragon book.  It is likely to be slow and use a lot of memory,
but it might be worth offering as an additional option.

It is probably possible to extend cse to a few very frequent cases
without so much expense.

For example, it is not very hard to handle cse through if-then
statements with no else clauses.  Here's how to do it.  On reaching a
label, notice that the label's use-count is 1 and that the last
preceding jump jumps conditionally to this label.  Now you know it
is a simple if-then statement.  Remove from the hash table
all the expressions that were entered since that jump insn
and you can continue with cse.

It is probably not hard to handle cse from the end of a loop
around to the beginning, and a few loops would be greatly sped
up by this.

* Iteration variables and strength reduction.

The red dragon book describes standard techniques for these kinds
of loop optimization.  But be careful!  These optimization techniques
don't always make the code better.  You need to avoid performing
the standard transformations unless they are greatly worth while.

In many common cases it is possible to deduce that an iteration
variable is always positive during the loop.  This information
may make it possible to use decrement-and-branch instructions
whose branch conditions are inconvenient.  For example, the 68000
`dbra' instruction branches if the value was not equal to zero.
Therefore, it is not applicable to `for (i = 10; i >= 0; i--)'
unless the compiler can know that I will never be negative
before it is decremented.

* Using constraints on values.

Many operations could be simplified based on knowledge of the
minimum and maximum possible values of a register at any particular time.
These limits could come from the data types in the tree, via rtl generation,
or they can be deduced from operations that are performed.  For example,
the result of an `and' operation one of whose operands is 7 must be in
the range 0 to 7.  Compare instructions also tell something about the
possible values of the operand, in the code beyond the test.

Value constraints can be used to determine the results of a further
comparison.  They can also indicate that certain `and' operations are
redundant.  Constraints might permit a decrement and branch
instruction that checks zeroness to be used when the user has
specified to exit if negative.

* Smarter reload pass.

The reload pass as currently written can reload values only into registers
that are reserved for reloading.  This means that in order to use a
register for reloading it must spill everything out of that register.

It would be straightforward, though complicated, for reload1.c to keep
track, during its scan, of which hard registers were available at each
point in the function, and use for reloading even registers that were
free only at the point they were needed.  This would avoid much spilling
and make better code.

* Change the type of a variable.

Sometimes a variable is declared as `int', it is assigned only once
from a value of type `char', and then it is used only by comparison
against constants.  On many machines, better code would result if
the variable had type `char'.  If the compiler could detect this
case, it could change the declaration of the variable and change
all the places that use it.

* Order of subexpressions.

It might be possible to make better code by paying attention
to the order in which to generate code for subexpressions of an expression.

* Better code for switch statements.

If a switch statement has only a few cases, a sequence of conditional
branches is generated for it, rather than a jump table.  It would
be better to output a binary tree of branches.

* Distributive law.

The C expression *(X + 4 * (Y + C)) compiles better on certain
machines if rewritten as *(X + 4*C + 4*Y) because of known addressing
modes.  It may be tricky to determine when, and for which machines, to
use each alternative.

* Jump-execute-next.

Many recent machines have jumps which optionally execute the following
instruction before the instruction jumped to, either conditionally or
unconditionally.  To take advantage of this capability requires a new
compiler pass that would reorder instructions when possible.  After
reload may be a good place for it.

On some machines, the result of a load from memory is not available
until after the following instruction.  The easiest way to support
these machines is to output each RTL load instruction as two assembler
instructions, the second being a no-op.  Putting useful instructions
after the load instructions may be a similar task to putting them
after jump instructions.

* Pipeline scheduling.

On many machines, code gets faster if instructions are reordered
so that pipelines are kept full.  Doing the best possible job of this
requires knowing which functional units each kind of instruction executes
in and how long the functional unit stays busy with it.  Then the
goal is to reorder the instructions to keep many functional units
busy but never feed them so fast they must wait.

2. Simpler porting.

Right now, describing the target machine's instructions is done
cleanly, but describing its addressing mode is done with several
ad-hoc macro definitions.  Porting would be much easier if there were
an RTL description for addressing modes like that for instructions.
Tools analogous to genflags and genrecog would generate macros from
this description.

There would be one pattern in the address-description file for each
kind of addressing, and this pattern would have:

  * the RTL expression for the address
  * C code to verify its validity (since that may depend on
    the exact data).
  * C code to print the address in assembler language.
  * C code to convert the address into a valid one, if it is not valid.
    (This would replace LEGITIMIZE_ADDRESS).
  * Register constraints for all indeterminates that appear
    in the RTL expression.

3. Other languages.

Front ends for Pascal, Fortran, Algol, Cobol, Modula-2 and Ada are
desirable.

Pascal, Modula-2 and Ada require the implementation of functions
within functions.  Some of the mechanisms for this already exist.

4. Generalize the machine model.

Some new compiler features may be needed to do a good job on machines
where static data needs to be addressed using base registers.

Some machines have two stacks in different areas of memory, one used
for scalars and another for large objects.  The compiler does not
now have a way to understand this.