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GCC编译器源代码

address calculation.  Based on this information, they should decide
when it is desirable to eliminate one iteration variable and create
another in its place.

It should be possible to compute what the value of an iteration
variable will be at the end of the loop, and eliminate the variable
within the loop by computing that value at the loop end.

When a loop has a simple increment that adds 1,
instead of jumping in after the increment,
decrement the loop count and jump to the increment.
This allows aob insns to be used.

* 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.

* Better handling for very sparse switches.

There may be cases where it would be better to compile a switch
statement to use a fixed hash table rather than the current
combination of jump tables and binary search.

* 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.

* More code motion.

Consider hoisting common code up past conditional branches or
tablejumps.

* Trace scheduling.

This technique is said to be able to figure out which way a jump
will usually go, and rearrange the code to make that path the
faster one.

* 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.

Some work has been done on this, in combine.c.

* Can optimize by changing if (x) y; else z; into z; if (x) y;
if z and x do not interfere and z has no effects not undone by y.
This is desirable if z is faster than jumping.

* For a two-insn loop on the 68020, such as
  foo:	movb	a2@+,a3@+
	jne	foo
it is better to insert dbeq d0,foo before the jne.
d0 can be a junk register.  The challenge is to fit this into
a portable framework: when can you detect this situation and
still be able to allocate a junk register?

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. More extensions.

* Generated unique labels.  Have some way of generating distinct labels
for use in extended asm statements.  I don't know what a good syntax would
be.

* A way of defining a structure containing a union, in which the choice of
union alternative is controlled by a previous structure component.

Here is a possible syntax for this.

struct foo {
  enum { INT, DOUBLE } code;
  auto union { case INT: int i; case DOUBLE: double d;} value : code;
};

* Allow constructor expressions as lvalues, like this:

	(struct foo) {a, b, c} = foo();

This would call foo, which returns a structure, and then store the
several components of the structure into the variables a, b, and c.

5. 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.

6. Useful warnings.

* Warn about statements that are undefined because the order of
evaluation of increment operators makes a big difference.  Here is an
example:

    *foo++ = hack (*foo);

7. Better documentation of how GCC works and how to port it.

Here is an outline proposed by Allan Adler.

I.    Overview of this document
II.   The machines on which GCC is implemented
    A. Prose description of those characteristics of target machines and
       their operating systems which are pertinent to the implementation
       of GCC.
	i. target machine characteristics
	ii. comparison of this system of machine characteristics with
	    other systems of machine specification currently in use
    B. Tables of the characteristics of the target machines on which
       GCC is implemented.
    C. A priori restrictions on the values of characteristics of target 
       machines, with special reference to those parts of the source code
       which entail those restrictions
	i. restrictions on individual characteristics 
        ii. restrictions involving relations between various characteristics
    D. The use of GCC as a cross-compiler 
	i. cross-compilation to existing machines
	ii. cross-compilation to non-existent machines
    E. Assumptions which are made regarding the target machine
	i.  assumptions regarding the architecture of the target machine
	ii. assumptions regarding the operating system of the target machine
	iii. assumptions regarding software resident on the target machine
	iv. where in the source code these assumptions are in effect made
III.   A systematic approach to writing the files tm.h and xm.h
    A. Macros which require special care or skill
    B. Examples, with special reference to the underlying reasoning
IV.    A systematic approach to writing the machine description file md
    A. Minimal viable sets of insn descriptions
    B. Examples, with special reference to the underlying reasoning
V.     Uses of the file aux-output.c
VI.    Specification of what constitutes correct performance of an 
       implementation of GCC
    A. The components of GCC
    B. The itinerary of a C program through GCC
    C. A system of benchmark programs
    D. What your RTL and assembler should look like with these benchmarks
    E. Fine tuning for speed and size of compiled code
VII.   A systematic procedure for debugging an implementation of GCC
    A. Use of GDB
	i. the macros in the file .gdbinit for GCC
	ii. obstacles to the use of GDB
	    a. functions implemented as macros can't be called in GDB
    B. Debugging without GDB
	i. How to turn off the normal operation of GCC and access specific
	   parts of GCC
    C. Debugging tools
    D. Debugging the parser
	i. how machine macros and insn definitions affect the parser
    E. Debugging the recognizer
	i. how machine macros and insn definitions affect the recognizer

ditto for other components

VIII. Data types used by GCC, with special reference to restrictions not 
      specified in the formal definition of the data type
IX.   References to the literature for the algorithms used in GCC