From patchwork Sun Nov 4 11:42:52 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Gerald Pfeifer X-Patchwork-Id: 197001 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from sourceware.org (server1.sourceware.org [209.132.180.131]) by ozlabs.org (Postfix) with SMTP id 76BDD2C00C1 for ; Sun, 4 Nov 2012 22:43:13 +1100 (EST) Comment: DKIM? See http://www.dkim.org DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; d=gcc.gnu.org; s=default; x=1352634195; h=Comment: DomainKey-Signature:Received:Received:Received:Received:Date: From:To:Subject:Message-ID:MIME-Version:Content-Type: Mailing-List:Precedence:List-Id:List-Unsubscribe:List-Archive: List-Post:List-Help:Sender:Delivered-To; bh=aN6Ll9V6g9Mg9eSfTqH+ 1qTepsk=; b=a790nBD9ywqSQ+3fofmBrblDFimAIYhofhP0oTxkAQtFo0yUcdla lXkj+1gqTBhdQ05yeStTC16zuvqISikXtUXjGdUCkOIkXikiTLQvIgWHv4D7AJ1R 5ANBaQSR+YS144auzYZZ8opWtIb7TXvqtlvcTanAR4SOsxRgk3GBSok= Comment: DomainKeys? See http://antispam.yahoo.com/domainkeys DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=default; d=gcc.gnu.org; h=Received:Received:X-SWARE-Spam-Status:X-Spam-Check-By:Received:Received:Date:From:To:Subject:Message-ID:MIME-Version:Content-Type:X-IsSubscribed:Mailing-List:Precedence:List-Id:List-Unsubscribe:List-Archive:List-Post:List-Help:Sender:Delivered-To; b=CNe6p05sojuFVReDZNEZpUXUfMG8AhpeL83DY8ox9JaXqhlIfZwhXEthsNFLCi dtq7yMiolvoqvm1WlYROH5+vvJin3ClvO8xCo6T9IEG6ROMttKemJZcrhaKMhDZq Pl5ytA+PT2UdswPfaShUayX0kD2Pg78S6UcGRkZW6UfPk=; Received: (qmail 27862 invoked by alias); 4 Nov 2012 11:43:05 -0000 Received: (qmail 27845 invoked by uid 22791); 4 Nov 2012 11:43:04 -0000 X-SWARE-Spam-Status: No, hits=-48.7 required=5.0 tests=AWL, BAYES_50, SARE_OBFU_PART_CIA X-Spam-Check-By: sourceware.org Received: from ainaz.pair.com (HELO ainaz.pair.com) (209.68.2.66) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Sun, 04 Nov 2012 11:42:51 +0000 Received: from [192.168.0.129] (vie-188-118-240-184.dsl.sil.at [188.118.240.184]) by ainaz.pair.com (Postfix) with ESMTPSA id 3C8F33F40F; Sun, 4 Nov 2012 06:42:48 -0500 (EST) Date: Sun, 4 Nov 2012 12:42:52 +0100 (CET) From: Gerald Pfeifer To: gcc-patches@gcc.gnu.org, java-patches@gcc.gnu.org Subject: [wwwdocs,Java] Fix up java/papers/compiling.html Message-ID: MIME-Version: 1.0 X-IsSubscribed: yes Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Delivered-To: mailing list gcc-patches@gcc.gnu.org Looking at that page, I noticed a lot of broken navigation (broken images,...). This removes that navigation and also address the worst in terms of broken markup. Still bin/preprocess excludes this file from the regular treatment that our general web pages get; that'll be next, but it'll require a fair number of additional cleanups. Gerald 2012-11-03 Gerald Pfeifer * papers/compiling.html: Remove broken heading/navigation. Make a huge amount of markup fixes. Index: papers/compiling.html =================================================================== RCS file: /cvs/gcc/wwwdocs/htdocs/java/papers/compiling.html,v retrieving revision 1.1 diff -u -3 -p -r1.1 compiling.html --- papers/compiling.html 10 Mar 1999 17:41:24 -0000 1.1 +++ papers/compiling.html 3 Nov 2012 15:13:46 -0000 @@ -5,132 +5,23 @@ - - - - - - - - -
- Cygnus Solutions -
- - - - - - - - -
- - Cygnus - Technical Resources - -

About Cygnus - -

News and Press - -

Jobs at Cygnus - -

Products - -

Success Stories - -

Cygwin32EGCS Project - -

- - - - *News Image -

Compiling Java for Embedded Systems

By Per Bothner (bothner@cygnus.com)

-While a major factor in Java?s success is +While a major factor in Java's success is its use of portable bytecodes, we believe it cannot become a mainstream programming language without mainstream implementation techniques. Specifically, an optimizing, ahead-of-time compiler allows much better optimization along with much faster application start-up times than with JIT translators. Cygnus Solutions is writing a Java -front-end for the GNU compiler, gcc, in order to translate Java +front-end for the GNU Compiler Collection, GCC, in order to translate Java bytecodes to machine code. This uses a widely used, proven technology. In this paper, we discuss issues in implementing Java using traditional compiler, linker, and debugging technology; @@ -140,47 +31,50 @@ memory environments.

1 Java implementation

-Java (see Bibliography for JavaSpec) has +

Java (see Bibliography for JavaSpec) has become popular because it is a decent programming language, is buzzword-compliant (object-oriented and web-enabled), and because it is implemented by compiling to portable bytecodes (see Bibliography for JavaVMSpec). The traditional Java implementation model is to compile Java source code into -.class files containing machine-independent byte-code -instructions. These .class files are downloaded and +.class files containing machine-independent byte-code +instructions. These .class files are downloaded and interpreted by a browser or some other Java "Virtual Machine" (VM). See Figure 1 for a model of the traditional Java implementation.

-

+

Figure 1: Typical Java implementation

@@ -195,22 +89,22 @@ C or C++. There are three main drawbacks compared to conventional compilers:

    -

  • The compilation is done every time the application is executed, +
  • The compilation is done every time the application is executed, which means start-up times are much worse than pre-compiled -code.

    • +code. -

  • Since the JIT compiler has to run fast (it is run every time +
  • Since the JIT compiler has to run fast (it is run every time the application is run), it cannot do any non-trivial optimization. Only simple register allocation and peep-optimizations are practical. (Some have suggested that JIT could potentially produce faster code than a stand-alone compiler, since it can dynamically adjust to specific inputs and hardware. But the need for quick re-compilation will make it very difficult to make JIT faster in -practice.)

    • +practice.) -

  • The JIT compiler must remain in (virtual) memory while the +
  • The JIT compiler must remain in (virtual) memory while the application is executing. This memory may be quite costly in an -embedded application.

    • +embedded application.

While JIT compilers have an important place in a Java system, for @@ -231,7 +125,7 @@ system. See Figure 2 compilation process.

- +

Figure 2:  Compiled Java

@@ -239,7 +133,7 @@ compilation process.

2 Issues with embedded Java

-

Sun?s motto for Java is "Write once, run anywhere". As +

Sun's motto for Java is "Write once, run anywhere". As part of that, Sun has been pushing Java as also suitable for embedded systems, announcing specifications for "Embedded Java" and "Personal Java" specifications. The latter (just published @@ -261,7 +155,7 @@ because of its complexity will find Java

Top of Page

-

2.1 Advantages of Java

+

2.1 Advantages of Java

Java has a number of advantages for embedded systems. Using classes @@ -275,37 +169,38 @@ Java.

Java programs are generally more portable than C or C++ programs:

    -

  • The size of integer, floats, and character is defined by the -language.

    • +
  • The size of integer, floats, and character is defined by the +language.
    • -

  • The order and precision of expression evaluation is -defined.

    • +
  • The order and precision of expression evaluation is +defined.
    • -

  • Initial values of fields are defined, and the languages require +
  • Initial values of fields are defined, and the languages require that local variables be set before use in a way that the compiler can -check.

    • +check.

In fact, the only major non-determinacy in Java is due to time-dependencies between interacting threads. Safety-critical applications will find the following features very useful:

-
    -

  • More disciplined use of pointers, called "references" +
      +
    • More disciplined use of pointers, called "references" instead, provides "pointer safety" (no dangling references; all references are either null or point to an actual object; -de-referencing a null pointer raises an exception).

      • +de-referencing a null pointer raises an exception). -

    • Array indexing is checked, and an index out of bounds raises an -exception.

      • +
    • Array indexing is checked, and an index out of bounds raises an +exception.
      • -

    • Using exceptions makes it easier to separate out the normal +
    • Using exceptions makes it easier to separate out the normal case from error cases, and to handle the error in a disciplined -manner.

    +manner.
    • +

The portability of Java (including a portable binary format) means that an application can run on many hardware platforms, with no -porting effort (at least that?s the theory).

+porting effort (at least that's the theory).

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@@ -325,11 +220,11 @@ take 134 bytes, while the compiled instr are also pre-compiled; otherwise, 417 and 540 bytes are needed, respectively.) However, there is quite a bit of symbol table information necessary, bringing the actual size of the -Fib.class file up to 1227 bytes. How much space will +Fib.class file up to 1227 bytes. How much space will actually be used at run-time depends on how the symbolic (reflective) information is represented - but it does take a fair bit of space. (Pre-compiled code also needs space for the reflective data -structure ?about 520 bytes for this example.) Out tentative +structure, about 520 bytes for this example.) Our tentative conclusion is that the space advantage of bytecodes is minor at best, whereas the speed penalty is major.

@@ -341,8 +236,7 @@ whereas the speed penalty is major.

In addition to the space needed for the user code, there is also a large chunk of fixed code for the run-time environment. This includes -code for the standard libraries (such as java.lang), code +code for the standard libraries (such as java.lang), code for loading new classes, the garbage collector, and an interpreter or just-in-time compiler.

@@ -352,21 +246,21 @@ loading new classes at run-time, we can class files, and interpreting (or JIT-compiling) bytecodes. (If you have a dynamic loader, you could still down-load new classes, if you compile them off-line.) Similarly, some embedded applications might -not need support for Java?s Abstract Windowing Toolkit, or +not need support for Java's Abstract Windowing Toolkit, or networking, or decryption, while another might need some or all of these.

Depending on a conventional (static) linker to select only the code that is actually needed does not work, since a Java class can be referenced using a run-time String expression passed to the -Class.forName method. If that feature is not used, then a +Class.forName method. If that feature is not used, then a static linker can be used.

The Java run-time needs to have access to the name and type of every field and method of every class that is loaded. This is not needed for normal operation (especially not when using precompiled methods); however, a program can examine this information using the -java.lang.reflect package. Furthermore, +java.lang.reflect package. Furthermore, the Java Native Interface (JNI, a standard ABI for interfacing between C/C++ and Java) works by looking up fields and methods by name at run-time. Using the @@ -399,14 +293,14 @@ true that garbage collection usually tak execution time, and can lead to large unpredictable pauses. However, it is important to remember that is also an issue for manual heap allocations using malloc and free. There are many very poorly written -malloc/free implementations in common use, +malloc/free implementations in common use, just as there are inefficient implementations of garbage collection.

There are a number of incremental, parallel, or generational garbage collection algorithms that provide performance as good or -better than malloc/free. What is +better than malloc/free. What is difficult, however, is ensuring that pause times are -bounded?i.e., a small limit on +bounded, i.e. a small limit on the amount of time a new can take, even if garbage collection is triggered. The solution is to make sure to do a little piece of garbage collection on each allocation. Unfortunately, the only known @@ -418,34 +312,32 @@ real-time can be implemented at reasonab

3 Compiling Java

-

The core tool of Cygnus?s Java implementation is the -compiler. This is jc1, a new gcc front-end (see +

The core tool of Cygnus's Java implementation is the +compiler. This is jc1, a new GCC front-end (see Bibliography for GCC). This has similar structure -as existing front-ends (such as cc1plus, the C++ +as existing front-ends (such as cc1plus, the C++ front-end), and shares most of the code with them. The most unusual -aspect of jc1 is that its "parser" reads +aspect of jc1 is that its "parser" reads either Java source files or Java bytecode files. (The first release will only directly support bytecodes; parsing Java source will -be done by invoking Sun?s javac. A +be done by invoking Sun's javac. A future version will provide an integrated Java parser, largely for the sake of compilation -speed.) In any case, it is important that jc1 can read +speed.) In any case, it is important that jc1 can read bytecodes, for the following reasons:

    -

  • It is the natural way to get declarations of external classes; +
  • It is the natural way to get declarations of external classes; in this respect, a Java bytecode file is like a C++ pre-compiled -header file.

    • +header file. -

  • It is needed so we can support code produced from other tools +
  • It is needed so we can support code produced from other tools that produce Java bytecodes (such as the Kawa Scheme-to-Java-bytecode -compiler; see Bibliography for Kawa).

    • +compiler; see Bibliography for Kawa). -

  • Some libraries are (unfortunately) distributed as Java -bytecodes without source.

    • +
  • Some libraries are (unfortunately) distributed as Java +bytecodes without source.

Much of the work of the compiler is the same whether we are @@ -463,7 +355,7 @@ Transforming bytecodes.

3.1 Transforming bytecodes

-

This section describes how jc1 works.

+

This section describes how jc1 works.

The executable content of a bytecode file contains a vector of bytecode instructions for each (non-native) method. The bytecodes are @@ -473,52 +365,49 @@ local variables are initialized with the local variable or stack "slot" is a word big enough for either a 32-bit integer, a float, or an object reference (pointer). (Two slots are used for 64-bits doubles and longs.) The -slots are not typed; i.e., at one point, a slot might contain +slots are not typed; i.e., at one point, a slot might contain an integer value, and, at another point, the same slot might contain an object reference. However, you cannot store an integer in a slot, and then retrieve the same bits re-interpreted as an object reference. Moreover, at any given program point, each slot has a unique type can be determined using static data flow. (The type may be "unassigned", in which case you are not allowed to read the -slot?s value.) These restrictions are part of Java security model, +slot's value.) These restrictions are part of Java security model, and are enforced by the Java bytecode verifier. We do a similar -analysis in jc1, which lets us know for +analysis in jc1, which lets us know for every program point, the current stack pointer, and the type of every local variable and stack slot.

-

Internally gcc uses two main +

Internally GCC uses two main representations: The tree representation is at the level of an abstract syntax tree, and is used to represent high-level (fully-typed) expressions, declarations, and -types. The rtl (Register Transform +types. The rtl (Register Transform Language) form is used to represent instructions, instruction patterns, and machine-level calculations in general. Constant folding is done using -trees, but otherwise most optimizations +trees, but otherwise most optimizations are done at the -rtl level.

+rtl level.

The basic strategy for translating Java stack-oriented bytecodes is that we create a dummy local variable for each Java local variable or -stack slot. These are mapped to gcc "virtual -registers," and standard gcc register +stack slot. These are mapped to GCC "virtual +registers," and standard GCC register allocation later assigns each virtual register to a hard register or a stack location. This makes it easy to map each opcode into -tree-nodes or rtl to manipulate the virtual +tree-nodes or rtl to manipulate the virtual registers.

-

As an example, consider how to compile iadd, which adds +

As an example, consider how to compile iadd, which adds the top two ints on the stack. For illustration, assume the stack pointer is 3, and virtual registers 50, 51, and 52 are associated with -stack slots 0, 1, and 2. The code generated by jc1 is the +stack slots 0, 1, and 2. The code generated by jc1 is the following:

- reg51 := vreg51 + vreg52 + reg51 := vreg51 + vreg52

Note that the stack exists only at compile-time. There is no stack, stack pointer, or stack operations in the emitted code.

@@ -527,66 +416,66 @@ stack pointer, or stack operations in th compilers:

    -

  • We would like to do constant folding, which is done at the -tree level. However, tree nodes are -typed.

    • +
  • We would like to do constant folding, which is done at the +tree level. However, tree nodes are +typed.
    • -

  • The simple-minded approach uses lots of virtual registers, and +
  • The simple-minded approach uses lots of virtual registers, and the code generated is very bad. Running the optimizer (with the --O flag) fixes the generated code, but you still get a lot +-O flag) fixes the generated code, but you still get a lot of useless stack slots. It would be nice not to have to run the optimizer, and if you do, not to make unnecessary work for -it.

    • +it. -

  • The rtl representation is semi-typed, since it +
  • The rtl representation is semi-typed, since it distinguishes the various machine "modes" such as pointer, and different-sized integers and floats. This causes problems because a given Java stack slot may have different types at different points -in the code.

    • +in the code.

The last problem we solve by using a separate virtual register for each machine mode. For example, for local variable slot 5 we might use -vreg40 when it contains an integer, and vreg41 +vreg40 when it contains an integer, and vreg41 when it points an object reference. This is safe, because the Java verifier does not allow storing an integer in an object slot and later reusing the same value as an object reference or float.

The other two problems we solve by modeling the stack as a stack of -tree nodes, and not storing the results in their +tree nodes, and not storing the results in their "home" virtual registers unless we have to. Thus jc1actually -does the following for iadd:

+does the following for iadd:

-

+

tree arg1 = pop_value (int_type_node);
tree arg2 = pop_value (int_type_node);
push_value (fold (build (PLUS_EXPR, int_type_node, arg1, -arg2)));

+arg2)));

-

The build function is the standard gcc function for -creating tree-nodes, while fold is the standard function -for doing constant folding. The functions pop_value and -push_value are specific to jc1, and keep track -of which stack location corresponds to which tree node. No -code is actually generated?yet.

+

The build function is the standard GCC function for +creating tree-nodes, while fold is the standard function +for doing constant folding. The functions pop_value and +push_value are specific to jc1, and keep track +of which stack location corresponds to which tree node. No +code is actually generated yet.

This works for straight-line code (i.e. within a basic block). When we come to a branch or a branch target, we have to flush the stack of -tree nodes, and make sure the value of each stack slot gets +tree nodes, and make sure the value of each stack slot gets saved in its "home" virtual register. The stack is usually empty at branches and branch targets, so this does not happen very -often. Otherwise, we only emit actual rtl instructions to +often. Otherwise, we only emit actual rtl instructions to evaluate the expressions when we get to a side-effecting operation (such as a store or a method call).

Since we only allocate a virtual register when we need one, we are now using fewer virtual registers, which leads to better code. We also -get the benefits of gcc constant folding, plus the existing +get the benefits of GCC constant folding, plus the existing machinery for selecting the right instructions for addition and other expressions.

The result is that we end up generating code using the same -mechanisms that the gcc C and C++ front-ends do, and +mechanisms that the GCC C and C++ front-ends do, and therefore we can expect similar code quality.

Top of Page

@@ -617,26 +506,26 @@ for embedded systems.

A possible solution is to emit something like:

-

+ static const char FooClassData[] = "\xCa\xfe\xba\xbe...";
static {
LoadClassFromByteArray(FooClassData);
Patch up method to point to code;
-

+
-

The code marked static is compiled into +

The code marked static is compiled into a dummy function that is executed at program startup. This can be handled using whatever mechanism is used to execute C++ static initializers. This dummy function reads the meta-data in external format from -FooClassData, creates the internal +FooClassData, creates the internal representation, and enters the class into the class table. It then patches up the method descriptors so that they point to the compiled code.

This works, but it is rather wasteful in terms of memory and startup time. We need space for both the external representation (in -FooClassData) and the internal representation, and we have +FooClassData) and the internal representation, and we have to spend time creating the latter from the former. It is much better if we can have the compiler directly create the internal representation. If we use initialized static data, we can have the @@ -647,18 +536,18 @@ symbol table.

Consider the following example class:

-

+ public class Foo extends Bar {
    public int a;
    public int f(int j) { return a+j; }
};
-

+

That class compiles into something like the following:

-

int Foo_f (Foo* this, int j)
+int Foo_f (Foo* this, int j)
{ return this->a + j; }

struct Method Foo_methods[1] = {{
@@ -679,8 +568,8 @@ struct Class Foo_class = {

static {
     RegisterClass (&Foo_class, "Foo");
-}

- +} +

Thus startup is fast, and does not require any dynamic allocation.

@@ -690,13 +579,13 @@ static {

3.3 Static references

-

A class may make references to static +

A class may make references to static fields and methods of another class. If we can assume that the other -class will also be jc1-compiled, then -jc1 can emit a direct reference to the -external static field or method (just like -a C++ compiler would). That is, a call to a static method can be compiled as a direct +class will also be jc1-compiled, then +jc1 can emit a direct reference to the +external static field or method (just like +a C++ compiler would). That is, a call to a +static method can be compiled as a direct function call. If you want to make a static call from a pre-compiled class to a known class that you do not know is pre-compiled, you can try using extra indirection and a "trampoline" stub @@ -719,22 +608,22 @@ time.

3.4 Linking

-

The jc1 program creates standard +

The jc1 program creates standard assembler files that are processed by the standard unmodified GNU assembler. The resulting object files can be placed in a dynamic or static library, or linked (together with the run-time system) into an executable, using a standard linker. The only linker extension we really need is support for static initializers, but we already have -that (since gcc supports C++).

+that (since GCC supports C++).

While we do not need any linker modification, there are some that may be desirable. Here are some ideas.

-

Java needs a lot of names at run-time ?for classes, +

Java needs a lot of names at run-time for classes, files, local variables, methods, type signatures, and source -files. These are represented in the constant pool of .class files as CONSTANT_Utf8 values. Compilation removes the +files. These are represented in the constant pool of +.class files as +CONSTANT_Utf8 values. Compilation removes the need for many of these names (because it resolves many symbolic field and method references). However, names are still needed for the meta-data. Two different class files may generate two references the @@ -765,20 +654,20 @@ able to dynamically load new bytecoded c needed in some embedded environments. Basically, the appropriate Java run-time environment is a Java Virtual Machine.

-

It is possible to have jc1 produce code +

It is possible to have jc1 produce code compatible with the Java Native Interface ABI. Such code could run under any Java VM that implements the JNI. However, the JNI has relatively high overhead, so if you are not concerned about binary portability it is better to use a more low-level ABI, similar to the -VM?s internal calling convention. (If you are concerned about -portability, use .class files.) While we +VM's internal calling convention. (If you are concerned about +portability, use .class files.) While we plan to support the portable JNI, we will also support such a lower-level ABI. Certainly the standard Java functionality (such as -that in java.lang will be compiled to the +that in java.lang will be compiled to the lower-level ABI.

A low-level ABI is inherently dependent on a specific VM. We are -using Kaffe?a free Java VM, written by Tim Wilkinson +using Kaffe, a free Java VM, written by Tim Wilkinson (see Bibliography for Kaffe), with help from volunteers around the "Net." Kaffe uses either a JIT compiler on many common platforms, or a conventional bytecode @@ -790,8 +679,8 @@ calling convention).

We are making many enhancements to make Kaffe a more suitable target for pre-compiled code. One required change is to add a hook so that pre-compiled and pre-allocated classes can be added to the global -table of loaded classes. This means implementing the RegisterClass function.

+table of loaded classes. This means implementing the +RegisterClass function.

Other changes are not strictly needed, but are highly desirable. The original Kaffe meta-data had some redundant @@ -809,8 +698,8 @@ replacing linked lists by arrays.

4.1 Debugging

-

Our debugging strategy for Java is to enhance gdb (the GNU debugger) so it can understand +

Our debugging strategy for Java is to enhance +GDB (the GNU debugger) so it can understand Java-compiled code. This follows from our philosophy of treating Java like other programming languages. This also makes it easier to debug multi-language applications (C and Java).

@@ -822,45 +711,44 @@ Java expressions and primitive types are C++.

Adding support for Java objects is somewhat more work. Getting, -setting, and printing of Object fields is +setting, and printing of Object fields is basically the same as for C++. Printing an Object reference can be -done using a format similar to that used by the default toString method ?the class followed by the -address such as java.io.DataInput@cf3408. Sometimes you instead +done using a format similar to that used by the default +toString method the class followed by the +address such as +java.io.DataInput@cf3408. Sometimes you instead want to print the contents of the object, rather than its address (or identity). Strings should, by default, be printed using -their contents, rather than their address. For other objects, gdb can invoke the toString method to get a printable +their contents, rather than their address. For other objects, +GDB can invoke the toString method to get a printable representation, and print that. However, there should be different options to get different styles of output.

-

gdb can evaluate a general +

GDB can evaluate a general user-supplied expression, including a function call. For Java, this means we must add support for invoking a method in the program we are -debugging. Thus, gdb has to be able to +debugging. Thus, GDB has to be able to know the structure of the Java meta-data so it can find the right -method. Alternatively, gdb could invoke -functions in the VM to do the job on its behalf.

+method. Alternatively, GDB could invoke +functions in the VM to do the job on its behalf.

-

gdb has an internal representation of +

GDB has an internal representation of the types of the variables and functions in the program being debugged. Those are read from the symbol-table section of the executable file. To some extent this information duplicates the -meta-data that we already need in the program?s address space. We +meta-data that we already need in the program's address space. We can save some file space if we avoid putting duplicate meta-data in -the symbol table section, and instead extend gdb -so it can get the information it needs from the running -process. This also makes gdb start-up faster, since it makes it easier -to only create type information when needed.

+the symbol table section, and instead extend GDB +so it can get the information it needs from the running +process. This also makes GDB start-up faster, since it makes it easier +to only create type information when needed.

Potentially duplicated meta-data includes the source line numbers. This is because a Java program needs to be able to do a stack trace, even without an external debugger. Ideally, the stack trace should include source line numbers. Therefore, it is best to put the line numbers in a special read-only section of the executable. This -would be pointed to by the method meta-data, where both gdb and the +would be pointed to by the method meta-data, where both GDB and the internal Java stack dumper can get at it. (For embedded systems one would probably leave out line numbers in the run-time, and only keep it in the debug section of the executable file.)

@@ -873,13 +761,13 @@ we will want to debug bytecodes that hav the VM. This problem is eased if the VM uses JIT (as Kaffe does), since in that case the representation of dynamically-(JIT-)compiled Java code is the same as pre-compiled code. However, we still need to -provide hooks so that gdb knows when a new +provide hooks so that GDB knows when a new class is loaded into the VM.

-

Long-term, it might be useful to download Java code into gdb itself (so we can extend gdb using Java), but that requires integrating a -Java evaluator into gdb.

+

Long-term, it might be useful to download Java code into +GDB itself (so we can extend GDB +using Java), but that requires integrating a +Java evaluator into GDB.

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@@ -890,34 +778,34 @@ Java evaluator into One problem with Java is the lack of profiling tools. This makes it difficult to track down the "hot-spots" in an application. Using GCC to compile Java to native code lets us use -existing profiling tools, such as gprof, -and the gcov coverage analyzer.

+existing profiling tools, such as gprof, +and the gcov coverage analyzer.

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5 Status

-

As of early July 1997, jc1 was able to +

As of early July 1997, jc1 was able to compile a simple test program, which calculates the Fibonacci numbers (up to 36), both iteratively and recursively (which is slow!), and prints the results. No manual tinkering was needed with the assembly -code generated, which was assembled and linked in with kaffe (a modified pre-0.9.1 snapshot) as the +code generated, which was assembled and linked in with +kaffe (a modified pre-0.9.1 snapshot) as the run-time engine. On a SparcStation 5 running Solaris2, it took 16 seconds to execute. In comparison, the same program dynamically -compiled by Kaffe?s JIT takes 26 seconds, and Sun?s JDK 1.1 +compiled by Kaffe's JIT takes 26 seconds, and Sun's JDK 1.1 takes 88 seconds to run the same program.

These numbers are encouraging, but they need some context. Start-up times (for class loading and just-in-time compilation) should in all cases be fairly minor, since the execution time is dominated by -recursive calls. In the jc1-compiled case, +recursive calls. In the jc1-compiled case, only the actually test class (calculating Fibonacci plus the main methods that prints the result) is compiled by jc1; all the other -classes loaded (including the classes for I/O) are compiled by kaffe?s JIT-compiler. This means there would -be some slight extra speed up if all the classes were jc1-compiled. Do note that the test-program uses +classes loaded (including the classes for I/O) are compiled by +kaffe's JIT-compiler. This means there would +be some slight extra speed up if all the classes were +jc1-compiled. Do note that the test-program uses simple C-like features that are easy to compile: integer arithmetic, simple control structures, and direct recursion. The results cannot be directly generalized to larger programs that use object-oriented @@ -926,16 +814,15 @@ features more heavily.

The basic structure of the compiler works, but there is still quite a bit of work to do. Many of the byte-codes are not supported yet, and neither is exception handling. Only some very simple Java-specific -optimizations are implemented. (Of course, jc1 benefits from the existing -language-independent optimizations in gcc.)

+optimizations are implemented. (Of course, +jc1 benefits from the existing +language-independent optimizations in GCC.)

The Kaffe VM works on most Java code. We have enhanced it in various ways, and modified the data structures to be simpler and more suitable for being emitted by the compiler.

-

The Java support in the gdb debugger is +

The Java support in the GDB debugger is partially written, but there is still quite a bit to go before it is user-friendly. We have not started work on our own library implementation or source-code compiler.

@@ -951,38 +838,32 @@ Spring 1997.

Bibliography

-

[GCC] Using GNU CC
+

[GCC] Using GNU CC
Richard Stallman
Free Software Foundation, 1996.

-

[JavaSpec] The Java Language -Specification
- +

[JavaSpec] The Java Language +Specification
James Gosling, Bill Joy, Guy Steele
Addison-Wesley, 1996.

-

[JavaVMSpec] The Java Virtual Machine -Specification
+

[JavaVMSpec] The Java Virtual Machine +Specification
Tim Lindholm, Frank Yellin
Addison-Wesley, 1996.

-

[Kaffe] Kaffe?a virtual machine to run -Java code
-Tim Wilkinson
+

[Kaffe] Kaffe, a virtual machine to run +Java code
+Tim Wilkinson
http://www.kaffe.org/

-

[Kawa] Kawa, the Java-based Scheme System
-Per Bothner
+ +

[Kawa] Kawa, the Java-based Scheme System
+Per Bothner
http://www.cygnus.com/~bothner/kawa.html

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-