Re: What is byte-code ?

ralph@inputplus.co.uk (Ralph Corderoy)
9 May 2005 22:31:28 -0400

          From comp.compilers

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[17 earlier articles]
Re: What is byte-code ? nathan.moore@sdc.cox.net (Nathan Moore) (2005-04-02)
Re: What is byte-code ? slimick@venango.upb.pitt.edu (John Slimick) (2005-04-11)
Re: What is byte-code ? kers@hpl.hp.com (Chris Dollin) (2005-04-11)
Re: What is byte-code ? anton@mips.complang.tuwien.ac.at (2005-04-11)
Re: What is byte-code ? nathan.moore@cox.net (Nathan Moore) (2005-04-16)
Re: What is byte-code ? gah@ugcs.caltech.edu (glen herrmannsfeldt) (2005-04-16)
Re: What is byte-code ? ralph@inputplus.co.uk (2005-05-09)
Re: What is byte-code ? gah@ugcs.caltech.edu (glen herrmannsfeldt) (2005-05-13)
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From: ralph@inputplus.co.uk (Ralph Corderoy)
Newsgroups: comp.compilers
Date: 9 May 2005 22:31:28 -0400
Organization: Compilers Central
References: 05-03-015 05-03-125 05-04-005 05-04-037
Keywords: VM, interpreter

Hi Anton,


Anton Ertl <anton@mips.complang.tuwien.ac.at> wrote:
> Nathan Moore <nathan.moore@sdc.cox.net> writes:
> > This will save you about 2 jumps per instructoin since the jump tables
> > that are usually spit out by compilers are:
> >
> > jump ip+(constant*case)
> > jump CODE_THAT_ACTUALLY_DOES_CASE_0
> > jump CODE_THAT_ACTUALLY_DOES_CASE_1
> > ...
>
> I have never seen that kind of code generated. Which compiler does it
> that way?


I've seen the Norcroft and gcc C compilers targetted at ARM generate
such code.


                ldrb r1, [r11, #0]
                cmp r1, #7
                addls pc, pc, r1, lsl #2 ;
                b case_8
                b case_0
                b case_1
                b case_2
                b case_3
                b case_4
                b case_5
                b case_6


        .case_7
                ldr r1, [r11, #&028]
                mov r0, r11


With a 32-bit word, pc is already two words ahead of the current
instruction due to pipe-lining. This is the idiomatic jump table in ARM
and assembly programmers use it too.


> Big, dense switch statements have been translated into a range check,
> an array access (for the target address), and an indirect jump to the
> target in the code I looked at (usually by gcc).


The assembler above only works when the relative jump destination can be
encoded along with the b instruction in the 32-bit word.


Cheers,


--
Ralph Corderoy. http://inputplus.co.uk/ralph/ http://troff.org/


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