[Jilc-dev] Fw: IL Structs

[Gaurav Vaish]

Hello,

    Here's a relevant portion of a mail from Rhys Weatherly (man behind dotGNU)
where I recently discussed with him about how to emit il code.


Cheers,
Gaurav Vaish
http://home.iitk.ac.in/student/gvaish
http://calendar.yahoo.com/mastergaurav
---------------------------------


----- Original Message -----
From: "Rhys Weatherley" <address@hidden>
To: "Gaurav Vaish" <address@hidden>
Sent: Monday, March 11, 2002 07:11
Subject: Re: IL Structs


: Gaurav Vaish wrote:
:
: >     Besides, I have one more query - I am still a little confused about how
to
: > do label management (IL_xxxx) while converting. Reading from a PE file and
: > dumping it in il file is easy, but when converting from java code, the
labels
: > will not remain same and I need to keep track of the labels, esp those that
are
: > pointed by "br.s" and conditional jumps.
:
: If you are outputting IL source, to be fed into ilasm, then
: you can cheat a little.  Generate a label based on the
: offset within the Java method.  Then, whenever you see
: a Java branch, you use the Java offset to generate the
: label name.  e.g. consider this code to determine if an
: integer variable is non-zero or not (var != 0).
:
: 0000: iload var
: 0002: ifne 0008
: 0005: iconst_0
: 0006: goto 000A
: 0009: iconst_1
: 000A:
:
: When you generate the IL, you'll do this:
:
: L0000: ldloc var'
: L0002: brtrue.s L0008
: L0005: ldc.i4.0
: L0006: br.s L000A
: L0009: ldc.i4.1
: L000A:
:
: Note: the "brtrue.s" instruction is actually 2 bytes long,
: not 3.  But it doesn't matter because the assembler treats
: the labels as symbolic: it generates the real offsets itself.
:
: Knowing the exact IL offsets is only a problem if you want
: to generate IL bytecode directly, rather than through an
: assembler.  In that case, you will need to keep a table
: that maps Java offsets to IL offsets, and back-patch
: any opcodes that refer to later instructions.
:
: Cheers,
:
: Rhys.
: