🚀 go-pugleaf

In article <1gt7r0INNhrd@usenet.INS.CWRU.Edu> an375@cleveland.Freenet.Edu (Jason Baker) writes:
>>(gcc can compile shared text programs if it is given the -mbaserel
>>parameter)

>	I was wondering what that meant.  The new mintlib readme metions 
>-mpcrel (or something).  What does this mean; under what conditions will
>gcc generate non-pc-relative code?  Does the new gcc support both shared 
>and private data in the same program?

The -mbaserel and -mpcrel options are independent, they just both happened
to be added at the same time. The pc-relative stuff is very primitive, and
of only marginal use. Ordinarily GCC generates long-absolute addresses for
everything (except stack slots and register variables, of course). The
PC-relative option causes GCC to try pc-relative addressing for any items
that reside in the text segment. There's no error-checking in the compiler
though, so it's possible to wind up with a pc-relative reference that
doesn't fit in the available 16 bit offset. (Oh, correction, GCC always uses
pc-relative code for generating the jump table for switch statements...)

The base-relative option causes all static and global variable references to
be made using a 16 bit offset from a base register (currently it's configured
to use register a4). Using this setup, there are no absolute references to
data or bss from the code, which allows a single copy of the code to be used
by multiple independent processes. In this approach, all data is private.
However, any items declared as "const" will go in the text segment, and so can
be considered to be shared initialized data (except that it's read-only data).
The compiler support for base-relative also doesn't do any error checking, so
it's also possible to generate code that won't successfully link. (The linker
performs all of the checks to see if all the addresses fit in their given
16 bit offsets.)

I'm working on an enhancement to make the linker perform the pc-relative code
transformations, since it will have enough information to do a better job at
it, as well as an improved version of the base-relative code generation that
will be less affected by size constraints...
-- 
  -- Howard Chu @ Jet Propulsion Laboratory, Pasadena, CA

All true wisdom is conveyed in one-line witticisms.