fix memory regions with displaced BSS data

[Nathan Sidwell]

This patch fixes a problem with memory regions and bss-like sections.  consider 
the following linker script:

  SECTIONS {
    .text : { *(.text) } >ram AT>rom
    .data : { *(.data) } >ram
    .bss : { *(.bss) } >ram
    .trail : { LONG(5) } >ram

Here, we're storing an initial image in ROM, intending to copy it to RAM in 
startup code.  Clearly we don't want an image of the bss section in ROM, and the 
  .bss section does not cause the advancement of the allocation point (the LMA). 
  So .trail ends up with an LMA just after .data.  This coincides with .bss, and 
so the overlap checking code issues an error message.  A real script may have 
multiple data/bss-like tuples for different physical RAMs.

Notice that the overlap checking code checks LMAs because:
       /* We must check the sections' LMA addresses not their VMA
	 addresses because overlay sections can have overlapping VMAs
	 but they must have distinct LMAs.  */

which is fine for overlays, but breaks this usage.  the attached patch relaxes 
the check slightly further by only checking LOAD sections, rather than (nearly) 
all ALLOC sections.

Ideally we'd do two checks, one on LMAs, for LOAD sections and one on VMAs 
skipping overlay sections.  Unfortunately, AFAICT, we don't know at this point 
what sections are overlay sections -- that information is in the linker script 
statement tree, not the output section list.

built and tested for i686-elf, ok?

nathan

-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::         CodeSourcery

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