[LLVMdev] [lld] Current ways to position memory sections (e.g. .text, .data, .bss) with lld?

[Rafael Auler]

Hi Ed,

It looks like lld is failing at mapping two sections far apart from each
other into two different segments. Since it puts these two sections (.text
and .data) in the same ELF segment, the segment is forced to be huge
because the start addresses of these sections are far apart from each
other. I would begin by investigating how
TargetLayout<ELFT>::assignSectionsToSegments() works and go from there.
This function lives at lld/lib/ReaderWriter/ELF/TargetLayout.cpp. I don't
believe you can try any other flags to try to get this done without fixing
this logic.

Rafael Auler

On Wed, Jul 1, 2015 at 12:00 PM, ed at modk.it <ed at modk.it> wrote:

> Hi All,
>
> Congratulations on the major progress on the llvm linker lld over the past
> year including the new linker script support.  This really makes it
> possible to ditch binutils altogether.  It looks like lld's MEMORY sections
> are currently parsed but not evaluated, but so far that hasn't been a
> problem.
>
> The only snag is I can't figure out how to define the start of the .data
> section in a sane way.  Let's say we want our flash (.text) section to
> start at address 0x20000 with a max size of 0x8000 bytes and a data section
> start at 0x20005000 with a max length of 0x3000 bytes.  We'd usually
> accomplish this with a combination of MEMORY and SECTION entries in the
> linker script tied together by region aliases, the "AT" directive and the
> ">" operator:
>
> MEMORY
>
> {
>
>     flash (rx) : ORIGIN = 0x20000, LENGTH = 0x8000
>
>     ram (rwx) : ORIGIN = 0x20005000, LENGTH = 0x00003000
>
> }
>
>
>  REGION_ALIAS("REGION_TEXT", flash);
>
> REGION_ALIAS("REGION_RAM", ram);
>
>
>  SECTIONS
>
> {
>
>     .text :
>
>     {
>
>         ...
>
>     } > REGION_TEXT
>
>
>
>     _etext = .;
>
>     .data :
>
>     {
>
>        ...
>
>     } > REGION_RAM AT > REGION_TEXT
>
>
> }
>
>
> But the MEMORY entries don't seem to be evaluated and ">", "AT", and
> "REGION_ALIAS" don't seem to be hooked up either.  Command line options
> like "-Tdata=org=0x20005000" or "--section-start=.data= 0x20005000" are
> also not working.
>
>
> However, the following linker script gets us close:
>
>
> SECTIONS
>
>     {
>
>         . = 0x20000;
>
>         .text :
>
>         {
>
>            ...
>
>         }
>
>
>
>         _etext = .;
>
>         . = 0x20005000;
>
>         _data = .;
>
>         .data :
>
>         {
>
>            ...
>
>         }
>
> }
>
>
> The only problem is the resulting elf and binary files are HUGE (e.g. elf
> is 500mb vs 50k) as the linker seems to be filling the space between the
> end of text and the beginning of data.  A "llvm-readobj -s" on the
> resulting elf is nearly identical to one created with binutils except
> binults-ld puts .data at Address: 0x20005000 Offset: 0xD000 and llvm-lld
> has data at Address: 0x20005000 Offset: 0x1FFED000 (based on start address
> of 0x18000.)
>
> Any thoughts on a supported linker flag or linker script option that can
> help here?  Or a source file to dive into to get something working?  I'd be
> willing to take a pass at completing the MEMORY section implementation if
> that's the most sane way to move forward but would love a pointer in the
> right direction.
>
>
> Thanks,
>
> Ed
>
>
>
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