ld lma assignment
Alan Modra
amodra@bigpond.net.au
Tue Aug 22 11:51:00 GMT 2006
On Wed, Jul 26, 2006 at 02:39:14PM +0930, Alan Modra wrote:
> While playing with overlays recently, I noticed an annoying linker
> script requirement. An output section following an overlay must have
> its lma specified (with AT or AT>). As must any section following one
> with an lma adjustment, because ld defaults to setting lma equal to
> vma and this may cause lma overlap.
Fixes doco for this change, and warns on backward movement of VMA
unless accompanied with an explicit LMA assignment. I guess I should
have added the news item when I committed the first patch to raise
awareness of the change in default lma. Apologies for that, I wasn't
trying to slip a change in under the radar! I still feel it's a good
change for 2.18, one that shouldn't affect too many scripts, and those
that it does are easily fixed.
* NEWS: Mention LMA default change.
* ld.texinfo (Output Section LMA): Update default description.
(Location Counter): Clarify backward movement.
* ldlang.c (lang_size_sections_1): Leave non-alloc sections with
default lma equal to vma. Warn on backward movement of dot.
Index: ld/NEWS
===================================================================
RCS file: /cvs/src/src/ld/NEWS,v
retrieving revision 1.66
diff -u -p -r1.66 NEWS
--- ld/NEWS 4 Aug 2006 14:53:26 -0000 1.66
+++ ld/NEWS 22 Aug 2006 04:39:19 -0000
@@ -1,4 +1,10 @@
-*- text -*-
+* The default output section LMA has changed for allocatable sections from
+ being equal to VMA, to keeping the difference between LMA and VMA the same as
+ the previous output section in the same region. This is a more useful
+ default when using overlays and other cases where you specify an LMA
+ differing from the VMA for some sections.
+
* New switch: --print-gc-sections to list any sections removed by garabge
collection.
Index: ld/ld.texinfo
===================================================================
RCS file: /cvs/src/src/ld/ld.texinfo,v
retrieving revision 1.171
diff -u -p -r1.171 ld.texinfo
--- ld/ld.texinfo 4 Aug 2006 14:53:26 -0000 1.171
+++ ld/ld.texinfo 22 Aug 2006 04:39:24 -0000
@@ -3764,15 +3764,20 @@ Every section has a virtual address (VMA
an output section description sets the VMA (@pxref{Output Section
Address}).
-The linker will normally set the LMA equal to the VMA. You can change
-that by using the @code{AT} keyword. The expression @var{lma} that
-follows the @code{AT} keyword specifies the load address of the
-section.
+The expression @var{lma} that follows the @code{AT} keyword specifies
+the load address of the section.
Alternatively, with @samp{AT>@var{lma_region}} expression, you may
specify a memory region for the section's load address. @xref{MEMORY}.
Note that if the section has not had a VMA assigned to it then the
linker will use the @var{lma_region} as the VMA region as well.
+
+If neither @code{AT} nor @code{AT>} is specified for an allocatable
+section, the linker will set the LMA such that the difference between
+VMA and LMA for the section is the same as the preceding output
+section in the same region. If there is no preceding output section
+or the section is not allocatable, the linker will set the LMA equal
+to the VMA.
@xref{Output Section Region}.
@cindex ROM initialized data
@@ -4586,7 +4591,9 @@ anywhere that an ordinary symbol is allo
@cindex holes
Assigning a value to @code{.} will cause the location counter to be
moved. This may be used to create holes in the output section. The
-location counter may never be moved backwards.
+location counter may not be moved backwards inside an output section,
+and may not be moved backwards outside of an output section if so
+doing creates areas with overlapping LMAs.
@smallexample
SECTIONS
Index: ld/ldlang.c
===================================================================
RCS file: /cvs/src/src/ld/ldlang.c,v
retrieving revision 1.234
diff -u -p -r1.234 ldlang.c
--- ld/ldlang.c 16 Aug 2006 08:31:45 -0000 1.234
+++ ld/ldlang.c 22 Aug 2006 04:39:28 -0000
@@ -4400,16 +4400,30 @@ lang_size_sections_1
lma = align_power (lma, os->section_alignment);
os->bfd_section->lma = lma;
}
- else if (r->last_os != NULL)
+ else if (r->last_os != NULL
+ && (os->bfd_section->flags & SEC_ALLOC) != 0)
{
bfd_vma lma;
asection *last;
last = r->last_os->output_section_statement.bfd_section;
- /* If dot moved backwards (which is invalid according
- to ld docs) then leave lma equal to vma. This
- keeps users of buggy ld scripts happy. */
- if (dot >= last->vma)
+
+ /* A backwards move of dot should be accompanied by
+ an explicit assignment to the section LMA (ie.
+ os->load_base set) because backwards moves normally
+ create overlapping LMAs. */
+ if (dot < last->vma)
+ {
+ einfo (_("%P: warning: dot moved backwards before `%s'\n"),
+ os->name);
+
+ /* If dot moved backwards then leave lma equal to
+ vma. This is the old default lma, which might
+ just happen to work when the backwards move is
+ sufficiently large. Nag anyway, so people fix
+ their linker scripts. */
+ }
+ else
{
/* If the current vma overlaps the previous section,
then set the current lma to that at the end of
@@ -4440,9 +4454,16 @@ lang_size_sections_1
lma region. We use this to set the lma for
following sections. Overlays or other linker
script assignment to lma might mean that the
- default lma == vma is incorrect. */
+ default lma == vma is incorrect.
+ To avoid warnings about dot moving backwards when using
+ -Ttext, don't start tracking sections until we find one
+ of non-zero size or with lma set differently to vma. */
if (((os->bfd_section->flags & SEC_HAS_CONTENTS) != 0
|| (os->bfd_section->flags & SEC_THREAD_LOCAL) == 0)
+ && (os->bfd_section->flags & SEC_ALLOC) != 0
+ && (os->bfd_section->size != 0
+ || os->bfd_section->vma != os->bfd_section->lma
+ || r->last_os != NULL)
&& os->lma_region == NULL
&& !link_info.relocatable)
r->last_os = s;
--
Alan Modra
IBM OzLabs - Linux Technology Centre
More information about the Binutils
mailing list