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Re: Commit: ARM: Document -munaligned-access


Hi H-P,

*cough*

Sounds like a call for a note in changes.html, to warn people
that they have to turn on the alignment feature in their startup
code (for whatever OS) for their ARMv6-or-later targets, if they
have not already done so.

What a good idea :-)



Maybe something like:


Index: changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-4.7/changes.html,v
retrieving revision 1.113
diff -p -u -r1.113 changes.html
--- changes.html	5 Jun 2012 11:03:53 -0000	1.113
+++ changes.html	15 Jun 2012 02:04:46 -0000
@@ -43,6 +43,19 @@

</li>

+    <li>On ARM, when compiling for ARMv6 (but not ARMv6-M), ARMv7-A,
+    ARMv7-R, or ARMv7-M, the new option
+    <code>-munaligned-access</code> is active by default, which for
+    some source codes generates code that accesses memory on unaligned
+    adresses.  This will require the kernel of those systems to enable
+    such accesses (controlled by CP15 register <code>c1</code>, refer
+    to ARM documentation).  Alternatively or for compatibility with
+    kernels where unaligned accesses are not supported, all code has
+    to be compiled with <code>-mno-unaligned-access</code>.
+    Linux/ARM in official releases has automatically and
+    unconditionally supported unaligned accesses as emitted by GCC due
+    to this option being active since Linux version 2.6.28.</li>
+
      <li>Support on ARM for the legacy floating-point accelerator (FPA) and
      the mixed-endian floating-point format that it used has been obsoleted.
      The ports that still use this format have been obsoleted as well.



<http://gcc.gnu.org/ml/gcc-patches/2012-07/msg00282.html>


Sorry, but I couldn't miss the opportunity to ping this now that
the ARM maintainers have this issue in their L1-cache.  A simple
yes or no would do; I don't see what could possibly need any
contemplation regarding this note.

Agreed - in fact agreed so much that as an ARM maintainer I say "approved - please apply".


Cheers
  Nick


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