.align may cause data to be interpreted as instructions

Taras Kondratiuk [off-list ref] writes:

On 10/16/2013 10:25 PM, Dave Martin wrote:

quoted

Unfortunately, objdump can and does get confused about data/instruction
boundaries, so the output you see above may be misleading.

Displaying the symbol table with --special-syms will list the magic
symbols that mark the instruction and data boundaries, to help debug
this kind of situation.


However, in this case, I think you've found a bug in the assembler,
as shown below.

Before linking, the final $a symbol (indicating the start of ARM
instructions) is at address 8, so in this case objdump is correct
to show 0x12345678 as an instruction.

After linking, the mapping symbols ($[atd]) remain as before, and
the linker has byteswapped this "instruction" (as it should).

This is likely related to the magic for inserting the extensible
NOP-padding fragment which implements the .align in code sections.
That is code, and requires a $a mapping symbol, but that somehow
goes AWOL or gets displaced after the alignment padding ...

I can't quite get my head around what is going on in
binutils/gas/config/tc-arm.c.  We would need to understand that
before we can identify a reliable workaround.

Thanks for confirming the issue.
Does it makes sense to file a GCC bug?

It seems like a binutils (gas) bug to me.

-- 
M?ns Rullg?rd
mans at mansr.com