Re: Why does gas put a stop bit in a bundle?

[Jim Wilson <wilson at tuliptree dot org>]

H. J. Lu wrote:
> GAS adds a stop bit at the last bundle in the current section
> automatically.
> Why does gas do that? I'd like to turn it off.

Because we can't do dependency violation checking across section 
switches.  Thus, in order to be safe, we must emit a stop bit when 
exiting the text section.  Doing otherwise will result in code that 
silently fails at run time.  Consider what happens if someone has C code 
with an extended asm that switches to an alternate code section and then 
back to text.

Section switching also causes problems for generation of unwind info. 
It also used to cause trouble for dwarf2 compaction, but I think Richard 
Henderson fixed that problem.

I believe the best long term solution for this is to modify IA-64 gas to 
insist on structured assembly language.  Gas should emit an error if it 
sees code that isn't inside a function.  Also, gas should emit an error 
if there is a section switch inside a function.  If we do this, then all 
of the dependency violation, unwind info, and dwarf2 problems go away. 
It isn't hard to follow these rules, it just takes a little thought. 
The IA-64 linux kernel is written this way for instance, because they 
have had problems in the past with secction switching in the middle of 
functions breaking DV checking and/or unwind info.

Jim