[Linux-ia64] segv with gas using gcc 3.2

[Linux-ia64] segv with gas using gcc 3.2

[Robert K Gjertsen]

This is a multipart message in MIME format.
--=_alternative 0033359286256CBD_Content-Type: text/plain; charset="us-ascii"

I'm seeing gas yak/segv on some assembly code that we use
to avoid pulling extraneous instructions into the
instruction cache (avoid debugging code when debugging is turned off).
This used to work with gcc 2.96 but now dies with gcc 3.2
when optimizations are turned on (-O or higher). I'm trying to reduce this
to a smaller and manageable case and also figure out whether
I'm doing something ill-advised given that we are making
some assumptions on how the code is ordered in the remote
section (works OK on i386 with gcc 3.2).
Just seeing if someone may have some insight on my example below.

Thanks,

--Rob

I use something like the following where the
assembly code before and after is usually a gcc asm
macro for convenience (and this isn't all bundled together
in a gcc asm statement like you see in spinlock.h with read_lock
... perhaps it has to be done that way with IA64),
and this is done numerous times in a particular file.


br.cond.sptk.few 2f
".section .text.trace,"ax
2: 
 
# various instructions for my debugging/tracing code

br.cond.sptk.many 1f
.previous
1:

--=_alternative 0033359286256CBD_Content-Type: text/html; charset="us-ascii"


<br><font size=2 face="Courier New">I'm seeing gas yak/segv on some assembly code that we use</font>
<br><font size=2 face="Courier New">to avoid pulling extraneous instructions into the</font>
<br><font size=2 face="Courier New">instruction cache (avoid debugging code when debugging is turned off).</font>
<br><font size=2 face="Courier New">This used to work with gcc 2.96 but now dies with gcc 3.2</font>
<br><font size=2 face="Courier New">when optimizations are turned on (-O or higher). I'm trying to reduce this</font>
<br><font size=2 face="Courier New">to a smaller and manageable case and also figure out whether</font>
<br><font size=2 face="Courier New">I'm doing something ill-advised given that we are making</font>
<br><font size=2 face="Courier New">some assumptions on how the code is ordered in the remote</font>
<br><font size=2 face="Courier New">section (works OK on i386 with gcc 3.2).</font>
<br><font size=2 face="Courier New">Just seeing if someone may have some insight on my example below.</font>
<br>
<br><font size=2 face="Courier New">Thanks,</font>
<br>
<br><font size=2 face="Courier New">--Rob</font>
<br>
<br><font size=2 face="Courier New">I use something like the following where the</font>
<br><font size=2 face="Courier New">assembly code before and after is usually a gcc asm</font>
<br><font size=2 face="Courier New">macro for convenience (and this isn't all bundled together</font>
<br><font size=2 face="Courier New">in a gcc asm statement like you see in spinlock.h with read_lock</font>
<br><font size=2 face="Courier New">... perhaps it has to be done that way with IA64),</font>
<br><font size=2 face="Courier New">and this is done numerous times in a particular file.</font>
<br>
<br>
<br><font size=2 face="Courier New">br.cond.sptk.few 2f<br>
&quot;.section .text.trace,&quot;ax<br>
2: &nbsp; &nbsp; &nbsp;</font>
<br><font size=2 face="Courier New">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </font>
<br><font size=2 face="Courier New"># various instructions for my debugging/tracing code</font>
<br>
<br><font size=2 face="Courier New">br.cond.sptk.many 1f<br>
.previous<br>
1:</font>
<br>
--=_alternative 0033359286256CBD_=--

Re: [Linux-ia64] segv with gas using gcc 3.2

[David Mosberger]

>>>>> On Wed, 29 Jan 2003 09:21:09 -0600, Robert K Gjertsen <gjertsen@us.ibm.com> said:

  Rob> I'm seeing gas yak/segv on some assembly code that we use to
  Rob> avoid pulling extraneous instructions into the instruction
  Rob> cache (avoid debugging code when debugging is turned off).
  Rob> This used to work with gcc 2.96 but now dies with gcc 3.2 when
  Rob> optimizations are turned on (-O or higher). I'm trying to
  Rob> reduce this to a smaller and manageable case and also figure
  Rob> out whether I'm doing something ill-advised given that we are
  Rob> making some assumptions on how the code is ordered in the
  Rob> remote section (works OK on i386 with gcc 3.2).  Just seeing if
  Rob> someone may have some insight on my example below.

Perhaps the block-reordering is causing the problems for you?
Might want to try -fno-reorder-blocks.

A general comment: I'd highly discourage to switch sections within a
procedure.  The problem is that the unwind info will be all wrong
otherwise.

	--david

Re: [Linux-ia64] segv with gas using gcc 3.2

[Jim Wilson]

It isn't clear to me what you are doing, since you mention problems with
compiler optimization, but then state that you are using asm macros instead
of gcc extended asm statements.  You can't use asm macros in C code, so I
don't understand what you are trying to say.

Gcc has support for automatically moving rarely used sections of code out
of line.  You can use __builtin_expect in a condition to indicate whether
it is expected to be true or false.  When gcc sees that a section of code
is not expected to be executed, then it tries to move it someplace out of
the way.  This happens in the block reordering pass.  This might be a better
solution that what you are doing now.  This is used in various places if you
want to see examples.  I think it is used in the kernel and glibc.  There is
also a little info in the gcc documentation.

On IA-64, changing sections in the middle of the function is a very bad idea.
In addition to the unwind problems that David mentioned, you will also get
bad insn bundling (unnecessary nops and stop bits).  You may also get bad
debug info.  I think the IA-64 assembler should emit an error if someone
switches sections in the middle of a function, but I never got a chance to
try to implement that.

Jim