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<div class="moz-cite-prefix">On 11/08/13 11:34, Richard Smith wrote:<br>
</div>
<blockquote
cite="mid:CAOfiQqnEWegxvRw6H1_TkHymjMjmGPRNJ7e_njtbvSVpb6OpEw@mail.gmail.com"
type="cite">
<div dir="ltr">On Thu, Nov 7, 2013 at 6:29 PM, Behan Webster <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:behanw@converseincode.com" target="_blank">behanw@converseincode.com</a>></span>
wrote:<br>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="im">On 11/07/13 01:47, Joerg Sonnenberger
wrote:<br>
> On Wed, Nov 06, 2013 at 05:46:43PM -0800, Behan
Webster wrote:<br>
>> Another way would be to introduce a new builtin
in parallel to others<br>
>> like __builtin_return_address(). Essentially
adding a<br>
>> __builtin_stack_pointer() which does what the
above code does. The idea<br>
>> would be to get something like this added to
both clang and gcc in order<br>
>> to make this work across compilers, and across
arches.<br>
> You still haven't said what the intended behavior
of this builtin is<br>
> when the function containing it is inlined.<br>
</div>
The intent in all situations is for it to use the current
value in the<br>
stack pointer register. It doesn't matter whether it is
inlined or not.<br>
It should be the immediate value of R13 on ARM or esp on
X86_64 (etc for<br>
other arches).<br>
<br>
The idea is to preclude the need to use ASM to move the
value in the<br>
stack pointer to another register in order to use it in C
code (which is<br>
inefficient).<br>
<br>
Essentially I'm looking for a better solution than adding
a C symbol<br>
name to the stack register.<br>
<br>
Does that make sense?</blockquote>
<div><br>
</div>
<div>Not yet. Why do you want this? What will you do with
it? What semantics do you want the returned pointer to
have? How can correct code ever do anything with this
pointer? (Remember that the backend is in principle
allowed to modify the stack pointer at any time, including
between the point where you call this intrinsic and the
point where you use its result.)</div>
</div>
</div>
</div>
</blockquote>
Its used by the Linux kernel in several situations, but mostly for
threading, and various debug and stack tracing code.<br>
<br>
Essentially the kernel code primarily uses the stack pointer by
assigning it a C symbol name like this.<br>
<br>
register unsigned long current_sp asm ("sp");<br>
<br>
The value of that named register is read and stored elsewhere, or is
used to calculate the beginning or end of the stack in order to find
the current threadinfo or the current pt_regs (previous dump of the
CPU registers). Essentially in the case of ARM, r13 is used directly
in the resulting code.<br>
<br>
gcc allows you to do the above in order to access the value of the
stack pointer register (in the way I describe), however clang does
not unless you then add the following.<br>
<br>
register unsigned long current_sp asm ("sp");<br>
asm("" : "=r(current_sp));<br>
<br>
... which works, but is ugly. Since my goal is to be able to compile
the Linux kernel with both clang and gcc, in a way which is the most
efficient with each compiler, and which doesn't make the code worse,
the above it's going to cut it. <br>
<br>
Instead I'd like to see __builtin_stack_pointer() added to both
clang and gcc. It's easier to read, would provide read only access
to the register value (which is safer), and mirrors the other
__builtin functions (which in the Linux kernel are often called
together: __builtin_frame_address() and __builtin_return_address()<br>
<br>
For instance:<br>
<br>
frame.fp = (unsigned long)__builtin_frame_address(0);<br>
frame.sp = current_sp;<br>
frame.lr = (unsigned long)__builtin_return_address(0);<br>
frame.pc = (unsigned long)return_address;<br>
<br>
No doubt there is a more optimal way than the patch I sent.<br>
<br>
Behan<br>
<pre class="moz-signature" cols="72">--
Behan Webster
<a class="moz-txt-link-abbreviated" href="mailto:behanw@converseincode.com">behanw@converseincode.com</a></pre>
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