Project Home
Project Home		 Source Code
Source Code		 Documents
Documents		 Wiki
Wiki		 Discussion Forums
Discussions		 Project Information
Project Info
Core Development Tools/Discussion/General/how does arm-gcc use fp register?/List of Posts
Forum Topic - how does arm-gcc use fp register?: (7 Items)
   
Update
Yao Zhao(deleted)
01/14/2009 2:05 PM
post20070
how does arm-gcc use fp register?   
I tried with -O2 to compile an test application but it still uses fp register, is this normal?
but I tried on arm host linux with native gcc 4.1.2 then I didn't see fp is used.
Ryan Mansfield(deleted)
01/15/2009 9:16 AM
post20112
Re: how does arm-gcc use fp register?   
Our arm tools use a different ABI than most Linux arm toolschains. We follow apcs-gnu, and they typically follow the 
EABi (aapcs-linux). By default, our gcc uses -mapcs-frame which means the complaint apcs frame will be emitted. The only
 case the frame pointer is not required is in a leaf function.

Regards,

Ryan Mansfield
Yao Zhao(deleted)
01/15/2009 9:51 AM
post20116
Re: how does arm-gcc use fp register?   
> Our arm tools use a different ABI than most Linux arm toolschains. We follow 
> apcs-gnu, and they typically follow the EABi (aapcs-linux). By default, our 
> gcc uses -mapcs-frame which means the complaint apcs frame will be emitted. 
> The only case the frame pointer is not required is in a leaf function.
> 
> Regards,
> 
> Ryan Mansfield


Thanks Ryan!
It seems that apcs is out of date? apcs save at least fp,ip,lr,pc but aapcs didn't when omit-frame-pointer is enabled. I
 didn't understand why pc is saved in stack with apcs? what is that for? Also when I debug arm Linux  binary gdb still 
can show backtrace, how is that done if no fp is saved.
Yao Zhao(deleted)
01/15/2009 10:52 AM
post20133
Re: how does arm-gcc use fp register?   
looks like apcs-gnu will not save fp when -fomit-frame-pointer is enabled with leaf function.
but I tried -mno-apcs-frame then it will not save so many registers any more so this should be faster. why we enabled 
this default? In gcc document -mno-apcs-frame is default.
Ryan Mansfield(deleted)
01/15/2009 11:35 AM
post20138
Re: how does arm-gcc use fp register?   
Yao Zhao wrote:
> looks like apcs-gnu will not save fp when -fomit-frame-pointer is enabled with leaf function.
> but I tried -mno-apcs-frame then it will not save so many registers any more so this should be faster. why we enabled 
this default? In gcc document -mno-apcs-frame is default.

-mno-apcs-frame would mean debugging would not always work properly. 
-mno-apcs-frame is the default for targets that follow the newer ABIs.

Regards,

Ryan Mansfield
Ryan Mansfield(deleted)
01/15/2009 11:25 AM
post20135
Re: how does arm-gcc use fp register?   
Yao Zhao wrote:
> Thanks Ryan!
> It seems that apcs is out of date? 

Yes, apcs is old but for us to switch would mean breaking binary 
compatibility with all existing ARM binaries.

apcs save at least fp,ip,lr,pc but aapcs didn't when omit-frame-pointer 
is enabled. I didn't understand why pc is saved in stack with apcs? what 
is that for?

There was a historical reason for saving the PC but I can't recall at 
the moment but it's rarely needed. It still gets saved because the ABI 
says so.

Also when I debug arm Linux  binary gdb still can show backtrace, how is 
that done if no fp is saved.

The eabi requires per function unwind tables.

Regards,

Ryan Mansfield
Yao Zhao(deleted)
01/15/2009 2:44 PM
post20164
Re: how does arm-gcc use fp register?   
thanks!
now understood!

Return


  A subsidiary of BlackBerry             All content ©2004-2013, QNX Software Systems