[Bug target/29969] should use floating point registers for block copies

[Bug target/29969] should use floating point registers for block copies

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29969

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |olegendo at gcc dot gnu.org

--- Comment #4 from Oleg Endo <olegendo at gcc dot gnu.org> 2012-07-15 18:43:59 UTC ---
(In reply to comment #3)
> (In reply to comment #2)
> > This could be a disaster if floating point exceptions are enabled, as it would
> > trigger an exception whenever some part of the block was an invalid floating
> > point number.
> > One would at least need to save/restore the floating point exception flags
> > around such a block copy.
> 
> If that is an issue is target specific.  First, if all floating point loads
> detect loading invalid floating point numbers, and sencond, if floating point
> exceptions are used for the target in practical terms.
> E.G. for the SH4 (at least SH4-100 / SH4-200), you get always a floating
> point exception if the exception is enabled and it could happen for any
> input data, so it is not very popular to enable it.

Jörn, I know it's been a while since you worked on the SH port...

According to SH4, SH4A and SH2A manuals these are the following exceptions
that could be triggered by FMOV instructions:

SH4 / SH4A:
 - Data TLB miss exception
 - Data protection violation exception
 - Initial page write exception
 - Data address error

SH2A:
 - Address error

... plus the 'FPU disabled exception'.
As far as I understand the manuals, FPU exceptions are only raised for
FPU arithmetic, not for loads/stores.
If this is true, using 64 bit fmov for block moves could be beneficial.
Still, I'd not enable this by default but allow the user to turn it on
with an -m option.

[Bug target/29969] should use floating point registers for block copies

[amylaar at gcc dot gnu dot org]

------- Comment #3 from amylaar at gcc dot gnu dot org  2010-06-10 19:58 -------
(In reply to comment #2)
> This could be a disaster if floating point exceptions are enabled, as it would
> trigger an exception whenever some part of the block was an invalid floating
> point number.
> One would at least need to save/restore the floating point exception flags
> around such a block copy.

If that is an issue is target specific.  First, if all floating point loads
detect loading invalid floating point numbers, and sencond, if floating point
exceptions are used for the target in practical terms.
E.G. for the SH4 (at least SH4-100 / SH4-200), you get always a floating
point exception if the exception is enabled and it could happen for any
input data, so it is not very popular to enable it.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29969

[Bug target/29969] should use floating point registers for block copies

[marc dot mengel at gmail dot com]

------- Comment #2 from marc dot mengel at gmail dot com  2010-06-10 16:43 -------
This could be a disaster if floating point exceptions are enabled, as it would
trigger an exception whenever some part of the block was an invalid floating
point number.
One would at least need to save/restore the floating point exception flags
around such a block copy.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29969

[Bug target/29969] should use floating point registers for block copies

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-11-25 23:36 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-11-25 23:36:47
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29969