Hi Koakuma,
On Sat, Mar 16, 2024 at 02:59:42PM +0000, Koakuma wrote:
> Hello, first time poster so apologies if I posted to the wrong list.
>
> Anyone knows why the SPARC makefiles (arch/sparc/Makefile and
> arch/sparc/vdso/Makefile) set `-fcall-used-g5` and `-fcall-used-g7`
> in their CFLAGS?
sparc32 uses:
-fcall-used-g5 -fcall-used-g7
sparc64 uses:
-ffixed-g4 -ffixed-g5 -fcall-used-g7
For sparc64:
-ffixed-g4 is added because sparc64 uses the g4 register to hold the
pointer to the current task. See:
arch/sparc/include/asm/current.h line 18.
g2, g5, g7 all have their specific use or assumptions.
>From arch/sparc/include/asm/ttable.h:
* Further note that we cannot use the g2, g4, g5, and g7 alternate
* globals in the spill routines, check out the save instruction in
* arch/sparc64/kernel/etrap.S to see what I mean about g2, and
* g4/g5 are the globals which are preserved by etrap processing
* for the caller of it. The g7 register is the return pc for
* etrap. Finally, g6 is the current thread register so we cannot
* us it in the spill handlers either. Most of these rules do not
* apply to fill processing, only g6 is not usable.
*/
Looking at https://github.com/gcc-mirror/gcc/blob/master/gcc/config/sparc/sparc.h
I read that:
On v9 systems:
g1,g5 are free to use as temporaries, and are free to use between calls
...
g6-g7 are reserved for the operating system (or application in
embedded case).
Based on the above I would assume gcc do not change behaviour with or
without -fcall-used-g7.
I do not have a sparc64 system at my hands - and for this qemu may not
cut it. But it would be super if someone with a working sparc64 target
could verify if the kernel could be built and works without
-fcall-used-g7.
For sparc32 the above file says:
g5 through g7 are reserved for the operating system.
So again - it looks like -fcall-used-g5 -fcall-used-g7 should have no
effect here and verification on a real target would be nice.
Sam
