Le 08/07/2020 à 16:00, Mathieu Desnoyers a écrit :
----- On Jul 8, 2020, at 8:33 AM, Mathieu Desnoyers
mathieu.desnoy...@efficios.com wrote:
----- On Jul 7, 2020, at 8:59 PM, Segher Boessenkool seg...@kernel.crashing.org
wrote:
[...]
So perhaps you have code like
int *p;
int x;
...
asm ("lwz %0,%1" : "=r"(x) : "m"(*p));
We indeed have explicit "lwz" and "stw" instructions in there.
where that last line should actually read
asm ("lwz%X1 %0,%1" : "=r"(x) : "m"(*p));
Indeed, turning those into "lwzx" and "stwx" seems to fix the issue.
There has been some level of extra CPP macro coating around those instructions
to
support both ppc32 and ppc64 with the same assembly. So adding %X[arg] is not
trivial.
Let me see what can be done here.
I did the following changes which appear to generate valid asm.
See attached corresponding .S output.
I grepped for uses of "m" asm operand in Linux powerpc code and noticed it's
pretty much
always used with e.g. "lwz%U1%X1". I could find one blog post discussing that
%U is about
update flag, and nothing about %X. Are those documented ?
As far as I can see, %U is mentioned in
https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html in the
powerpc subpart, at the "m" constraint.
For the %X I don't know.
Christophe
Although it appears to generate valid asm, I have the feeling I'm relying on
undocumented
features here. :-/
