Hi All,
I have found an issue related with the condition register usage, while
migrating to GCC from DIAB compiler. The RTOS used is AMX and the platform
is PowerPC.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
if (argVal < MACRO_VALUE) // exp-1
{
cmplwi cr4, r3, 0x1
bgt+ cr4, 0xaabb
....
....
}
...
x = os_wrapper_fn(); // Modifies CR4
...
if (argVal < MACRO_VALUE) // exp-2
{
bgt+ cr4, 0xccdd
....
....
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
1. The comparison result of expression "(argVal < MACRO_VALUE)" is stored in
CR4 initially (exp-1). Since the same expression is used again (exp-2) and
the 'argVal' is not modified in between these expressions, GCC does not
re-evaluate the expression, instead uses the already evaluated result from
CR4 for branch decision.
But in between these two comparison statements, there is a system call where
the CR4 register is used for expression evaluation. So when the CR4 register
is reused in the second if statement (exp-2), the evaluation result goes
wrong.
The above comparison is made in several places w/o any changes in 'argVal'
value, so it seems to be a fairly obvious and valid optimisation to take
result of "(argVal < MACRO_VALUE)" into CR4 and keep it there and just
branch on it later. Diab does not do that but gcc does.
My concern is if GCC assumes 'CR4' is preserved across function calls, and
e.g. my RTOS call would not preserve it.
I have tried using volatile qualifier on 'argVal' which stops optimization
and thus comparison expression is evaluated again instead of reusing the
result from CR4.
In this case, things go fine, but I still have this concern that there can
be many such places in code where this issue can come up. So I am thinking
of the following options:
Option (1) Not to use CR1..CR3, by taking those out of GCC use, which limits
the optimization potential. Can anyone please advise if there is any option
to restrict the CR usage?
Option (2) To tell GCC that each of the OS calls modifies condition code
register.Is there a pragma to define registers which an external function
will trash?
Or do we have any other option to solve this issue? Can anyone please help?
Thanks-in-advance.
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