Hi,
I am looking at best approach for
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43721 - Failure to optimise
(a/b) and (a%b) into single __aeabi_idivmod call in ARM architecture
In sumary, the following c code results in __aeabi_idivmod() call and
one __aeabi_idiv() call even though the former already calculates the
quotient.
int q = a / b;
int r = a % b;
return q + r;
My question is what would be the best way to handle it. As I see there
are few options with some issues.
1. Handling in gimple level, try to reduce the operations to equivalent
of this. We should do this for the targets without integer divide.
{q, r} = a % b;
Gimple assign stmts have only one lhs operation (?). Therefore, lhs has
to be made 64bit to signify return values of R0 and R1 returned
together. I am not too sure of any implications on other architectures here.
2. Handling in expand_divmod. Here, when we see a div or mod operation,
we will have to do a linear search to see if there is a valid equivalent
operation to combine. If we find one, we can generate __aeabi_idivmod()
and cache the result for the equivalent operation. As I see, this can
get messy and might not be acceptable.
3. An RTL pass to process and combine these library calls. Possibly
using cse. I am still looking at this.
4. Ramana tried a prototype to do the same using target pattens. He has
ruled this out. (if you want more info, please refer to at
https://code.launchpad.net/~ramana/gcc-linaro/divmodsi4-experiments)
Any suggestion for best way to handle this?
Thanks,
Kugan
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