Failure to optimise (a/b) and (a%b) into single __aeabi_idivmod call

[Kugan]

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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Re: Failure to optimise (a/b) and (a%b) into single __aeabi_idivmod call

[Matthew Gretton-Dann]

Kugan,

I don't have the source code to hand but how are the
sin()/cos()->sincos() optimizations handled?

Thanks,

Matt

On 5 June 2013 11:44, Kugan  wrote:
> 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



-- 
Matthew Gretton-Dann
Linaro Toolchain Working Group
matthew.gretton-d...@linaro.org

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