[cfe-dev] Restrict for values that are not parameters

[Hal Finkel]

----- Original Message -----
> From: "Richard Smith" <richard at metafoo.co.uk>
> To: "Mark Schimmel" <Mark.Schimmel at synopsys.com>
> Cc: cfe-dev at cs.uiuc.edu
> Sent: Friday, March 6, 2015 8:21:26 PM
> Subject: Re: [cfe-dev] Restrict for values that are not parameters
> 
> On Fri, Mar 6, 2015 at 12:14 PM, Mark Schimmel <
> Mark.Schimmel at synopsys.com > wrote:
> 
> 
> I cannot get Clang to add noalias via a cast operation. This code
> does not produce any warnings, and I believe it’s legal, but the
> restrict qualifier is lost. Am I incorrect about this code being
> legal? Any idea why the qualifier is stripped? (It is a contrived
> example. You could imagine address checking and two separate loops.)
> 
> 
> We have no way to model this use of 'restrict' in our IR. Putting the
> 'restrict' on a function parameter should work. If you have an
> address checks and two separate loops, you could try factoring the
> 'noalias' loop out to a separate function.

To this I'll add that the necessary IR extensions for this have already been discussed on the llvmdev list (see this thread for the details: http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-November/078786.html), and I'm planning to implement them this month. So we should have better support for this soon.

 -Hal

> 
> 
> 
> #define NOALIAS __restrict
> 
> //#define NOALIAS __attribute__((noalias))
> 
> 
> 
> void move(
> 
> int* velocity_x, int* velocity_y, int* velocity_z,
> 
> int* position_x,int* position_y,int* position_z,
> 
> int* acceleration_x, int* acceleration_y, int* acceleration_z,
> 
> int time_step,
> 
> int count,
> 
> int stride )
> 
> {
> 
> {
> 
> int* NOALIAS r_acceleration_x = (int* NOALIAS)acceleration_x;
> 
> 
> The cast on the right-hand side does nothing; per C11 6.7.3/4, "The
> properties associated with qualified types are meaningful only for
> expressions that are lvalues."
> 
> 
> 
> 
> 
> 
> 
> 
> 
> int* NOALIAS r_velocity_x = (int* NOALIAS)velocity_x;
> 
> int* NOALIAS r_position_x = (int* NOALIAS)position_x;
> 
> int* NOALIAS r_acceleration_y = (int* NOALIAS)acceleration_y;
> 
> int* NOALIAS r_velocity_y = (int* NOALIAS)velocity_y;
> 
> int* NOALIAS r_position_y = (int* NOALIAS)position_y;
> 
> int* NOALIAS r_acceleration_z = (int* NOALIAS)acceleration_z;
> 
> int* NOALIAS r_velocity_z = (int* NOALIAS)velocity_z;
> 
> int* NOALIAS r_position_z = (int* NOALIAS)position_z;
> 
> 
> 
> for (int i=0;i<count*stride;i+=stride)
> 
> {
> 
> r_velocity_x[i] += r_acceleration_x[i] * time_step;
> 
> r_velocity_y[i] += r_acceleration_y[i] * time_step;
> 
> r_velocity_z[i] += r_acceleration_z[i] * time_step;
> 
> r_position_x[i] += r_velocity_x[i] * time_step;
> 
> r_position_y[i] += r_velocity_y[i] * time_step;
> 
> r_position_z[i] += r_velocity_z[i] * time_step;
> 
> }
> 
> }
> 
> }
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory