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Re: Re: Improving code generation in the nvptx back end


On 02/17/2017 05:09 AM, Thomas Schwinge wrote:

> On Fri, 17 Feb 2017 14:00:09 +0100, I wrote:
>> [...] for "normal" functions there is no reason to use the
>> ".param" space for passing arguments in and out of functions.  We can
>> then get rid of the boilerplate code to move ".param %in_ar*" into ".reg
>> %ar*", and the other way round for "%value_out"/"%value".  This will then
>> also simplify the call sites, where all that code "evaporates".  That's
>> actually something I started to look into, many months ago, and I now
>> just dug out those changes, and will post them later.
>>
>> (Very likely, the PTX "JIT" compiler will do the very same thing without
>> difficulty, but why not directly generate code that is less verbose to
>> read?)
> 
> Using my WIP patch, the generated PTX code changes/is simplified as
> follows:
> 
>      // BEGIN GLOBAL FUNCTION DECL: f
>     -.visible .func (.param.f32 %value_out) f (.param.u32 %in_ar0, .param.u64 %in_ar1);
>     +.visible .func (.reg.f32 %value_out) f (.reg.u32 %ar0, .reg.u64 %ar1);
>     
>      // BEGIN GLOBAL FUNCTION DEF: f
>     -.visible .func (.param.f32 %value_out) f (.param.u32 %in_ar0, .param.u64 %in_ar1)
>     +.visible .func (.reg.f32 %value_out) f (.reg.u32 %ar0, .reg.u64 %ar1)
>      {
>             .reg.f32 %value;
>     -       .reg.u32 %ar0;
>     -       ld.param.u32 %ar0, [%in_ar0];
>     -       .reg.u64 %ar1;
>     -       ld.param.u64 %ar1, [%in_ar1];
>             .reg.f64 %r23;
>             .reg.f32 %r24;
>             .reg.u32 %r25;
>     @@ -34,15 +30,15 @@ $L3:
>                     mov.f32 %r24, 0f00000000;
>      $L1:
>                     mov.f32 %value, %r24;
>     -       st.param.f32    [%value_out], %value;
>     +       mov.f32 %value_out, %value;
>             ret;
>      }
>     
>      // BEGIN GLOBAL FUNCTION DECL: main
>     -.visible .func (.param.u32 %value_out) main (.param.u32 %in_ar0, .param.u64 %in_ar1);
>     +.visible .func (.reg.u32 %value_out) main (.reg.u32 %ar0, .reg.u64 %ar1);
>     
>      // BEGIN GLOBAL FUNCTION DEF: main
>     -.visible .func (.param.u32 %value_out) main (.param.u32 %in_ar0, .param.u64 %in_ar1)
>     +.visible .func (.reg.u32 %value_out) main (.reg.u32 %ar0, .reg.u64 %ar1)
>      {
>             .reg.u32 %value;
>             .local .align 8 .b8 %frame_ar[32];
>     @@ -70,13 +66,9 @@ $L1:
>                     st.u64  [%frame+24], %r29;
>                     add.u64 %r31, %frame, 16;
>             {
>     -               .param.f32 %value_in;
>     -               .param.u32 %out_arg1;
>     -               st.param.u32 [%out_arg1], %r26;
>     -               .param.u64 %out_arg2;
>     -               st.param.u64 [%out_arg2], %r31;
>     -               call (%value_in), f, (%out_arg1, %out_arg2);
>     -               ld.param.f32    %r32, [%value_in];
>     +               .reg.f32 %value_in;
>     +               call (%value_in), f, (%r26, %r31);
>     +               mov.f32 %r32, %value_in;
>             }
>                     setp.eq.f32     %r33, %r32, 0f00000000;
>             @%r33   bra     $L5;
>     @@ -89,17 +81,13 @@ $L5:
>                     st.u64  [%frame+24], %r36;
>                     mov.u32 %r34, 1;
>             {
>     -               .param.f32 %value_in;
>     -               .param.u32 %out_arg1;
>     -               st.param.u32 [%out_arg1], %r34;
>     -               .param.u64 %out_arg2;
>     -               st.param.u64 [%out_arg2], %r31;
>     -               call (%value_in), f, (%out_arg1, %out_arg2);
>     -               ld.param.f32    %r39, [%value_in];
>     +               .reg.f32 %value_in;
>     +               call (%value_in), f, (%r34, %r31);
>     +               mov.f32 %r39, %value_in;
>             }
>                     setp.neu.f32    %r40, %r39, 0f3f800000;
>             @%r40   bra     $L6;
>                     mov.u32 %value, 0;
>     -       st.param.u32    [%value_out], %value;
>     +       mov.u32 %value_out, %value;
>             ret;
>      }
> 
> (Not yet directly using "%value_out" instead of the intermediate "%value"
> register.)
> 
> Is such a patch something to pursue to completion?

Are you trying to optimize acc routines in general? I'm not sure how
frequently they are used at the moment.

Also, while .param values may be overkill for routines, they are
addressable. Looking at section 5.1.6.1 in the PTX reference manual, you
can have something like this:

.entry foo ( .param .b32 N, .param .align 8 .b8 buffer[64] )
{
  .reg .u32 %n;
  .reg .f64 %d;
  ld.param.u32 %n, [N];
  ld.param.f64
  ...

Granted, this is an entry function to be called from the host, but the
same usage is applicable inside routines.

This gives me an idea. While working on the firstprivate changes, I
noticed that GCC packs all of the offloaded function arguments into a
structure, which the nvptx run time plugin uploads to a special data
mapping prior to calling cuLaunchKernel. That's inefficient in
application that launch a lot of small offloaded regions because those
data transfers require an additional hardware synchronization. In light
of this observation, I had originally proposed that we teach GCC how to
invoke the offloaded region with individual arguments for each offloaded
variable instead of a pointer to a packed struct, because cuLaunchKernel
supports the former more efficiently. However, given that param values
can be addressable values, it would probably be more straightforward to
just pass in the struct containing the offloaded variables by value
instead of by reference.

I'd need to look more closely at the PTX ISA to see if this is even
feasible.

Cesar


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