Re: [og7] vector_length extension part 2: Generalize state propagation and synchronization

[Cesar Philippidis <cesar@codesourcery.com>]

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On 03/22/2018 07:23 AM, Tom de Vries wrote:
> On 03/02/2018 05:55 PM, Cesar Philippidis wrote:
> 
>>     (nvptx_declare_function_name): Emit a .maxntid directive hint and
>>     call nvptx_init_oacc_workers.
> 
>> +
>> +  /* Emit a .maxntid hint to help the PTX JIT emit SYNC branches.  */
>> +  if (lookup_attribute ("omp target entrypoint", DECL_ATTRIBUTES (decl))
>> +      && lookup_attribute ("oacc function", DECL_ATTRIBUTES (decl)))
>> +      s << ".maxntid " << cfun->machine->axis_dim[0] << ", "
>> +    << cfun->machine->axis_dim[1] << ", 1\n";
>> +
> 
> This change:
> ...
>  // BEGIN FUNCTION DEF: main$_omp_fn$0
>  .entry main$_omp_fn$0 (.param .u64 %in_ar0)
> +  .maxntid 32, 32, 1
> ...
> needs to be an individual patch.

cfun->machine->axis_dims is something new to the vector length changes,
so I hard-coded .maxntid to size '32, 32, 1' for og7 as an interim solution.

>> +  /* Emit a .maxntid hint to help the PTX JIT emit SYNC branches.  */
> 
> 'Help' is too strongly formulated, given that there's no clear link
> between the semantics of the directive, and the observed effect.
> 
> Use "seems to have the effect" or some such formulation.
> 
> Also, list in the comment a JIT driver version, and sm_ version and a
> testcase for which this is required.
> 
> Also, guard it with WORKAROUND_PTXJIT_BUG_3 (_2 is already taken in trunk.)

Sounds reasonable. I'll commit the patch to og7 once the regression
testing has completed.

Thanks,
Cesar

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From b89ec8060de3affb94b580be3260381028d4c183 Mon Sep 17 00:00:00 2001
From: Cesar Philippidis <cesar@codesourcery.com>
Date: Thu, 22 Mar 2018 08:05:53 -0700
Subject: [PATCH] add .maxntid hint

---
 gcc/config/nvptx/nvptx.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/gcc/config/nvptx/nvptx.c b/gcc/config/nvptx/nvptx.c
index eff87732c4b..9fb2bcd6852 100644
--- a/gcc/config/nvptx/nvptx.c
+++ b/gcc/config/nvptx/nvptx.c
@@ -76,6 +76,7 @@
 #include "target-def.h"
 
 #define WORKAROUND_PTXJIT_BUG 1
+#define WORKAROUND_PTXJIT_BUG_3 1
 
 /* Define dimension sizes for known hardware.  */
 #define PTX_VECTOR_LENGTH 32
@@ -1219,6 +1220,15 @@ nvptx_declare_function_name (FILE *file, const char *name, const_tree decl)
      stream, in order to share the prototype writing code.  */
   std::stringstream s;
   write_fn_proto (s, true, name, decl);
+
+#if WORKAROUND_PTXJIT_BUG_3
+  /* Emitting a .maxntid seems to have the effect of encouraging the
+     PTX JIT emit SYNC branches.  */
+  if (lookup_attribute ("omp target entrypoint", DECL_ATTRIBUTES (decl))
+      && lookup_attribute ("oacc function", DECL_ATTRIBUTES (decl)))
+      s << ".maxntid 32, 32, 1\n";
+#endif
+
   s << "{\n";
 
   bool return_in_mem = write_return_type (s, false, result_type);
-- 
2.14.3