nvptx: Make 'nvptx_uniform_warp_check' fit for non-full-warp execution, via 'vote.all.pred'

For example, this allows for '-muniform-simt' code to be executed
single-threaded, which currently fails (device-side 'trap'): the '0xffffffff'
bitmask isn't correct if not all 32 threads of a warp are active.  The same
issue/fix, I suppose but have not verified, would apply if we were to allow for
OpenACC 'vector_length' smaller than 32, for example for OpenACC 'serial'.

We use 'nvptx_uniform_warp_check' only for PTX ISA version less than 6.0.
Otherwise we're using 'nvptx_warpsync', which emits 'bar.warp.sync 0xffffffff',
which evidently appears to do the right thing.  (I've tested '-muniform-simt'
code executing single-threaded.)

The change that I proposed on 2022-12-15 was to emit PTX code to calculate
'(1 << %ntid.x) - 1' as the actual bitmask to use instead of '0xffffffff'.
This works, but the PTX JIT generates SASS code to do this computation.

In turn, this change now uses PTX 'vote.all.pred' -- which even simplifies upon
the original code a little bit, see the following examplary SASS 'diff' before
vs. after this change:

    [...]
              /*[...]*/                   SYNC                                                        (*"BRANCH_TARGETS .L_x_332"*)        }
      .L_x_332:
    -         /*[...]*/                   VOTE.ANY R9, PT, PT ;
    +         /*[...]*/                   VOTE.ALL P1, PT ;
    -         /*[...]*/                   ISETP.NE.U32.AND P1, PT, R9, -0x1, PT ;
    -         /*[...]*/              @!P1 BRA `(.L_x_333) ;
    +         /*[...]*/               @P1 BRA `(.L_x_333) ;
              /*[...]*/                   BPT.TRAP 0x1 ;
      .L_x_333:
    -         /*[...]*/               @P1 EXIT ;
    +         /*[...]*/              @!P1 EXIT ;
    [...]

	gcc/
	* config/nvptx/nvptx.md (nvptx_uniform_warp_check): Make fit for
	non-full-warp execution, via 'vote.all.pred'.
	gcc/testsuite/
	* gcc.target/nvptx/nvptx.exp
	(check_effective_target_default_ptx_isa_version_at_least_6_0):
	New.
	* gcc.target/nvptx/uniform-simt-2.c: Adjust.
	* gcc.target/nvptx/uniform-simt-5.c: New.

4 files changed, 39 insertions, 9 deletions

diff --git a/gcc/config/nvptx/nvptx.md b/gcc/config/nvptx/nvptx.md
index ef7e3fb..7878a3b 100644
--- a/gcc/config/nvptx/nvptx.md
+++ b/gcc/config/nvptx/nvptx.md
@@ -2316,14 +2316,11 @@
   {
     const char *insns[] = {
       "{",
-      "\\t"		  ".reg.b32"	    "\\t" "%%r_act;",
-      "%.\\t"		  "vote.ballot.b32" "\\t" "%%r_act,1;",
-      "\\t"		  ".reg.pred"	    "\\t" "%%r_do_abort;",
-      "\\t"		  "mov.pred"	    "\\t" "%%r_do_abort,0;",
-      "%.\\t"		  "setp.ne.b32"	    "\\t" "%%r_do_abort,%%r_act,"
-						  "0xffffffff;",
-      "@ %%r_do_abort\\t" "trap;",
-      "@ %%r_do_abort\\t" "exit;",
+      "\\t"		".reg.pred"	"\\t" "%%r_sync;",
+      "\\t"		"mov.pred"	"\\t" "%%r_sync, 1;",
+      "%.\\t"		"vote.all.pred" "\\t" "%%r_sync, 1;",
+      "@!%%r_sync\\t"	"trap;",
+      "@!%%r_sync\\t"	"exit;",
       "}",
       NULL
     };
diff --git a/gcc/testsuite/gcc.target/nvptx/nvptx.exp b/gcc/testsuite/gcc.target/nvptx/nvptx.exp
index 97aa7ae..3151381 100644
--- a/gcc/testsuite/gcc.target/nvptx/nvptx.exp
+++ b/gcc/testsuite/gcc.target/nvptx/nvptx.exp
@@ -49,6 +49,11 @@ proc check_effective_target_default_ptx_isa_version_at_least { major minor } {
     return $res
 }
 
+# Return 1 if code by default compiles for at least PTX ISA version 6.0.
+proc check_effective_target_default_ptx_isa_version_at_least_6_0 { } {
+    return [check_effective_target_default_ptx_isa_version_at_least 6 0]
+}
+
 # Return 1 if code with PTX ISA version major.minor or higher can be run.
 proc check_effective_target_runtime_ptx_isa_version_at_least { major minor } {
     set name runtime_ptx_isa_version_${major}_${minor}
diff --git a/gcc/testsuite/gcc.target/nvptx/uniform-simt-2.c b/gcc/testsuite/gcc.target/nvptx/uniform-simt-2.c
index b1eee0d..1d83c49 100644
--- a/gcc/testsuite/gcc.target/nvptx/uniform-simt-2.c
+++ b/gcc/testsuite/gcc.target/nvptx/uniform-simt-2.c
@@ -17,4 +17,4 @@ f (void)
 
 /* { dg-final { scan-assembler-times "@%r\[0-9\]*\tatom.global.cas" 1 } } */
 /* { dg-final { scan-assembler-times "shfl.idx.b32" 1 } } */
-/* { dg-final { scan-assembler-times "vote.ballot.b32" 1 } } */
+/* { dg-final { scan-assembler-times "vote.all.pred" 1 } } */
diff --git a/gcc/testsuite/gcc.target/nvptx/uniform-simt-5.c b/gcc/testsuite/gcc.target/nvptx/uniform-simt-5.c
new file mode 100644
index 0000000..cd6ea82
--- /dev/null
+++ b/gcc/testsuite/gcc.target/nvptx/uniform-simt-5.c
@@ -0,0 +1,28 @@
+/* Verify that '-muniform-simt' code may be executed single-threaded.
+
+   { dg-do run }
+   { dg-options {-save-temps -O2 -muniform-simt} } */
+
+enum memmodel
+{
+  MEMMODEL_RELAXED = 0
+};
+
+unsigned long long int v64;
+unsigned long long int *p64 = &v64;
+
+int
+main()
+{
+  /* Trigger uniform-SIMT processing.  */
+  __atomic_fetch_add (p64, v64, MEMMODEL_RELAXED);
+
+  return 0;
+}
+
+/* Per 'omp_simt_exit':
+     - 'nvptx_warpsync'
+       { dg-final { scan-assembler-times {bar\.warp\.sync\t0xffffffff;} 1 { target default_ptx_isa_version_at_least_6_0 } } }
+     - 'nvptx_uniform_warp_check'
+       { dg-final { scan-assembler-times {vote\.all\.pred\t%r_sync, 1;} 1 { target { ! default_ptx_isa_version_at_least_6_0 } } } }
+*/