[nvptx] Use bit-bucket operand for atom insns

For an atomic fetch operation that doesn't use the result:
...
  __atomic_fetch_add (p64, v64, MEMMODEL_RELAXED);
...
we currently emit:
...
  atom.add.u64 %r26, [%r25], %r27;
...

Detect the REG_UNUSED reg-note for %r26, and emit instead:
...
  atom.add.u64 _, [%r25], %r27;
...

Likewise for all atom insns.

Tested on nvptx.

gcc/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	PR target/104815
	* config/nvptx/nvptx.cc (nvptx_print_operand): Handle 'x' operand
	modifier.
	* config/nvptx/nvptx.md: Use %x0 destination operand in atom insns.

gcc/testsuite/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	PR target/104815
	* gcc.target/nvptx/atomic-bit-bucket-dest.c: New test.

3 files changed, 50 insertions, 6 deletions

diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index 6ca99a6..14911bd 100644
--- a/gcc/config/nvptx/nvptx.cc
+++ b/gcc/config/nvptx/nvptx.cc
@@ -2835,7 +2835,8 @@ nvptx_mem_maybe_shared_p (const_rtx x)
    S -- print a shuffle kind specified by CONST_INT
    t -- print a type opcode suffix, promoting QImode to 32 bits
    T -- print a type size in bits
-   u -- print a type opcode suffix without promotions.  */
+   u -- print a type opcode suffix without promotions.
+   x -- print a destination operand that may also be a bit bucket.  */
 
 static void
 nvptx_print_operand (FILE *file, rtx x, int code)
@@ -2863,6 +2864,14 @@ nvptx_print_operand (FILE *file, rtx x, int code)
 
   switch (code)
     {
+    case 'x':
+      if (current_output_insn != NULL
+	  && find_reg_note (current_output_insn, REG_UNUSED, x) != NULL_RTX)
+	{
+	  fputs ("_", file);
+	  return;
+	}
+      goto common;
     case 'B':
       if (SYMBOL_REF_P (XEXP (x, 0)))
 	switch (SYMBOL_DATA_AREA (XEXP (x, 0)))
diff --git a/gcc/config/nvptx/nvptx.md b/gcc/config/nvptx/nvptx.md
index 8079763..1cbf197 100644
--- a/gcc/config/nvptx/nvptx.md
+++ b/gcc/config/nvptx/nvptx.md
@@ -2050,7 +2050,7 @@
   ""
   {
     const char *t
-      = "%.\\tatom%A1.cas.b%T0\\t%0, %1, %2, %3;";
+      = "%.\\tatom%A1.cas.b%T0\\t%x0, %1, %2, %3;";
     return nvptx_output_atomic_insn (t, operands, 1, 4);
   }
   [(set_attr "atomic" "true")])
@@ -2076,7 +2076,7 @@
 	return "";
       }
     const char *t
-      = "%.\tatom%A1.exch.b%T0\t%0, %1, %2;";
+      = "%.\tatom%A1.exch.b%T0\t%x0, %1, %2;";
     return nvptx_output_atomic_insn (t, operands, 1, 3);
   }
   [(set_attr "atomic" "true")])
@@ -2166,7 +2166,7 @@
 	return "";
       }
     const char *t
-      = "%.\\tatom%A1.add%t0\\t%0, %1, %2;";
+      = "%.\\tatom%A1.add%t0\\t%x0, %1, %2;";
     return nvptx_output_atomic_insn (t, operands, 1, 3);
   }
   [(set_attr "atomic" "true")])
@@ -2196,7 +2196,7 @@
 	return "";
       }
     const char *t
-      = "%.\\tatom%A1.add%t0\\t%0, %1, %2;";
+      = "%.\\tatom%A1.add%t0\\t%x0, %1, %2;";
     return nvptx_output_atomic_insn (t, operands, 1, 3);
   }
   [(set_attr "atomic" "true")])
@@ -2226,7 +2226,7 @@
 	return "";
       }
     const char *t
-      = "%.\\tatom%A1.<logic>.b%T0\\t%0, %1, %2;";
+      = "%.\\tatom%A1.<logic>.b%T0\\t%x0, %1, %2;";
     return nvptx_output_atomic_insn (t, operands, 1, 3);
   }
 
diff --git a/gcc/testsuite/gcc.target/nvptx/atomic-bit-bucket-dest.c b/gcc/testsuite/gcc.target/nvptx/atomic-bit-bucket-dest.c
new file mode 100644
index 0000000..7e3ffce
--- /dev/null
+++ b/gcc/testsuite/gcc.target/nvptx/atomic-bit-bucket-dest.c
@@ -0,0 +1,35 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -misa=sm_35" } */
+
+enum memmodel
+{
+  MEMMODEL_RELAXED = 0
+};
+
+unsigned long long int *p64;
+unsigned long long int v64;
+
+int
+main()
+{
+  __atomic_fetch_add (p64, v64, MEMMODEL_RELAXED);
+  __atomic_fetch_and (p64, v64, MEMMODEL_RELAXED);
+  __atomic_fetch_or (p64, v64, MEMMODEL_RELAXED);
+  __atomic_fetch_xor (p64, v64, MEMMODEL_RELAXED);
+  __atomic_exchange_n (p64, v64, MEMMODEL_RELAXED);
+
+  {
+    unsigned long long expected = v64;
+    __atomic_compare_exchange_n (p64, &expected, 0, 0, MEMMODEL_RELAXED,
+				 MEMMODEL_RELAXED);
+  }
+
+  return 0;
+}
+
+/* { dg-final { scan-assembler-times "atom.add.u64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "atom.and.b64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "atom.or.b64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "atom.xor.b64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "atom.exch.b64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "atom.cas.b64\[\t \]+_," 1 } } */