RISC-V: Zihintpause: add __builtin_riscv_pause

The Zihintpause extension uses an opcode from the 'fence' opcode range
to add a true hint instruction (i.e. if it is not supported on any
given platform, the 'fence' that is encoded will not enforce any
specific ordering on memory accesses) for entering a low-power state
(e.g. in an idle thread).  We expose this new instruction through a
machine-dependent builtin to allow generating it without a requirement
for any inline assembly.

Given that the encoding of 'pause' is valid (as a 'fence' encoding)
even for processors that do not (yet) support Zihintpause, we make
this builtin available without any further TARGET_* constraints.

gcc/ChangeLog:

	* config/riscv/riscv-builtins.cc (struct riscv_builtin_description):
	add the pause machine-dependent builtin with no result and no
	arguments; mark it as always present (pause is a true hint
	that encodes into a fence-insn, if not supported with the new
	pause semantics).
	* config/riscv/riscv-ftypes.def: Add type for void -> void.
	* config/riscv/riscv.md (riscv_pause): Add risc_pause and
	UNSPECV_PAUSE
	* doc/extend.texi: Document __builtin_riscv_pause.
	* optabs.cc (maybe_gen_insn): Allow nops == 0 (void -> void).

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/builtin_pause.c: New test.

6 files changed, 28 insertions, 3 deletions

diff --git a/gcc/config/riscv/riscv-builtins.cc b/gcc/config/riscv/riscv-builtins.cc
index 021f6c6..24ae22c 100644
--- a/gcc/config/riscv/riscv-builtins.cc
+++ b/gcc/config/riscv/riscv-builtins.cc
@@ -88,8 +88,6 @@ struct riscv_builtin_description {
 };
 
 AVAIL (hard_float, TARGET_HARD_FLOAT || TARGET_ZFINX)
-
-
 AVAIL (clean32, TARGET_ZICBOM && !TARGET_64BIT)
 AVAIL (clean64, TARGET_ZICBOM && TARGET_64BIT)
 AVAIL (flush32, TARGET_ZICBOM && !TARGET_64BIT)
@@ -100,6 +98,7 @@ AVAIL (zero32,  TARGET_ZICBOZ && !TARGET_64BIT)
 AVAIL (zero64,  TARGET_ZICBOZ && TARGET_64BIT)
 AVAIL (prefetchi32, TARGET_ZICBOP && !TARGET_64BIT)
 AVAIL (prefetchi64, TARGET_ZICBOP && TARGET_64BIT)
+AVAIL (always,     (!0))
 
 /* Construct a riscv_builtin_description from the given arguments.
 
@@ -148,7 +147,8 @@ static const struct riscv_builtin_description riscv_builtins[] = {
   #include "riscv-cmo.def"
 
   DIRECT_BUILTIN (frflags, RISCV_USI_FTYPE, hard_float),
-  DIRECT_NO_TARGET_BUILTIN (fsflags, RISCV_VOID_FTYPE_USI, hard_float)
+  DIRECT_NO_TARGET_BUILTIN (fsflags, RISCV_VOID_FTYPE_USI, hard_float),
+  DIRECT_NO_TARGET_BUILTIN (pause, RISCV_VOID_FTYPE, always),
 };
 
 /* Index I is the function declaration for riscv_builtins[I], or null if the
diff --git a/gcc/config/riscv/riscv-ftypes.def b/gcc/config/riscv/riscv-ftypes.def
index c2b45c6..bf2d307 100644
--- a/gcc/config/riscv/riscv-ftypes.def
+++ b/gcc/config/riscv/riscv-ftypes.def
@@ -27,6 +27,7 @@ along with GCC; see the file COPYING3.  If not see
         argument type.  */
 
 DEF_RISCV_FTYPE (0, (USI))
+DEF_RISCV_FTYPE (0, (VOID))
 DEF_RISCV_FTYPE (1, (VOID, USI))
 DEF_RISCV_FTYPE (1, (VOID, VOID_PTR))
 DEF_RISCV_FTYPE (1, (SI, SI))
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index 532289d..0469882 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -97,6 +97,9 @@
   UNSPECV_INVAL
   UNSPECV_ZERO
   UNSPECV_PREI
+
+  ;; Zihintpause unspec
+  UNSPECV_PAUSE
 ])
 
 (define_constants
@@ -1913,6 +1916,11 @@
   "TARGET_ZIFENCEI"
   "fence.i")
 
+(define_insn "riscv_pause"
+  [(unspec_volatile [(const_int 0)] UNSPECV_PAUSE)]
+  ""
+  "pause")
+
 ;;
 ;;  ....................
 ;;
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 608bbe1..b1dd39e 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -21102,6 +21102,10 @@ processors.
 Returns the value that is currently set in the @samp{tp} register.
 @end deftypefn
 
+@deftypefn {Built-in Function}  void __builtin_riscv_pause (void)
+Generates the @code{pause} (hint) machine instruction.
+@end deftypefn
+
 @node RX Built-in Functions
 @subsection RX Built-in Functions
 GCC supports some of the RX instructions which cannot be expressed in
diff --git a/gcc/optabs.cc b/gcc/optabs.cc
index 2d9ca4e..bbe3bd7 100644
--- a/gcc/optabs.cc
+++ b/gcc/optabs.cc
@@ -7962,6 +7962,8 @@ maybe_gen_insn (enum insn_code icode, unsigned int nops,
 
   switch (nops)
     {
+    case 0:
+      return GEN_FCN (icode) ();
     case 1:
       return GEN_FCN (icode) (ops[0].value);
     case 2:
diff --git a/gcc/testsuite/gcc.target/riscv/builtin_pause.c b/gcc/testsuite/gcc.target/riscv/builtin_pause.c
new file mode 100644
index 0000000..9250937
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/builtin_pause.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" }  */
+
+void test_pause()
+{
+  __builtin_riscv_pause ();
+}
+
+/* { dg-final { scan-assembler "pause" } } */
+