RISC-V: Implement __builtin_thread_pointer

RISC-V has a dedicate register for thread pointer which is specified in psABI
doc, so we could support __builtin_thread_pointer in straightforward way.

Note: clang/llvm was supported __builtin_thread_pointer for RISC-V port
recently.
- https://reviews.llvm.org/rGaabc24acf0d5f8677bd22fe9c108581e07c3e180

gcc/ChangeLog:

	* config/riscv/riscv.md (get_thread_pointer<mode>): New.
	(TP_REGNUM): Ditto.
	* doc/extend.texi (Target Builtins): Add RISC-V built-in section.
	Document __builtin_thread_pointer.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/read-thread-pointer.c: New.

gcc/config/riscv/riscv.md

@@ -70,6 +70,7 @@
 (define_constants
   [(RETURN_ADDR_REGNUM		1)
    (GP_REGNUM 			3)
+   (TP_REGNUM			4)
    (T0_REGNUM			5)
    (T1_REGNUM			6)
    (S0_REGNUM			8)
@@ -2515,6 +2516,13 @@
   DONE;
 })
 
+;; Named pattern for expanding thread pointer reference.
+(define_expand "get_thread_pointer<mode>"
+  [(set (match_operand:P 0 "register_operand" "=r")
+	(reg:P TP_REGNUM))]
+  ""
+{})
+
 (include "sync.md")
 (include "peephole.md")
 (include "pic.md")

gcc/doc/extend.texi

@@ -13859,6 +13859,7 @@ instructions, but allow the compiler to schedule those calls.
 * PowerPC Hardware Transactional Memory Built-in Functions::
 * PowerPC Atomic Memory Operation Functions::
 * PowerPC Matrix-Multiply Assist Built-in Functions::
+* RISC-V Built-in Functions::
 * RX Built-in Functions::
 * S/390 System z Built-in Functions::
 * SH Built-in Functions::
@@ -21471,6 +21472,16 @@ vec_t __builtin_vsx_xvcvspbf16 (vec_t);
 vec_t __builtin_vsx_xvcvbf16sp (vec_t);
 @end smallexample
 
+@node RISC-V Built-in Functions
+@subsection RISC-V Built-in Functions
+
+These built-in functions are available for the RISC-V family of
+processors.
+
+@deftypefn {Built-in Function} {void *} __builtin_thread_pointer (void)
+Returns the value that is currently set in the @samp{tp} register.
+@end deftypefn
+
 @node RX Built-in Functions
 @subsection RX Built-in Functions
 GCC supports some of the RX instructions which cannot be expressed in

gcc/testsuite/gcc.target/riscv/read-thread-pointer.c

@@ -0,0 +1,7 @@
+/* { dg-do compile } */
+
+void *get_tp()
+{
+    return __builtin_thread_pointer ();
+}
+/* { dg-final { scan-assembler "mv\[ \t\]*[at][0-9]+,tp" } } */