[PATCH] RISC-V: Implement TLS Descriptors.

[Tatsuyuki Ishi <ishitatsuyuki@gmail.com>]

This implements TLS Descriptors (TLSDESC) as specified in [1].

In TLSDESC instruction sequence, the first instruction relocates against
the target TLS variable, while subsequent instructions relocates against
the address of the first. Such usage of labels are not well-supported
within GCC. Due to this, the 4-instruction sequence is implemented as a
single RTX insn.

For now, keep defaulting to the traditional TLS model, but this can be
revisited once toolchain and libc support ships.

[1]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373
---
No regression in binutils and gcc tests for rv64gc, tested alongside the
binutils and glibc implementation (posted at the same time). During
testing, the default TLS dialect was changed to TLSDESC.

This contribution is made on behalf of Blue Whale Systems, which has
copyright assignment on file with the FSF.

 gcc/config/riscv/riscv-opts.h   |  6 ++++++
 gcc/config/riscv/riscv-protos.h |  5 +++--
 gcc/config/riscv/riscv.cc       | 34 +++++++++++++++++++++++++++++----
 gcc/config/riscv/riscv.h        |  3 +++
 gcc/config/riscv/riscv.md       | 22 ++++++++++++++++++++-
 gcc/config/riscv/riscv.opt      | 14 ++++++++++++++
 6 files changed, 77 insertions(+), 7 deletions(-)

diff --git a/gcc/config/riscv/riscv-opts.h b/gcc/config/riscv/riscv-opts.h
index 378a17699cd..db03f35430a 100644
--- a/gcc/config/riscv/riscv-opts.h
+++ b/gcc/config/riscv/riscv-opts.h
@@ -319,4 +319,10 @@ enum riscv_entity
 #define TARGET_VECTOR_VLS                                                      \
   (TARGET_VECTOR && riscv_autovec_preference == RVV_SCALABLE)
 
+/* TLS types.  */
+enum riscv_tls_type {
+  TLS_TRADITIONAL,
+  TLS_DESCRIPTORS
+};
+
 #endif /* ! GCC_RISCV_OPTS_H */
diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
index 472c00dc439..9b7471f7591 100644
--- a/gcc/config/riscv/riscv-protos.h
+++ b/gcc/config/riscv/riscv-protos.h
@@ -33,9 +33,10 @@ enum riscv_symbol_type {
   SYMBOL_TLS,
   SYMBOL_TLS_LE,
   SYMBOL_TLS_IE,
-  SYMBOL_TLS_GD
+  SYMBOL_TLS_GD,
+  SYMBOL_TLSDESC,
 };
-#define NUM_SYMBOL_TYPES (SYMBOL_TLS_GD + 1)
+#define NUM_SYMBOL_TYPES (SYMBOL_TLSDESC + 1)
 
 /* Classifies an address.
 
diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 49062bef9fc..4ff0adbbb1e 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -799,6 +799,7 @@ static int riscv_symbol_insns (enum riscv_symbol_type type)
     case SYMBOL_ABSOLUTE: return 2; /* LUI + the reference.  */
     case SYMBOL_PCREL: return 2; /* AUIPC + the reference.  */
     case SYMBOL_TLS_LE: return 3; /* LUI + ADD TP + the reference.  */
+    case SYMBOL_TLSDESC: return 6; /* 4-instruction call + ADD TP + the reference.  */
     case SYMBOL_GOT_DISP: return 3; /* AUIPC + LD GOT + the reference.  */
     default: gcc_unreachable ();
     }
@@ -1601,6 +1602,16 @@ static rtx riscv_tls_add_tp_le (rtx dest, rtx base, rtx sym)
     return gen_tls_add_tp_lesi (dest, base, tp, sym);
 }
 
+/* Instruction sequence to call the TLS Descriptor resolver.  */
+
+static rtx riscv_tlsdesc (rtx sym, rtx seqno)
+{
+  if (Pmode == DImode)
+    return gen_tlsdescdi (sym, seqno);
+  else
+    return gen_tlsdescsi (sym, seqno);
+}
+
 /* If MODE is MAX_MACHINE_MODE, ADDR appears as a move operand, otherwise
    it appears in a MEM of that mode.  Return true if ADDR is a legitimate
    constant in that context and can be split into high and low parts.
@@ -1734,7 +1745,7 @@ riscv_call_tls_get_addr (rtx sym, rtx result)
 static rtx
 riscv_legitimize_tls_address (rtx loc)
 {
-  rtx dest, tp, tmp;
+  rtx dest, tp, tmp, a0;
   enum tls_model model = SYMBOL_REF_TLS_MODEL (loc);
 
 #if 0
@@ -1750,9 +1761,24 @@ riscv_legitimize_tls_address (rtx loc)
       /* Rely on section anchors for the optimization that LDM TLS
 	 provides.  The anchor's address is loaded with GD TLS. */
     case TLS_MODEL_GLOBAL_DYNAMIC:
-      tmp = gen_rtx_REG (Pmode, GP_RETURN);
-      dest = gen_reg_rtx (Pmode);
-      emit_libcall_block (riscv_call_tls_get_addr (loc, tmp), dest, tmp, loc);
+      if (TARGET_TLSDESC)
+	{
+	  static unsigned seqno;
+	  tp = gen_rtx_REG (Pmode, THREAD_POINTER_REGNUM);
+	  a0 = gen_rtx_REG (Pmode, GP_ARG_FIRST);
+	  dest = gen_reg_rtx (Pmode);
+
+	  emit_insn (riscv_tlsdesc (loc, GEN_INT (seqno)));
+	  emit_insn (gen_add3_insn (dest, a0, tp));
+	  seqno++;
+	}
+      else
+	{
+	  tmp = gen_rtx_REG (Pmode, GP_RETURN);
+	  dest = gen_reg_rtx (Pmode);
+	  emit_libcall_block (riscv_call_tls_get_addr (loc, tmp), dest, tmp,
+			      loc);
+	}
       break;
 
     case TLS_MODEL_INITIAL_EXEC:
diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index e18a0081297..7cf1365ec08 100644
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -1122,4 +1122,7 @@ extern void riscv_remove_unneeded_save_restore_calls (void);
 #define OPTIMIZE_MODE_SWITCHING(ENTITY) (TARGET_VECTOR)
 #define NUM_MODES_FOR_MODE_SWITCHING {VXRM_MODE_NONE, riscv_vector::FRM_NONE}
 
+/* Check TLS Descriptors mechanism is selected.  */
+#define TARGET_TLSDESC (riscv_tls_dialect == TLS_DESCRIPTORS)
+
 #endif /* ! GCC_RISCV_H */
diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
index b456fa6abb3..bddd92323ad 100644
--- a/gcc/config/riscv/riscv.md
+++ b/gcc/config/riscv/riscv.md
@@ -47,7 +47,7 @@
   UNSPEC_TLS_LE
   UNSPEC_TLS_IE
   UNSPEC_TLS_GD
-
+  UNSPEC_TLSDESC
   ;; High part of PC-relative address.
   UNSPEC_AUIPC
 
@@ -121,6 +121,14 @@
    (T1_REGNUM			6)
    (S0_REGNUM			8)
    (S1_REGNUM			9)
+   (A0_REGNUM			10)
+   (A1_REGNUM			11)
+   (A2_REGNUM			12)
+   (A3_REGNUM			13)
+   (A4_REGNUM			14)
+   (A5_REGNUM			15)
+   (A6_REGNUM			16)
+   (A7_REGNUM			17)
    (S2_REGNUM			18)
    (S3_REGNUM			19)
    (S4_REGNUM			20)
@@ -1869,6 +1877,18 @@
   [(set_attr "got" "load")
    (set_attr "mode" "<MODE>")])
 
+(define_insn "tlsdesc<mode>"
+  [(set (reg:P A0_REGNUM)
+	    (unspec:P
+			[(match_operand:P 0 "symbolic_operand" "")
+             (match_operand:P 1 "const_int_operand")]
+			UNSPEC_TLSDESC))
+   (clobber (reg:SI T0_REGNUM))]
+  "TARGET_TLSDESC"
+  ".LT%1: auipc\ta0, %%tlsdesc_hi(%0)\;<load>\tt0,%%tlsdesc_load_lo(.LT%1)(a0)\;addi\ta0,a0,%%tlsdesc_add_lo(.LT%1)\;jalr\tt0,t0,%%tlsdesc_call(.LT%1)"
+  [(set_attr "type" "multi")
+   (set_attr "mode" "<MODE>")])
+
 (define_insn "auipc<mode>"
   [(set (match_operand:P           0 "register_operand" "=r")
 	(unspec:P
diff --git a/gcc/config/riscv/riscv.opt b/gcc/config/riscv/riscv.opt
index 6304efebfd5..40b3ebf2a99 100644
--- a/gcc/config/riscv/riscv.opt
+++ b/gcc/config/riscv/riscv.opt
@@ -311,3 +311,17 @@ Enum(riscv_autovec_lmul) String(m8) Value(RVV_M8)
 -param=riscv-autovec-lmul=
 Target RejectNegative Joined Enum(riscv_autovec_lmul) Var(riscv_autovec_lmul) Init(RVV_M1)
 -param=riscv-autovec-lmul=<string>	Set the RVV LMUL of auto-vectorization in the RISC-V port.
+
+Enum
+Name(tls_type) Type(enum riscv_tls_type)
+The possible TLS dialects:
+
+EnumValue
+Enum(tls_type) String(trad) Value(TLS_TRADITIONAL)
+
+EnumValue
+Enum(tls_type) String(desc) Value(TLS_DESCRIPTORS)
+
+mtls-dialect=
+Target RejectNegative Joined Enum(tls_type) Var(riscv_tls_dialect) Init(TLS_TRADITIONAL) Save
+Specify TLS dialect.
\ No newline at end of file
-- 
2.34.1