[PATCH] RISC-V: Add support for addi that compresses to c.nop.

[Jim Wilson <jimw at sifive dot com>]

This fixes a bug reported against the github riscv/riscv-binutils-gdb project.
    https://github.com/riscv/riscv-binutils-gdb/issues/135

This fixes a minor oversight.  An addi x0,x0,0 is a nop, and should compress
to c.nop when RVC support is enabled, but without this patch we always get
a 4-byte nop.  With this patch, we get the expected 2-byte nop.

This was tested with rv{32,64}-{elf,linux} make check-{binutils,gas,ld}.  There
were no regressions.

Committed.

Jim

	gas/
	* testsuite/gas/riscv/c-zero-imm.s: Test addi that compresses to c.nop.
	* testsuite/gas/riscv/c-zero-imm.d: Likewise.
	opcodes/
	* riscv-opc.c (match_c_nop): New.
	(riscv_opcodes) <addi>: Handle an addi that compresses to c.nop.
---
 gas/testsuite/gas/riscv/c-zero-imm.d | 5 +++--
 gas/testsuite/gas/riscv/c-zero-imm.s | 1 +
 opcodes/riscv-opc.c                  | 8 ++++++++
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/gas/testsuite/gas/riscv/c-zero-imm.d b/gas/testsuite/gas/riscv/c-zero-imm.d
index ac47e8061d..c1389662e8 100644
--- a/gas/testsuite/gas/riscv/c-zero-imm.d
+++ b/gas/testsuite/gas/riscv/c-zero-imm.d
@@ -11,6 +11,7 @@ Disassembly of section .text:
 [ 	]+2:[ 	]+4581[ 	]+li[ 	]+a1,0
 [ 	]+4:[ 	]+8a01[ 	]+andi[ 	]+a2,a2,0
 [ 	]+6:[ 	]+8a81[ 	]+andi[ 	]+a3,a3,0
-[ 	]+8:[ 	]+00070713[ 	]+mv[ 	]+a4,a4
-[ 	]+c:[ 	]+0781[ 	]+addi[ 	]+a5,a5,0
+[ 	]+8:[ 	]+0001[ 	]+nop
+[ 	]+a:[ 	]+00070713[ 	]+mv[ 	]+a4,a4
+[ 	]+e:[ 	]+0781[ 	]+addi[ 	]+a5,a5,0
 #...
diff --git a/gas/testsuite/gas/riscv/c-zero-imm.s b/gas/testsuite/gas/riscv/c-zero-imm.s
index 650313d7cb..a07baa4995 100644
--- a/gas/testsuite/gas/riscv/c-zero-imm.s
+++ b/gas/testsuite/gas/riscv/c-zero-imm.s
@@ -4,6 +4,7 @@
 	c.li a1,0
 	andi a2,a2,0
 	c.andi a3,0
+	addi x0,x0,0
 	# Don't let this compress to a hint.
 	addi a4,a4,0
 	# These are hints.
diff --git a/opcodes/riscv-opc.c b/opcodes/riscv-opc.c
index 79e7214835..a4e4b26598 100644
--- a/opcodes/riscv-opc.c
+++ b/opcodes/riscv-opc.c
@@ -122,6 +122,13 @@ match_c_add_with_hint (const struct riscv_opcode *op, insn_t insn)
   return match_opcode (op, insn) && ((insn & MASK_CRS2) != 0);
 }
 
+static int
+match_c_nop (const struct riscv_opcode *op, insn_t insn)
+{
+  return (match_opcode (op, insn)
+	  && (((insn & MASK_RD) >> OP_SH_RD) == 0));
+}
+
 static int
 match_c_addi16sp (const struct riscv_opcode *op, insn_t insn)
 {
@@ -225,6 +232,7 @@ const struct riscv_opcode riscv_opcodes[] =
 {"bne",       "I",   "s,t,p",  MATCH_BNE, MASK_BNE, match_opcode, 0 },
 {"addi",      "C",   "Ct,Cc,CK", MATCH_C_ADDI4SPN, MASK_C_ADDI4SPN, match_c_addi4spn, INSN_ALIAS },
 {"addi",      "C",   "d,CU,Cj",  MATCH_C_ADDI, MASK_C_ADDI, match_rd_nonzero, INSN_ALIAS },
+{"addi",      "C",   "d,CU,0",    MATCH_C_NOP, MASK_C_ADDI | MASK_RVC_IMM, match_c_nop, INSN_ALIAS },
 {"addi",      "C",   "Cc,Cc,CL", MATCH_C_ADDI16SP, MASK_C_ADDI16SP, match_c_addi16sp, INSN_ALIAS },
 {"addi",      "I",   "d,s,j",  MATCH_ADDI, MASK_ADDI, match_opcode, 0 },
 {"add",       "C",   "d,CU,CV",  MATCH_C_ADD, MASK_C_ADD, match_c_add, INSN_ALIAS },
-- 
2.14.1