Allow for optional operands with non-zero default values.

ISA 2.07 (ie, POWER8) added the rfebb instruction which takes one operand
with the value of either a 0 or 1.  It also defines an extended mnemonic
with no operands (ie, "rfebb") that is supposed to be equivalent to "rfebb 1".
I implemented rfebb's lone operand with PPC_OPERAND_OPTIONAL, but the
problem is, optional operands that are ommitted always default to the
value 0, which is wrong in this case.  I have added support for allowing
non-zero default values by adding an additional flag PPC_OPERAND_OPTIONAL_VALUE
that specifies that the default operand value to be used is stored in the
SHIFT field of the operand field immediately following this one.

This fixes the rfebb issue.  I also fixed the mftb and mfcr instructions
so they use the same mechanism.  This allows us to flag invalid uses of
mfcr where we explicitly pass in a zero FXM value, like the use in a2.[sd].

include/opcode/

	* ppc.h (PPC_OPERAND_OPTIONAL_VALUE): New.
	(ppc_optional_operand_value): New inline function.

opcodes/
	* ppc-dis.h (skip_optional_operands): Use ppc_optional_operand_value.
	* ppc-opc.c (FXM4): Add non-zero optional value.
	(TBR): Likewise.
	(SXL): Likewise.
	(insert_fxm): Handle new default operand value.
	(extract_fxm): Likewise.
	(insert_tbr): Likewise.
	(extract_tbr): Likewise.

gas/
	* config/tc-ppc.c (md_assemble): Use ppc_optional_operand_value.
	Allow for optional operands without insert functions.

gas/testsuite/
	* gas/ppc/power8.d: Fixup rfebb test results.
	* gas/ppc/a2.s: Fix invalid mfcr test.
	* gas/ppc/a2.d: Likewise.

1 files changed, 13 insertions, 0 deletions

diff --git a/include/opcode/ppc.h b/include/opcode/ppc.h
index 5dc132e..5246365 100644
--- a/include/opcode/ppc.h
+++ b/include/opcode/ppc.h
@@ -380,6 +380,11 @@ extern const unsigned int num_powerpc_operands;
 
 /* This is a CR FIELD that does not use symbolic names.  */
 #define PPC_OPERAND_CR_REG (0x200000)
+
+/* This flag is only used with PPC_OPERAND_OPTIONAL.  If this operand
+   is omitted, then the value it should use for the operand is stored
+   in the SHIFT field of the immediatly following operand field.  */
+#define PPC_OPERAND_OPTIONAL_VALUE (0x400000)
 
 /* The POWER and PowerPC assemblers use a few macros.  We keep them
    with the operands table for simplicity.  The macro table is an
@@ -409,4 +414,12 @@ extern const int powerpc_num_macros;
 
 extern ppc_cpu_t ppc_parse_cpu (ppc_cpu_t, ppc_cpu_t *, const char *);
 
+static inline long
+ppc_optional_operand_value (const struct powerpc_operand *operand)
+{
+  if ((operand->flags & PPC_OPERAND_OPTIONAL_VALUE) != 0)
+    return (operand+1)->shift;
+  return 0;
+}
+
 #endif /* PPC_H */