aarch64: Check for register aliases before mnemonics

Previously we would not accept:

	A .req B

if A happened to be the name of an instruction.  Adding new
instructions could therefore invalidate existing register aliases.

I noticed this with a test that used "zero" as a register alias
for "xzr", where "zero" is now also the name of an SME instruction.
I don't have any evidence that "real" code is doing this, but it
seems at least plausible.

This patch switches things so that we check for register aliases
first.  It might slow down parsing slightly, but the difference
is unlikely to be noticeable.

Things like:

	b	.req + 0

still work, since create_register_alias checks for " .req ",
and with the input scrubber, we'll only keep whitespace after
.req if it's followed by another name.  If there's some valid
expression that I haven't thought about that is scrubbed to
" .req ", users could avoid the ambiguity by wrapping .req
in parentheses.

The new test for invalid aliases already passed.  I just wanted
something to exercise the !dot condition.

I can't find a way of exercising the (existing) p == base condition,
but I'm not brave enough to say that it can never happen.  If it does
happen, get_mnemonic_name would return an empty string.

gas/
	* config/tc-aarch64.c (opcode_lookup): Move mnemonic extraction
	code to...
	(md_assemble): ...here.  Check for register aliases first.
	* testsuite/gas/aarch64/register_aliases.d,
	testsuite/gas/aarch64/register_aliases.s: Test for a register
	alias called "zero".
	* testsuite/gas/aarch64/register_aliases_invalid.d,
	testsuite/gas/aarch64/register_aliases_invalid.l,
	testsuite/gas/aarch64/register_aliases_invalid.s: New test.

6 files changed, 38 insertions, 34 deletions

diff --git a/gas/config/tc-aarch64.c b/gas/config/tc-aarch64.c
index 9fc61f7..b6ed80e 100644
--- a/gas/config/tc-aarch64.c
+++ b/gas/config/tc-aarch64.c
@@ -5738,25 +5738,18 @@ lookup_mnemonic (const char *start, int len)
 }
 
 /* Subroutine of md_assemble, responsible for looking up the primary
-   opcode from the mnemonic the user wrote.  STR points to the
-   beginning of the mnemonic. */
+   opcode from the mnemonic the user wrote.  BASE points to the beginning
+   of the mnemonic, DOT points to the first '.' within the mnemonic
+   (if any) and END points to the end of the mnemonic.  */
 
 static templates *
-opcode_lookup (char **str)
+opcode_lookup (char *base, char *dot, char *end)
 {
-  char *end, *base, *dot;
   const aarch64_cond *cond;
   char condname[16];
   int len;
 
-  /* Scan up to the end of the mnemonic, which must end in white space,
-     '.', or end of string.  */
-  dot = 0;
-  for (base = end = *str; is_part_of_name(*end); end++)
-    if (*end == '.' && !dot)
-      dot = end;
-
-  if (end == base || dot == base)
+  if (dot == end)
     return 0;
 
   inst.cond = COND_ALWAYS;
@@ -5765,23 +5758,13 @@ opcode_lookup (char **str)
   if (dot)
     {
       cond = str_hash_find_n (aarch64_cond_hsh, dot + 1, end - dot - 1);
-      if (cond)
-	{
-	  inst.cond = cond->value;
-	  *str = end;
-	}
-      else
-	{
-	  *str = dot;
-	  return 0;
-	}
+      if (!cond)
+	return 0;
+      inst.cond = cond->value;
       len = dot - base;
     }
   else
-    {
-      *str = end;
-      len = end - base;
-    }
+    len = end - base;
 
   if (inst.cond == COND_ALWAYS)
     {
@@ -7870,7 +7853,6 @@ dump_opcode_operands (const aarch64_opcode *opcode)
 void
 md_assemble (char *str)
 {
-  char *p = str;
   templates *template;
   const aarch64_opcode *opcode;
   aarch64_inst *inst_base;
@@ -7893,14 +7875,28 @@ md_assemble (char *str)
   DEBUG_TRACE ("==============================");
   DEBUG_TRACE ("Enter md_assemble with %s", str);
 
-  template = opcode_lookup (&p);
+  /* Scan up to the end of the mnemonic, which must end in whitespace,
+     '.', or end of string.  */
+  char *p = str;
+  char *dot = 0;
+  for (; is_part_of_name (*p); p++)
+    if (*p == '.' && !dot)
+      dot = p;
+
+  if (p == str)
+    {
+      as_bad (_("unknown mnemonic -- `%s'"), str);
+      return;
+    }
+
+  if (!dot && create_register_alias (str, p))
+    return;
+
+  template = opcode_lookup (str, dot, p);
   if (!template)
     {
-      /* It wasn't an instruction, but it might be a register alias of
-         the form alias .req reg directive.  */
-      if (!create_register_alias (str, p))
-	as_bad (_("unknown mnemonic `%s' -- `%s'"), get_mnemonic_name (str),
-		str);
+      as_bad (_("unknown mnemonic `%s' -- `%s'"), get_mnemonic_name (str),
+	      str);
       return;
     }
 
diff --git a/gas/testsuite/gas/aarch64/register_aliases.d b/gas/testsuite/gas/aarch64/register_aliases.d
index eab6387..8d614b4 100644
--- a/gas/testsuite/gas/aarch64/register_aliases.d
+++ b/gas/testsuite/gas/aarch64/register_aliases.d
@@ -10,3 +10,4 @@ Disassembly of section \.text:
    8:	f94003b1 	ldr	x17, \[x29\]
    c:	f90003b0 	str	x16, \[x29\]
   10:	f94003b1 	ldr	x17, \[x29\]
+  14:	f900001f 	str	xzr, \[x0\]
diff --git a/gas/testsuite/gas/aarch64/register_aliases.s b/gas/testsuite/gas/aarch64/register_aliases.s
index fcd0650..856be56 100644
--- a/gas/testsuite/gas/aarch64/register_aliases.s
+++ b/gas/testsuite/gas/aarch64/register_aliases.s
@@ -3,9 +3,10 @@
 	fp 	.req 	x29
 	ip0	.req 	x16
 	ip1	.req 	x17
+	zero	.req	xzr
 	add	ip0, ip0, lr
 	str 	ip0, [fp]
 	ldr	ip1, [fp]
 	str 	IP0, [fp]
 	ldr	IP1, [fp]
-
+	str	zero, [x0]
diff --git a/gas/testsuite/gas/aarch64/register_aliases_invalid.d b/gas/testsuite/gas/aarch64/register_aliases_invalid.d
new file mode 100644
index 0000000..7c453ce
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/register_aliases_invalid.d
@@ -0,0 +1 @@
+#error_output: register_aliases_invalid.l
diff --git a/gas/testsuite/gas/aarch64/register_aliases_invalid.l b/gas/testsuite/gas/aarch64/register_aliases_invalid.l
new file mode 100644
index 0000000..6350049
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/register_aliases_invalid.l
@@ -0,0 +1,3 @@
+.*:
+.*: Error: unknown mnemonic `lr\.req' -- `lr\.req x30'
+.*: Error: unknown mnemonic `lr\.a' -- `lr\.a .req x30'
diff --git a/gas/testsuite/gas/aarch64/register_aliases_invalid.s b/gas/testsuite/gas/aarch64/register_aliases_invalid.s
new file mode 100644
index 0000000..2df2eaa
--- /dev/null
+++ b/gas/testsuite/gas/aarch64/register_aliases_invalid.s
@@ -0,0 +1,2 @@
+lr.req 	x30
+lr.a	.req	x30