Re: ARM gas: Maximum alignment

[Alan Modra <amodra at gmail dot com>]

On Sun, Jul 26, 2015 at 10:06:20AM +1000, Matthew Fernandez wrote:
>     // gas/config/tc-arm.c:2980 @ f0b0791b05ed335e5d74d843d828645805db1f9c
>     long max_alignment = 15;
> 
> My question is, why is this limit is 15 bits? I'm not aware of anything in
> the hardware/ELF specs that would prevent larger alignment. The limitation
> seems to vary across targets and it is not clear where the value of 15 for
> ARM is derived from. Please CC me directly in replies as I'm not on the
> list.

Alignment used to be limited to 32k a long time ago, probably because
older object file formats have restrictions on the maximum alignment
of a section.  tc-arm.c:s_align was copied from the generic code in
read.c before the 15 bit limit was lifted.

It seems like the only reason tc-arm.c needs its own copy of s_align
is that ".align 0" oddly means the same as ".align", ie. a default
alignment.

How does this rationalization look to the ARM maintainers?  This will
also give ARM support for 3 arg .align, specifying max skip bytes.

	* gas/config/tc-arm.c (s_align): Delete.
	(md_pseudo_table): Use s_align_ptwo for "align".
	* gas/config/tc-arm.h (TC_ALIGN_ZERO_IS_DEFAULT): Define.
	* read.c (s_align): Modify for TC_ALIGN_ZERO_IS_DEFAULT.

diff --git a/gas/config/tc-arm.c b/gas/config/tc-arm.c
index 7b3b5c9..9115eed 100644
--- a/gas/config/tc-arm.c
+++ b/gas/config/tc-arm.c
@@ -2969,53 +2969,6 @@ s_syntax (int unused ATTRIBUTE_UNUSED)
 
 /* Directives: sectioning and alignment.  */
 
-/* Same as s_align_ptwo but align 0 => align 2.	 */
-
-static void
-s_align (int unused ATTRIBUTE_UNUSED)
-{
-  int temp;
-  bfd_boolean fill_p;
-  long temp_fill;
-  long max_alignment = 15;
-
-  temp = get_absolute_expression ();
-  if (temp > max_alignment)
-    as_bad (_("alignment too large: %d assumed"), temp = max_alignment);
-  else if (temp < 0)
-    {
-      as_bad (_("alignment negative. 0 assumed."));
-      temp = 0;
-    }
-
-  if (*input_line_pointer == ',')
-    {
-      input_line_pointer++;
-      temp_fill = get_absolute_expression ();
-      fill_p = TRUE;
-    }
-  else
-    {
-      fill_p = FALSE;
-      temp_fill = 0;
-    }
-
-  if (!temp)
-    temp = 2;
-
-  /* Only make a frag if we HAVE to.  */
-  if (temp && !need_pass_2)
-    {
-      if (!fill_p && subseg_text_p (now_seg))
-	frag_align_code (temp, 0);
-      else
-	frag_align (temp, (int) temp_fill, 0);
-    }
-  demand_empty_rest_of_line ();
-
-  record_alignment (now_seg, temp);
-}
-
 static void
 s_bss (int ignore ATTRIBUTE_UNUSED)
 {
@@ -4691,7 +4644,7 @@ const pseudo_typeS md_pseudo_table[] =
   { "qn",          s_qn,          0 },
   { "unreq",	   s_unreq,	  0 },
   { "bss",	   s_bss,	  0 },
-  { "align",	   s_align,	  0 },
+  { "align",	   s_align_ptwo,  2 },
   { "arm",	   s_arm,	  0 },
   { "thumb",	   s_thumb,	  0 },
   { "code",	   s_code,	  0 },
diff --git a/gas/config/tc-arm.h b/gas/config/tc-arm.h
index 22e9027..98038be 100644
--- a/gas/config/tc-arm.h
+++ b/gas/config/tc-arm.h
@@ -245,6 +245,7 @@ arm_min (int am_p1, int am_p2)
 #define TC_FRAG_TYPE		struct arm_frag_type
 /* NOTE: max_chars is a local variable from frag_var / frag_variant.  */
 #define TC_FRAG_INIT(fragp)	arm_init_frag (fragp, max_chars)
+#define TC_ALIGN_ZERO_IS_DEFAULT 1
 #define HANDLE_ALIGN(fragp)	arm_handle_align (fragp)
 #define SUB_SEGMENT_ALIGN(SEG, FRCHAIN)			\
   ((!(FRCHAIN)->frch_next && subseg_text_p (SEG))		\
diff --git a/gas/read.c b/gas/read.c
index 816c255..000c75b 100644
--- a/gas/read.c
+++ b/gas/read.c
@@ -1471,6 +1471,11 @@ s_align (int arg, int bytes_p)
     {
       align = get_absolute_expression ();
       SKIP_WHITESPACE ();
+
+#ifdef TC_ALIGN_ZERO_IS_DEFAULT
+      if (arg > 0 && align == 0)
+	align = arg;
+#endif
     }
 
   if (bytes_p)

-- 
Alan Modra
Australia Development Lab, IBM