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Re: ARM gas: Maximum alignment
- From: Alan Modra <amodra at gmail dot com>
- To: Matthew Fernandez <matthew dot fernandez at gmail dot com>
- Cc: binutils at sourceware dot org
- Date: Mon, 27 Jul 2015 17:15:56 +0930
- Subject: Re: ARM gas: Maximum alignment
- Authentication-results: sourceware.org; auth=none
- References: <55B4247C dot 2070302 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