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Message-ID: <53875FE9.9080103@arm.com>
Date: Thu, 29 May 2014 16:27:00 -0000
From: Yufeng Zhang <Yufeng.Zhang@arm.com>
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To: Martin Storsjo <martin@martin.st>
CC: "binutils@sourceware.org" <binutils@sourceware.org>,  Marcus Shawcroft <Marcus.Shawcroft@arm.com>
Subject: Re: [PATCH] aarch64: Fix the documentation on :pg_hi21:
References: <1401364824-4630-1-git-send-email-martin@martin.st> <538725B9.8060102@arm.com>
In-Reply-To: <538725B9.8060102@arm.com>
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On 05/29/14 13:19, Yufeng Zhang wrote:
> Hi Martin,
>
> On 05/29/14 13:00, Martin Storsjo wrote:
>> The syntax described, including a leading '#', was not supported
>> in practice. (For :lo12: it is optional.)
>
> I believe this is an issue in the parser; '#' should be optional in
> either case.
>
> It looks like gas/config/tc-aarch64.c:parse_adrp () is missing the
> skip_past_char (&p, '#') at its beginning; see parse_half () for an examp=
le.

After spending some time this afternoon reading ARMv8 Architecture=20
Reference Manual (ARMARM) and having a discussion with a colleague, I=20
think I'll have to take back what I just said.  Sorry!

The A64 assembly language allows the optional # character to introduce=20
constant immediate operands, e.g. in LDR instructions with immediate offset:

   LDR <Wt>, [<Xn|SP>], #<simm>

and in the bitfield move instructions:

   BFM <Wd>, <Wn>, #<immr>, #<imms>

The ADRP instruction, however, has the following syntax:

   ADRP <Xd>, <label>

whose second operand is a program label whose 4KB page address is to be=20
calculated, and for such a operand, it doesn't make much sense to allow=20
the # character as an optional prefix, e.g. ADRP X0, #foo would look=20
weird and confusing.

So I think your original patch is the right one to go with, and my only=20
comment is to remove both # characters from the line of

"
   @samp{#:pg_hi21:} and @samp{#:lo12:} respectively.
"

Apologize again for the previous wrong direction.

Please get an approval from Marcus.

Thanks,
Yufeng


>
> Yufeng
>
>> ---
>>    gas/doc/c-aarch64.texi | 10 +++++-----
>>    1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/gas/doc/c-aarch64.texi b/gas/doc/c-aarch64.texi
>> index cce4f61..1f4ce4c 100644
>> --- a/gas/doc/c-aarch64.texi
>> +++ b/gas/doc/c-aarch64.texi
>> @@ -186,24 +186,24 @@ For example to load the 48-bit absolute address of=
 @var{foo} into x0:
>>    @cindex ADRP, ADD, LDR/STR group relocations, AArch64
>>    Relocations for @samp{ADRP}, and @samp{ADD}, @samp{LDR} or @samp{STR}
>>    instructions can be generated by prefixing the label with
>> -@samp{#:pg_hi21:} and @samp{#:lo12:} respectively.
>> +@samp{:pg_hi21:} and @samp{#:lo12:} respectively.
>>
>>    For example to use 33-bit (+/-4GB) pc-relative addressing to
>>    load the address of @var{foo} into x0:
>>
>>    @smallexample
>> -        adrp x0, #:pg_hi21:foo
>> +        adrp x0, :pg_hi21:foo
>>            add  x0, x0, #:lo12:foo
>>    @end smallexample
>>
>>    Or to load the value of @var{foo} into x0:
>>
>>    @smallexample
>> -        adrp x0, #:pg_hi21:foo
>> +        adrp x0, :pg_hi21:foo
>>            ldr  x0, [x0, #:lo12:foo]
>>    @end smallexample
>>
>> -Note that @samp{#:pg_hi21:} is optional.
>> +Note that @samp{:pg_hi21:} is optional.
>>
>>    @smallexample
>>            adrp x0, foo
>> @@ -212,7 +212,7 @@ Note that @samp{#:pg_hi21:} is optional.
>>    is equivalent to
>>
>>    @smallexample
>> -        adrp x0, #:pg_hi21:foo
>> +        adrp x0, :pg_hi21:foo
>>    @end smallexample
>>
>>    @node AArch64 Floating Point
>
>
>