* RE: Ping : H8/300 ABI Document
@ 2004-04-02 8:44 Anil Paranjape
2004-04-05 23:57 ` Jim Wilson
0 siblings, 1 reply; 4+ messages in thread
From: Anil Paranjape @ 2004-04-02 8:44 UTC (permalink / raw)
To: Jim Wilson; +Cc: gcc
Hi Jim,
>This is a maximum. Fields get natural alignment (alignment same as
>size) up to this maximum. So shorts are always 2 byte aligned, but
>4-byte ints get 2-byte alignment on H8/300 and 4-byte alignment on H8S.
>Structure alignment is the max of the alignment of all fields.
I agree to this.
In my example, there were two short variables as members of structure.
Each will be treated as separate member.
> Structure Alignment
> Unless __attribute__ ((packed)) is attached to the declaration of a struct, each structure member is aligned
> to a multiple of 2 bytes on H8/300 and of 4 bytes on H8/300H and H8S.
Hence as per above statement,
Each short variable(each structure member) should be aligned to multiple of 4 bytes on H8S.
This is not the case. Because as you have rightly said "shorts always have 2-byte alignment".
So size of structure is 4 bytes and alignment is 2 bytes.
I am suggesting to change the description of "structure alignment" to proper one.
Thanks and Regards,
Anil
-----Original Message-----
From: Jim Wilson [mailto:wilson@specifixinc.com]
Sent: Friday, April 02, 2004 2:29 AM
To: Anil Paranjape
Cc: gcc@gcc.gnu.org
Subject: Re: Ping : H8/300 ABI Document
Anil Paranjape wrote:
> Structure Alignment
> Unless __attribute__ ((packed)) is attached to the declaration of a struct, each structure member is aligned
> to a multiple of 2 bytes on H8/300 and of 4 bytes on H8/300H and H8S.
This is a maximum. Fields get natural alignment (alignment same as
size) up to this maximum. So shorts are always 2 byte aligned, but
4-byte ints get 2-byte alignment on H8/300 and 4-byte alignment on H8S.
Structure alignment is the max of the alignment of all fields.
Your suggested wording is not correct. The structure in your example
still has only 2-byte alignment even whwn compiled for H8S.
.comm _t,4,2
gives it a 4-byte size and 2-byte alignment. This is because shorts
always have 2-byte alignment.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
^ permalink raw reply [flat|nested] 4+ messages in thread
* RE: Ping : H8/300 ABI Document
2004-04-02 8:44 Ping : H8/300 ABI Document Anil Paranjape
@ 2004-04-05 23:57 ` Jim Wilson
0 siblings, 0 replies; 4+ messages in thread
From: Jim Wilson @ 2004-04-05 23:57 UTC (permalink / raw)
To: Anil Paranjape; +Cc: gcc
On Fri, 2004-04-02 at 00:43, Anil Paranjape wrote:
> I am suggesting to change the description of "structure alignment" to proper one.
Your suggested wording is wrong. I think the current wording is better
than your replacement, because your replacement says nothing about
alignment of fields. I think the current one can be improved by
changing "a multiple of" to "at most"
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Ping : H8/300 ABI Document
2004-03-30 13:10 Anil Paranjape
@ 2004-04-01 20:58 ` Jim Wilson
0 siblings, 0 replies; 4+ messages in thread
From: Jim Wilson @ 2004-04-01 20:58 UTC (permalink / raw)
To: Anil Paranjape; +Cc: gcc
Anil Paranjape wrote:
> Structure Alignment
> Unless __attribute__ ((packed)) is attached to the declaration of a struct, each structure member is aligned
> to a multiple of 2 bytes on H8/300 and of 4 bytes on H8/300H and H8S.
This is a maximum. Fields get natural alignment (alignment same as
size) up to this maximum. So shorts are always 2 byte aligned, but
4-byte ints get 2-byte alignment on H8/300 and 4-byte alignment on H8S.
Structure alignment is the max of the alignment of all fields.
Your suggested wording is not correct. The structure in your example
still has only 2-byte alignment even whwn compiled for H8S.
.comm _t,4,2
gives it a 4-byte size and 2-byte alignment. This is because shorts
always have 2-byte alignment.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
^ permalink raw reply [flat|nested] 4+ messages in thread
* Ping : H8/300 ABI Document
@ 2004-03-30 13:10 Anil Paranjape
2004-04-01 20:58 ` Jim Wilson
0 siblings, 1 reply; 4+ messages in thread
From: Anil Paranjape @ 2004-03-30 13:10 UTC (permalink / raw)
To: Kazu Hirata; +Cc: gcc
Hi,
Please review change suggested in H8 ABI document from following link,
http://gcc.gnu.org/ml/gcc/2004-03/msg01325.html
Regards,
Anil Paranjpe
-----Original Message-----
From: Anil Paranjape
Sent: Tuesday, March 23, 2004 3:24 PM
To: 'Kazu Hirata'; gnuh8@gnuh8.org.uk
Cc: 'gcc@gcc.gnu.org'
Subject: RE: ABI Document
Hi,
Please find following sentence from http://gcc.gnu.org/projects/h8300-abi.html,
Structure Alignment
Unless __attribute__ ((packed)) is attached to the declaration of a struct, each structure member is aligned
to a multiple of 2 bytes on H8/300 and of 4 bytes on H8/300H and H8S.
I think this is slightly misleading statement.
Consider following testcase,
struct test {
unsigned short a; /* Address 0,1 */
unsigned short b; /* Address 2,3 */
};
struct test t;
int main (void)
{
t.b = 1;
return 0;
}
Compiler options passed are
-ms -ms2600 -fverbose-asm -S
Generated assembly for above testcase is following,
.file "test.c"
.h8300s
.section .text
.align 1
.global _main
_main:
mov.l er6,@-er7 ;#,
mov.l er7,er6 ;#,
mov.w #1,r2 ;#, tmp17
mov.w r2,@_t+2 ;# tmp17, t.b
sub.w r2,r2 ;# <result>
mov.w r2,r0 ;# <result>, <result>
mov.l @er7+,er6 ;#,
rts
.size _main, .-_main
.comm _t,4,2
.end
.ident "GCC: (GNU) 3.4.0 20040303 (prerelease)"
Analysis:
Assuming above mentioned structure alignment is true then instruction "mov.w r2,@_t+2" used for accessing t.b in generated assembly should be wrong. The instruction should be "mov.w r2,@_t+4".
According to structure alignment every structure member is aligned to 4 bytes . So padding mentioned in following structure definition should be correct,
struct test {
unsigned short a; /* Address 0,1 (2,3 padding) */
unsigned short b; /* Address 4,5 (6,7 padding) */
};
But actually it is not the case. By default structure is aligned to 4 byte boundary.
So instruction for accessing t.b in generated assembly is correct.
The default alignment of structure is 4 bytes unless __attribute__ ((packed)) is used.
I suggest to change the text of structure alignment in ABI document like following,
Structure Alignment
Unless __attribute__ ((packed)) is attached to the declaration of a struct or union, each structure or union is aligned
to a multiple of 2 bytes on H8/300 and of 4 bytes on H8/300H and H8S. The compiler inserts padding to maintain the alignment of its members. The sizeof a structure is always multiple of its alignment and this may require tail padding.
Regards,
Anil Paranjpe
-----Original Message-----
From: Kazu Hirata [mailto:kazu@cs.umass.edu]
Sent: Monday, January 19, 2004 11:22 AM
To: gnuh8@gnuh8.org.uk
Subject: ABI Document
Hi,
I just added a document describing H8 ABI for GCC.
http://gcc.gnu.org/projects/h8300-abi.html
I'll try to make it complete over time. Suggestions, questions, etc,
are all welcome.
Maybe interrupt function handling needs to be described.
Kazu Hirata
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