From: "Anil Paranjape" <AnilP1@KPITCummins.com>
To: "Kazu Hirata" <kazu@cs.umass.edu>
Cc: <gcc@gcc.gnu.org>
Subject: Ping : H8/300 ABI Document
Date: Tue, 30 Mar 2004 13:10:00 -0000 [thread overview]
Message-ID: <69595093233BB547BB70CF5E492B63F204B96BAE@sohm.kpit.com> (raw)
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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next reply other threads:[~2004-03-30 6:13 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-03-30 13:10 Anil Paranjape [this message]
2004-04-01 20:58 ` Jim Wilson
2004-04-02 8:44 Anil Paranjape
2004-04-05 23:57 ` Jim Wilson
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