You can steer this behaviour using the packed and aligned attributess. By
setting the aligned attribute at 2 the compiler should generate efficient
code for elements which are word aligned.
----- Original Message -----
From: "Robert Kavaler" <[email protected]>
To: <[email protected]>
Sent: Friday, October 30, 2009 11:46 AM
Subject: Re: [Mspgcc-users] Possible code generation bug for packed structs
The older version generated the same code as if "packed" was not
specified. That version was either from 2003 or 2005. I see the
justification for doing the byte addressing, but I still think the
compiler could generate much better code. For example this:
mov.b &aa, r14
mov #aa+1, r12
mov.b @r12, r13
swpb r13
bis r14, r13
bis #llo(-32768), r13
mov.b r13, &aa
swpb r13
mov.b r13, @r12
can be reduced to this:
bis.b #llo(128), &aa+1
Perhaps when the code to implement the packed attribute was added (or
fixed) new peephole optimizations should have been added too. I guess
one has to be careful to use "packed" sparingly.
Robert
As far as I know, by putting the packed attribute on struct 'a' you are
removing the guarantee that the structure starts at an word aligned
address. The address of 'a' isn't known at compile time and so the
compiler must generate code that will work at all memory offsets (I
think?).
How did the compiler behave in the old version?
- Wayne
-----Original Message-----
From: Robert Kavaler [mailto:kava...@di...]
Sent: Friday, 30 October 2009 4:35 PM
To: mspgcc-us...@li...
Subject: [Mspgcc-users] Possible code generation bug for packed structs
A bug (or unfortunate feature) has been added to 3.2.3 version of the
compiler related to packed structures. Older versions of the compiler
(from 5 years ago) did not show this problem, but the latest CVS version
10/12/2009 does. The problem is that when the packed attribute is
applied to a struct the compiler generates very optimal code for
accesses to int aligned int variables. Here is a simple test program:
struct __attribute__ ((packed)) a { int a; };
struct a aa;
void testa() { aa.a |= 0x8000; }
struct b { int b;};
struct b bb;
void testb() { bb.b |= 0x8000; }
compiled by:
/opt/mspgcc/bin/msp430-gcc -O4 -mmcu=msp430x149 -S mspbug.c
yields:
.file "mspbug.c"
.arch msp430x149
/* Hardware multiplier registers: */
__MPY=0x130
__MPYS=0x132
__MAC=0x134
__MACS=0x136
__OP2=0x138
__RESLO=0x13a
__RESHI=0x13c
__SUMEXT=0x13e
.text
.p2align 1,0
.global testa
.type testa,@function
/***********************
* Function `testa'
***********************/
testa:
/* prologue: frame size = 0 */
.L__FrameSize_testa=0x0
.L__FrameOffset_testa=0x0
/* prologue end (size=0) */
mov.b &aa, r14
mov #aa+1, r12
mov.b @r12, r13
swpb r13
bis r14, r13
bis #llo(-32768), r13
mov.b r13, &aa
swpb r13
mov.b r13, @r12
ret
/* epilogue: not required */
/* function testa size 15 (14) */
.Lfe1:
.size testa,.Lfe1-testa
/********* End of function ******/
.p2align 1,0
.global testb
.type testb,@function
/***********************
* Function `testb'
***********************/
testb:
/* prologue: frame size = 0 */
.L__FrameSize_testb=0x0
.L__FrameOffset_testb=0x0
/* prologue end (size=0) */
bis #llo(-32768), &bb
ret
/* epilogue: not required */
/* function testb size 4 (3) */
.Lfe2:
.size testb,.Lfe2-testb
/********* End of function ******/
.comm aa,2,2
.comm bb,2,2
/*********************************************************************
* File mspbug.c: code size: 19 words (0x13)
* incl. words in prologues: 0, epilogues: 2
*********************************************************************/
Notice show the unpacked structure generates one instruction while the
packed structure generates 9 instructions to do the same thing. Without
the optimizer the 9 instructions grows to 16 instructions.
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