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* [Bug c/43003] New: Bitfield optimisation problem
@ 2010-02-08 22:29 zoltan at bendor dot com dot au
2010-02-08 22:32 ` [Bug middle-end/43003] " pinskia at gcc dot gnu dot org
2010-02-08 22:36 ` pinskia at gcc dot gnu dot org
0 siblings, 2 replies; 3+ messages in thread
From: zoltan at bendor dot com dot au @ 2010-02-08 22:29 UTC (permalink / raw)
To: gcc-bugs
The following C function:
typedef struct {
unsigned x : 1,
y : 1,
z : 1,
a : 3,
b : 18,
c : 5;
} FOO;
extern volatile FOO var;
void bar( void )
{
FOO x;
x.x = 1;
x.y = 0;
x.z = 1;
x.a = 0;
x.b = 55;
x.c = 0;
var = x;
}
static const FOO y = { 1,0,1,0,55,0 };
void baz( void )
{
var = y;
}
when compiled with -Os, -O2 or -O3 generates a very short function for bar():
ldr r3, .L3
ldr r2, .L3+4
str r2, [r3, #0]
bx lr
.align 2
.L3:
.word var
.word 3525
but a rather long one for baz:
ldr r3, .L7
ldr r2, [r3, #0]
orr r2, r2, #1
str r2, [r3, #0]
ldr r2, [r3, #0]
bic r2, r2, #2
str r2, [r3, #0]
ldr r2, [r3, #0]
orr r2, r2, #4
str r2, [r3, #0]
ldr r2, [r3, #0]
bic r2, r2, #56
str r2, [r3, #0]
ldr r2, [r3, #0]
bic r2, r2, #16711680
bic r2, r2, #61952
orr r2, r2, #3520
str r2, [r3, #0]
ldr r2, [r3, #0]
bic r2, r2, #520093696
str r2, [r3, #0]
bx lr
.align 2
.L7:
.word var
It seems that when the compiler sees that 'y' is a constant, it generates a
sequence of individual field loads for structure members for a temporary
structure (just like what bar() does), but then it does not run the optimizer
over the generated sequence.
--
Summary: Bitfield optimisation problem
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: zoltan at bendor dot com dot au
GCC host triplet: i586-gnu-linux
GCC target triplet: arm-elf-none
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43003
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/43003] Bitfield optimisation problem
2010-02-08 22:29 [Bug c/43003] New: Bitfield optimisation problem zoltan at bendor dot com dot au
@ 2010-02-08 22:32 ` pinskia at gcc dot gnu dot org
2010-02-08 22:36 ` pinskia at gcc dot gnu dot org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-08 22:32 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2010-02-08 22:32 -------
I think this is an issue with volatile really. Which was just fixed in the
last week or so.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43003
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/43003] Bitfield optimisation problem
2010-02-08 22:29 [Bug c/43003] New: Bitfield optimisation problem zoltan at bendor dot com dot au
2010-02-08 22:32 ` [Bug middle-end/43003] " pinskia at gcc dot gnu dot org
@ 2010-02-08 22:36 ` pinskia at gcc dot gnu dot org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-02-08 22:36 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2010-02-08 22:36 -------
Yep in fact the fix for PR 42898 fixed this one also.
*** This bug has been marked as a duplicate of 42898 ***
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |DUPLICATE
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43003
^ permalink raw reply [flat|nested] 3+ messages in thread
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2010-02-08 22:29 [Bug c/43003] New: Bitfield optimisation problem zoltan at bendor dot com dot au
2010-02-08 22:32 ` [Bug middle-end/43003] " pinskia at gcc dot gnu dot org
2010-02-08 22:36 ` pinskia at gcc dot gnu dot org
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