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* [Bug c/39255] New: bitfield alias generates incorrect assembly
@ 2009-02-20 23:45 jsworley at qwest dot net
2009-02-20 23:48 ` [Bug c/39255] " pinskia at gcc dot gnu dot org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: jsworley at qwest dot net @ 2009-02-20 23:45 UTC (permalink / raw)
To: gcc-bugs
Consider the following code:
typedef struct {
unsigned long A:16;
unsigned long B:48;
} bits;
int
wrong(unsigned long y)
{
bits *p = (bits *) &y;
if (y == 0 || p->A >= 512) {
return (1);
}
return (0);
}
int
right(unsigned long y)
{
bits *p = (bits *) &y;
if (y == 0 || p->A >= 512) {
return (-1);
}
return (0);
}
The *only* difference between right() and wrong() is the return value in the IF
clause. With -O2, the function right() compiles to correct, if suboptimal,
code. The function wrong(), however, generates the following:
wrong:
.prologue
.body
.mmi
cmp.ne p6, p7 = 0, r32
nop 0
addl r14 = 511, r0
.mmi
ld2 r15 = [r12] <<<<<< HUH?
st8 [r12] = r32 <<<<<< HUH?
addl r8 = 1, r0
.mib
nop 0
nop 0
(p7) br.ret.dpnt.many rp
;;
.mmi
cmp4.gtu p6, p7 = r15, r14
;;wrong:
.prologue
.body
.mmi
cmp.ne p6, p7 = 0, r32
nop 0
addl r14 = 511, r0
.mmi
ld2 r15 = [r12]
st8 [r12] = r32
addl r8 = 1, r0
.mib
nop 0
nop 0
(p7) br.ret.dpnt.many rp
;;
.mmi
cmp4.gtu p6, p7 = r15, r14
;;
(p6) addl r14 = 1, r0
(p7) mov r14 = r0
;;
.mib
nop 0
mov r8 = r14
br.ret.sptk.many rp
.endp wrong#
(p6) addl r14 = 1, r0
(p7) mov r14 = r0
;;
.mib
nop 0
mov r8 = r14
br.ret.sptk.many rp
.endp wrong#
It is clearly wrong to perform the load from the stack before the proper value
(r32) has been stored. When run, the value of p->A is the low 16 bits of
whatever was stored on the stack before.
--
Summary: bitfield alias generates incorrect assembly
Product: gcc
Version: 4.3.1
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jsworley at qwest dot net
GCC host triplet: i686-linux-gnu
GCC target triplet: ia64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/39255] bitfield alias generates incorrect assembly
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
@ 2009-02-20 23:48 ` pinskia at gcc dot gnu dot org
2009-02-20 23:56 ` pinskia at gcc dot gnu dot org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2009-02-20 23:48 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2009-02-20 23:47 -------
-O2 enables strict aliasing (type based aliasing). You need to use
-fno-strict-aliasing to disable that.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/39255] bitfield alias generates incorrect assembly
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
2009-02-20 23:48 ` [Bug c/39255] " pinskia at gcc dot gnu dot org
@ 2009-02-20 23:56 ` pinskia at gcc dot gnu dot org
2009-02-20 23:59 ` pinskia at gcc dot gnu dot org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2009-02-20 23:56 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from pinskia at gcc dot gnu dot org 2009-02-20 23:56 -------
*** This bug has been marked as a duplicate of 21920 ***
--
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=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/39255] bitfield alias generates incorrect assembly
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
2009-02-20 23:48 ` [Bug c/39255] " pinskia at gcc dot gnu dot org
2009-02-20 23:56 ` pinskia at gcc dot gnu dot org
@ 2009-02-20 23:59 ` pinskia at gcc dot gnu dot org
2009-02-21 0:26 ` jsworley at qwest dot net
2009-02-21 0:44 ` pinskia at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2009-02-20 23:59 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from pinskia at gcc dot gnu dot org 2009-02-20 23:59 -------
Note in 4.4 and above, GCC generates "correct" even with strict aliasing
enabled and it generates better code, it does not store on the stack at all.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/39255] bitfield alias generates incorrect assembly
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
` (2 preceding siblings ...)
2009-02-20 23:59 ` pinskia at gcc dot gnu dot org
@ 2009-02-21 0:26 ` jsworley at qwest dot net
2009-02-21 0:44 ` pinskia at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: jsworley at qwest dot net @ 2009-02-21 0:26 UTC (permalink / raw)
To: gcc-bugs
------- Comment #4 from jsworley at qwest dot net 2009-02-21 00:25 -------
Whether or not this is just an aliasing bug, I'd like to understand why
changing the return value from 1 to -1 makes the compiler generate correct
code.
--
jsworley at qwest dot net changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|DUPLICATE |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c/39255] bitfield alias generates incorrect assembly
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
` (3 preceding siblings ...)
2009-02-21 0:26 ` jsworley at qwest dot net
@ 2009-02-21 0:44 ` pinskia at gcc dot gnu dot org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2009-02-21 0:44 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from pinskia at gcc dot gnu dot org 2009-02-21 00:44 -------
Because of different optimizations happen. When having 1, it basically
creates:
if (y == 0)
return p->A >= 512;
return 0;
with -1, it keeps the branch.
This is normal with aliasing violations really.
Anyways the code is undefined so the compiler technically could do anything.
*** This bug has been marked as a duplicate of 21920 ***
--
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=39255
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2009-02-21 0:44 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2009-02-20 23:45 [Bug c/39255] New: bitfield alias generates incorrect assembly jsworley at qwest dot net
2009-02-20 23:48 ` [Bug c/39255] " pinskia at gcc dot gnu dot org
2009-02-20 23:56 ` pinskia at gcc dot gnu dot org
2009-02-20 23:59 ` pinskia at gcc dot gnu dot org
2009-02-21 0:26 ` jsworley at qwest dot net
2009-02-21 0:44 ` pinskia at gcc dot gnu dot org
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