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* [Bug inline-asm/20718] New: "+r" constraint with uninitialized value
@ 2005-04-01 16:39 jakub at gcc dot gnu dot org
2005-04-01 19:19 ` [Bug inline-asm/20718] " pinskia at gcc dot gnu dot org
2005-07-21 21:57 ` pbrook at gcc dot gnu dot org
0 siblings, 2 replies; 3+ messages in thread
From: jakub at gcc dot gnu dot org @ 2005-04-01 16:39 UTC (permalink / raw)
To: gcc-bugs
unsigned long foo (unsigned long *a, unsigned long *b,
unsigned long *c, int d)
{
unsigned long e, f;
if (d <= 0) return 0;
asm ("# registers %0 %1 %2 %3 %4 %5"
: "+a" (e),"+c" (d), "+r" (f)
: "r" (a), "r" (b), "r" (c));
return e;
}
at -O2 results in
# registers %rax %ecx %rdx %rdi %rsi %rdx
Note %2 and %5 being the same register, %rdx.
If f (and e) are initialized, both older GCCs and GCC4+ assign different
registers, but with unitialized values older GCCs seem to act as if "+r"
was instead "=&r" while GCC4+ treat it as "=r".
The library that used this (openssl) has been fixed to use "=&r" and "=&a"
instead, but I'd like to understand what is the desirable GCC behaviour
in this case. If it is undefined behaviour, so be it, but perhaps that
should be documented. Similarly if "r" (unitialized_var) among inputs means
GCC should allocate a unique register for that input or it doesn't have to.
--
Summary: "+r" constraint with uninitialized value
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: inline-asm
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jakub at gcc dot gnu dot org
CC: gcc-bugs at gcc dot gnu dot org,rth at gcc dot gnu dot
org
GCC target triplet: x86_64-*-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20718
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug inline-asm/20718] "+r" constraint with uninitialized value
2005-04-01 16:39 [Bug inline-asm/20718] New: "+r" constraint with uninitialized value jakub at gcc dot gnu dot org
@ 2005-04-01 19:19 ` pinskia at gcc dot gnu dot org
2005-07-21 21:57 ` pbrook at gcc dot gnu dot org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2005-04-01 19:19 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pinskia at gcc dot gnu dot org 2005-04-01 19:19 -------
This is not a regression and here is an example (compile on 3.2.3 with -O2 -
fomit-frame-pointer on i686-pc-linux-gnu and you get the same behavior as
pointed out here):
unsigned long foo (unsigned long *a, unsigned long *b,
unsigned long *c, int d)
{
unsigned long e, f,g,h;
// e = 0;f = 0;
if (d <= 0) return 0;
asm ("# registers %0 %1 %2 %3 %4 %5 %6 %7"
: "+a" (e),"+c" (d), "+r" (f), "+r"(g)
: "r" (a), "r" (b), "r" (c), "r"(h));
return e;
}
------ asm ----
#APP
# registers %eax %ecx %edx %ebp %edi %esi %ebx %edx
#NO_APP
Notice how edx are used for both input and output registers.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20718
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug inline-asm/20718] "+r" constraint with uninitialized value
2005-04-01 16:39 [Bug inline-asm/20718] New: "+r" constraint with uninitialized value jakub at gcc dot gnu dot org
2005-04-01 19:19 ` [Bug inline-asm/20718] " pinskia at gcc dot gnu dot org
@ 2005-07-21 21:57 ` pbrook at gcc dot gnu dot org
1 sibling, 0 replies; 3+ messages in thread
From: pbrook at gcc dot gnu dot org @ 2005-07-21 21:57 UTC (permalink / raw)
To: gcc-bugs
------- Additional Comments From pbrook at gcc dot gnu dot org 2005-07-21 21:06 -------
gcc4 behaviour seems fine to me.
A slightly simpler example
int foo(int a)
{
int b;
asm ("" : "+r" (b) : "r" (a));
return b;
}
Can be (and is) legitimately be transformed into
int foo(int a)
{
asm ("" : "+r" (a) : "r" (a));
return a;
}
In which case it's much more obvious that the compiler is going to allocate
the same register.
--
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20718
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2005-07-21 21:06 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2005-04-01 16:39 [Bug inline-asm/20718] New: "+r" constraint with uninitialized value jakub at gcc dot gnu dot org
2005-04-01 19:19 ` [Bug inline-asm/20718] " pinskia at gcc dot gnu dot org
2005-07-21 21:57 ` pbrook at gcc dot gnu dot org
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