On x86_64, the following function (extracted from gmp, the asms are from
its longlong.h)
void
mul_basecase (unsigned long * wp, unsigned long * up, long un,
unsigned long * vp, long vn)
{
long j;
unsigned long prod_low, prod_high;
unsigned long cy_dig;
unsigned long v_limb;
v_limb = vp[0];
cy_dig = 0;
for (j = un; j > 0; j--)
{
unsigned long u_limb, w_limb;
u_limb = *up++;
__asm__ ("mulq %3" : "=a" (prod_low), "=d" (prod_high)
: "%0" (u_limb), "rm" (v_limb));
__asm__ ("addq %5,%q1\n"
"\tadcq %3,%q0" : "=r" (cy_dig), "=&r" (w_limb)
: "0" (prod_high), "rme" (0), "%1" (prod_low),
"rme" (cy_dig));
*wp++ = w_limb;
}
}
produces with optimization:
#APP
# 16 "t.c" 1
mulq %rcx
# 0 "" 2
# 18 "t.c" 1
addq %rdx,%rax
adcq $0,%rdx
# 0 "" 2
#NO_APP
where you can see that cy_dig is allocated to the upper half of the mulq
result. It looks like that we are confused by rest_of_match_asm_constraints
which inserts
(insn 44 25 26 4 t.c:18 (set (reg/v:DI 67 [ cy_dig ])
(reg/v:DI 68 [ prod_high ])) -1 (nil))
after the mulq asm.
--
Summary: wrong code for multiple output asm, wrong df?
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: major
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rguenth at gcc dot gnu dot org
GCC target triplet: x86_64-*-*
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33552
