[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions
From: |
Al Viro |
Subject: |
Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions |
Date: |
Thu, 3 Jul 2014 07:51:04 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
More bugs: addl/v should sign-extend the result, as addl does.
As it is, we have
uint64_t helper_addlv(CPUAlphaState *env, uint64_t op1, uint64_t op2)
{
uint64_t tmp = op1;
op1 = (uint32_t)(op1 + op2);
if (unlikely((tmp ^ op2 ^ (-1UL)) & (tmp ^ op1) & (1UL << 31))) {
arith_excp(env, GETPC(), EXC_M_IOV, 0);
}
return op1;
}
IOW,
#include <stdio.h>
long r;
void __attribute__((noinline)) f(void)
{
asm __volatile(
"subl $31, 1, $0\n\t"
"addl $0, $0, $1\n\t"
"addl/v $0, $0, $0\n\t"
"subq $0, $1, $0\n\t"
"stq $0, %0\n\t"
: "=m"(r): :"$0", "$1");
}
main()
{
f();
printf("%ld\n", r);
}
ends up printing 0 on actual hardware (all variants) and 4294967296 on
qemu. Similar problem with subl/v -
#include <stdio.h>
long r;
void __attribute__((noinline)) f(void)
{
asm __volatile(
"subl $31, 1, $0\n\t"
"subl/v $31, 1, $1\n\t"
"subq $0, $1, $0\n\t"
"stq $0, %0\n\t"
: "=m"(r): :"$0", "$1");
}
main()
{
f();
printf("%ld\n", r);
}
prints 0 on actual hw and -4294967296 on qemu. What constraints do we have
on qemu host, anyway? Two's-complement, (int32_t)(uint32_t)x == x for any
int32_t x? helper_mullv() seems to assume that...
Oh, crap - our mull/v is sensitive to upper 32 bits of multiplicands.
If you put 1UL<<32 into one register, 1 into another and say mull/v,
result will be 0 and no overflow. qemu does
int64_t res = (int64_t)op1 * (int64_t)op2;
if (unlikely((int32_t)res != res)) {
arith_excp(env, GETPC(), EXC_M_IOV, 0);
}
return (int64_t)((int32_t)res);
which leads to overflow trap triggered for no good reason...
Incidentally, all those guys ({add,sub,mul}[lq]/v) *do* assign the result
(same as the variant without /v would) before entering the trap. So
arith_excp() is wrong here.
FWIW, why not just generate
trunc_i64_i32 tmp, va
trunc_i64_i32 tmp2, vb
muls2_i32 tmp2, tmp, tmp, tmp2
ext32s_i64 vc, tmp2
maybe_overflow_32 tmp
where maybe_overflow throws IOV unless tmp is 0 or -1? That would appear
to suffice for mull/v. mulq/v would be
muls2_i64 vc, tmp, va, vb
maybe_overflow_64 tmp
addl/v:
trunc_i64_i32 tmp, va
trunc_i64_i32 tmp2, vb
add2_i32 tmp2, tmp, tmp, zero, tmp2, zero
ext32s_i64 vc, tmp2
maybe_overflow_32 tmp
etc.
We'd need two helpers, differing only in argument type. Simple
if (unlikely(arg && ~arg))
arith_excp(env, GETPC(), EXC_M_IOV, 0);
would do. Not sure what flags would be needed in DEFINE_HELPER_... for
those, though. Comments?
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, (continued)
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/01
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/01
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/01
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/02
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions,
Al Viro <=
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Peter Maydell, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/03
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Richard Henderson, 2014/07/04
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/04
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/04
- Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions, Al Viro, 2014/07/05