Re: [Qemu-devel] [RFC] alpha qemu arithmetic exceptions

[Al Viro <viro@ZenIV.linux.org.uk>]

	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?