Re: [Qemu-stable] [PATCH 1/1] Fix conversion from uint64 to float128

[Petr Tesarik]

On Fri, 11 May 2018 09:10:52 +0200
Petr Tesarik <address@hidden> wrote:

> The significand is passed to normalizeRoundAndPackFloat128() as high
> first, low second. The current code passes the integer first, so the
> result is incorrectly shifted left by 64 bits.
> 
> This bug affects the emulation of s390x instruction CXLGBR (convert
> from logical 64-bit binary-integer operand to extended BFP result).

I forgot to add a simple reproducer:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
        unsigned long long x = atoll(argv[1]);
        long double d = x;

        printf("%llu -> %Lf\n", x, d);

        return 0;
}

On a real s390x, I get this:
linux-nig1:~ # ./cast 1
1 -> 1.000000

An s390x emulated with (unpatched) qemu-system-s390x gives:
linux-2s2c:~ # ./cast 1
1 -> 18446744073709551616.000000

That is 2^64.

Petr T