[Qemu-devel] [PATCH 1/1] Fix conversion from uint64 to float128

[Qemu-devel] [PATCH 1/1] Fix conversion from uint64 to float128

[Petr Tesarik]

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).

Cc: qemu-stable@nongnu.org
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
---
 fpu/softfloat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index 70e0c40a1c..386805475b 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -3139,7 +3139,7 @@ float128 uint64_to_float128(uint64_t a, float_status *status)
     if (a == 0) {
         return float128_zero;
     }
-    return normalizeRoundAndPackFloat128(0, 0x406E, a, 0, status);
+    return normalizeRoundAndPackFloat128(0, 0x406E, 0, a, status);
 }
 
 
-- 
2.13.6

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

[Petr Tesarik]

On Fri, 11 May 2018 09:10:52 +0200
Petr Tesarik <ptesarik@suse.com> 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

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

[Peter Maydell]

On 11 May 2018 at 08:10, Petr Tesarik <ptesarik@suse.com> 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).
>
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>
> ---
>  fpu/softfloat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index 70e0c40a1c..386805475b 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -3139,7 +3139,7 @@ float128 uint64_to_float128(uint64_t a, float_status *status)
>      if (a == 0) {
>          return float128_zero;
>      }
> -    return normalizeRoundAndPackFloat128(0, 0x406E, a, 0, status);
> +    return normalizeRoundAndPackFloat128(0, 0x406E, 0, a, status);
>  }

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM

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

[Richard Henderson]

On 05/11/2018 12:10 AM, Petr Tesarik 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).
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Petr Tesarik <ptesarik@suse.com>
> ---
>  fpu/softfloat.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Queued, thanks.


r~