Re: [Qemu-devel] [PATCH] target-i386: Fix regression with maxsd SSE2 instruction v2

[Peter Maydell]

On 8 November 2011 14:20, Jason Wessel <address@hidden> wrote:
> The maxsd instruction needs to take into account the sign of the
> numbers 64 bit numbers.  This is a regression that was introduced in
> 347ac8e356 (target-i386: switch to softfloat).
>
> The case that fails is:
>
> maxsd  %xmm1,%xmm0
>
> When xmm1 = 24 and xmm0 = -100
>
> This was found running the glib2 binding tests where it prints the message:
> /binding/transform:
> GLib-GObject-WARNING **: value "24.000000" of type `gdouble' is invalid or 
> out of range for property `value' of type `gdouble'
> aborting...
>
> Using a signed comparison fixes the problem.

This commit comment needs to be updated -- we're not using a
signed comparison, we're using a floating point comparison.

> diff --git a/target-i386/ops_sse.h b/target-i386/ops_sse.h
> index aa41d25..58f7bf5 100644
> --- a/target-i386/ops_sse.h
> +++ b/target-i386/ops_sse.h
> @@ -584,8 +584,8 @@ void helper_ ## name ## sd (Reg *d, Reg *s)\
>  #define FPU_SUB(size, a, b) float ## size ## _sub(a, b, &env->sse_status)
>  #define FPU_MUL(size, a, b) float ## size ## _mul(a, b, &env->sse_status)
>  #define FPU_DIV(size, a, b) float ## size ## _div(a, b, &env->sse_status)
> -#define FPU_MIN(size, a, b) (a) < (b) ? (a) : (b)
> -#define FPU_MAX(size, a, b) (a) > (b) ? (a) : (b)
> +#define FPU_MIN(size, a, b) float ## size ## _lt(a, b, &env->sse_status) ? 
> (a) : (b)
> +#define FPU_MAX(size, a, b) float ## size ## _lt(b, a, &env->sse_status) ? 
> (a) : (b)
>  #define FPU_SQRT(size, a, b) float ## size ## _sqrt(b, &env->sse_status)

(repeating my comments from the other thread):

Having mused about it a bit, I think that actually the macros
there do return the right answers for the special cases :-)

If (a,b) are +0,-0 in some order, then the _lt comparison will
treat them as equal and return 0, so we return b, as required.
If either of (a,b) are NaNs then the _lt comparison will raise
InvalidOp and return 0, so we return b.

That's a bit subtle, so I think it probably deserves a comment:

/* Note that the choice of comparison op here is important to get the
 * special cases right: for min and max Intel specifies that (-0,0),
 * (0,-0), (NaN, anything) and (anything, NaN) return the second argument.
 */

-- PMM