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Re: [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply
From: |
Richard Henderson |
Subject: |
Re: [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds |
Date: |
Thu, 14 Nov 2013 09:14:06 +1000 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 |
On 11/14/2013 06:49 AM, Tom Musta wrote:
> I have reviewed the code and the spec and I cannot see a flaw. The sequence
> is
> effectively this:
>
> - float64_muladd - performs proper FMA for 64 bit numbers)
> - float64_to_float32 - converts to single precision, including proper
> rounding
> - float32_to_float64
>
> The implementation of float64_muladd would seem to provide enough mantissa
> bits
> for proper handling of the case you describe. The only rounding occurs in the
> second step.
>
> I have also done quite a bit of random and targeted random testing using Power
> hardware to produce expected results. The targeted random tests followed your
> suggestion above: generate AxB + C where abs(exp(A) - exp(B)) = 23 and
> abs(exp(A) - exp(C)) = 46. Several million test patterns have been generated
> and played back through QEMU without any miscompares in the numerical results.
Here's an example that produces wrong results when rounding to double first.
Replace the portable math.h calls with ppc asm as necessary.
r~
$ cat z.c
#include <stdio.h>
#include <math.h>
float a = 65281;
float b = 257;
float c = 0x1p-29f;
int main()
{
double dd = fma(a, b, c);
float d = dd;
float e = fmaf(a, b, c);
printf("a = %a\n", a);
printf("b = %a\n", b);
printf("c = %a\n", c);
printf("dd= %a\n", dd);
printf("d = %a\n", d);
printf("e = %a\n", e);
return 0;
}
$ ./a.out
a = 0x1.fe02p+15
b = 0x1.01p+8
c = 0x1p-29
dd= 0x1.000001p+24
d = 0x1p+24
e = 0x1.000002p+24
- [Qemu-devel] [PATCH 07/14] VSX Stage 4: Add xsmulsp, (continued)
- [Qemu-devel] [PATCH 07/14] VSX Stage 4: Add xsmulsp, Tom Musta, 2013/11/06
- [Qemu-devel] [PATCH 08/14] VSX Stage 4: Add xsdivsp, Tom Musta, 2013/11/06
- [Qemu-devel] [PATCH 10/14] VSX Stage 4: Add xssqrtsp, Tom Musta, 2013/11/06
- [Qemu-devel] [PATCH 09/14] VSX Stage 4: Add xsresp, Tom Musta, 2013/11/06
- [Qemu-devel] [PATCH 11/14] VSX Stage 4: add xsrsqrtesp, Tom Musta, 2013/11/06
- [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Tom Musta, 2013/11/06
[Qemu-devel] [PATCH 13/14] VSX Stage 4: Add xscvsxdsp and xscvuxdsp, Tom Musta, 2013/11/06
[Qemu-devel] [PATCH 14/14] VSX Stage 4: Add xxleqv, xxlnand and xxlorc, Tom Musta, 2013/11/06
Re: [Qemu-devel] [PATCH 00/14] VSX Stage 4, Richard Henderson, 2013/11/07
Re: [Qemu-devel] [PATCH 00/14] VSX Stage 4, Andreas Färber, 2013/11/08