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<hr><p align="left"><small>发信人: plato (纯真年代), 信区: Embedded <br>
标 题: linux for ppc chapter 14 <br>
发信站: BBS 水木清华站 (Wed May 30 23:20:24 2001) <br>
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14. Floating Point <br>
Some of the embedded PowerPC processors do not have a floating point unit, a <br>
nd so must perform all floating point operations in software. Others do have <br>
a hardware floating point unit, and may perform operations in hardware. <br>
14.1 Software <br>
If your application has very intensive floating point requirements, you may <br>
need to switch to fixed point or choose a target processor which does have a <br>
n FPU. <br>
Floating point can be performed either by instruction emulation in the kerne <br>
l, or by compiling everything with -msoft-float. In particular, it's importa <br>
nt that all the libraries, whether dynamically or statically linked are comp <br>
iled with the same options as the binaries that use them. Unless you're usin <br>
g a toolkit where this has already been done for you, this will generally me <br>
an that if you wish to use -msoft-float to gain maximum performance, you nee <br>
d to (re)compile everything, including: <br>
All static and shared libraries <br>
The compiler's internal libraries, such as libgcc.a <br>
All executables <br>
You will almost certainly not get this right first time. The primary symptom <br>
is that printf gives bogus numbers for simple floating point values. <br>
However, if you succeed, you can save space by configuring the kernel withou <br>
t floating point emulation. <br>
See: http://lists.linuxppc.org/listarcs/linuxppc-embedded/199911/msg00056.ht <br>
ml <br>
To add kernel math emulation to the 2.2.13 kernel, see: http://lists.linuxpp <br>
c.org/listarcs/linuxppc-embedded/199912/msg00017.html <br>
The "paranoia" test should give no complaints if everything is working corre <br>
ctly. See: http://www.enseeiht.fr/NetLib/paranoia/index.html <br>
14.2 Hardware <br>
If your CPU has an FPU, you'll want to use it. Make sure that everything is <br>
consistently compiled for hardware floating point, especially if you've asse <br>
mbled your toolkit yourself, or are using an 8xx toolkit to compile for the <br>
8260. <br>
Programs compiled for hardware floating point can still run on a CPU without <br>
an FPU, provided the kernel is built with the floating point instruction em <br>
ulator. This incurs a further performance penalty on CPUs lacking an FPU. <br>
14.3 Mixed <br>
If you are using a single set of shared libraries, you cannot mix the two te <br>
chniques in the one system. If you link statically or try really hard by cre <br>
ating two sets of shared libraries, the two can co-exist since programs comp <br>
iled with -msoft-float never generate floating point instructions requiring <br>
kernel emulation. <br>
In general, you're better off choosing one or the other; and choosing hardwa <br>
re floating point with kernel instruction emulation is much easier to get wo <br>
rking correctly. <br>
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※ 来源:·BBS 水木清华站 smth.org·[FROM: 166.111.161.8] <br>
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