About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / arm / nwfpe / NOTES

Custom Search

Based on kernel version 4.13.3. Page generated on 2017-09-23 13:54 EST.

1	There seems to be a problem with exp(double) and our emulator.  I haven't
2	been able to track it down yet.  This does not occur with the emulator
3	supplied by Russell King.
5	I also found one oddity in the emulator.  I don't think it is serious but
6	will point it out.  The ARM calling conventions require floating point
7	registers f4-f7 to be preserved over a function call.  The compiler quite
8	often uses an stfe instruction to save f4 on the stack upon entry to a
9	function, and an ldfe instruction to restore it before returning.
11	I was looking at some code, that calculated a double result, stored it in f4
12	then made a function call. Upon return from the function call the number in
13	f4 had been converted to an extended value in the emulator.
15	This is a side effect of the stfe instruction.  The double in f4 had to be
16	converted to extended, then stored.  If an lfm/sfm combination had been used,
17	then no conversion would occur.  This has performance considerations.  The
18	result from the function call and f4 were used in a multiplication.  If the
19	emulator sees a multiply of a double and extended, it promotes the double to
20	extended, then does the multiply in extended precision.
22	This code will cause this problem:
24	double x, y, z;
25	z = log(x)/log(y);
27	The result of log(x) (a double) will be calculated, returned in f0, then
28	moved to f4 to preserve it over the log(y) call.  The division will be done
29	in extended precision, due to the stfe instruction used to save f4 in log(y).
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.