docs: arm: convert docs to ReST and rename to *.rst

[people/ms/linux.git] / Documentation / arm / nwfpe / NOTES

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