Diffstat (limited to 'Documentation/arm/nwfpe/NOTES')
1 files changed, 29 insertions, 0 deletions
diff --git a/Documentation/arm/nwfpe/NOTES b/Documentation/arm/nwfpe/NOTES
new file mode 100644
@@ -0,0 +1,29 @@
+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).