notes.rst (1493B)
1Notes 2===== 3 4There seems to be a problem with exp(double) and our emulator. I haven't 5been able to track it down yet. This does not occur with the emulator 6supplied by Russell King. 7 8I also found one oddity in the emulator. I don't think it is serious but 9will point it out. The ARM calling conventions require floating point 10registers f4-f7 to be preserved over a function call. The compiler quite 11often uses an stfe instruction to save f4 on the stack upon entry to a 12function, and an ldfe instruction to restore it before returning. 13 14I was looking at some code, that calculated a double result, stored it in f4 15then made a function call. Upon return from the function call the number in 16f4 had been converted to an extended value in the emulator. 17 18This is a side effect of the stfe instruction. The double in f4 had to be 19converted to extended, then stored. If an lfm/sfm combination had been used, 20then no conversion would occur. This has performance considerations. The 21result from the function call and f4 were used in a multiplication. If the 22emulator sees a multiply of a double and extended, it promotes the double to 23extended, then does the multiply in extended precision. 24 25This code will cause this problem: 26 27double x, y, z; 28z = log(x)/log(y); 29 30The result of log(x) (a double) will be calculated, returned in f0, then 31moved to f4 to preserve it over the log(y) call. The division will be done 32in extended precision, due to the stfe instruction used to save f4 in log(y).