#! | 29 lines | 23 code | 6 blank | 0 comment | 0 complexity | 6e5490e3fd7ce73630b782c18e9322a9 MD5 | raw file
Possible License(s): GPL-2.0, LGPL-2.0, AGPL-1.0
1There seems to be a problem with exp(double) and our emulator. I haven't 2been able to track it down yet. This does not occur with the emulator 3supplied by Russell King. 4 5I also found one oddity in the emulator. I don't think it is serious but 6will point it out. The ARM calling conventions require floating point 7registers f4-f7 to be preserved over a function call. The compiler quite 8often uses an stfe instruction to save f4 on the stack upon entry to a 9function, and an ldfe instruction to restore it before returning. 10 11I was looking at some code, that calculated a double result, stored it in f4 12then made a function call. Upon return from the function call the number in 13f4 had been converted to an extended value in the emulator. 14 15This is a side effect of the stfe instruction. The double in f4 had to be 16converted to extended, then stored. If an lfm/sfm combination had been used, 17then no conversion would occur. This has performance considerations. The 18result from the function call and f4 were used in a multiplication. If the 19emulator sees a multiply of a double and extended, it promotes the double to 20extended, then does the multiply in extended precision. 21 22This code will cause this problem: 23 24double x, y, z; 25z = log(x)/log(y); 26 27The result of log(x) (a double) will be calculated, returned in f0, then 28moved to f4 to preserve it over the log(y) call. The division will be done 29in extended precision, due to the stfe instruction used to save f4 in log(y).