diff options
Diffstat (limited to 'Documentation/arch/arm/nwfpe/notes.rst')
| -rw-r--r-- | Documentation/arch/arm/nwfpe/notes.rst | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/arch/arm/nwfpe/notes.rst b/Documentation/arch/arm/nwfpe/notes.rst new file mode 100644 index 0000000000..102e55af84 --- /dev/null +++ b/Documentation/arch/arm/nwfpe/notes.rst @@ -0,0 +1,32 @@ +Notes +===== + +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). |