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