RFR(M): 7133857: exp() and pow() should use the x87 ISA on x86

[Roland Westrelin]

While adding comments as suggested by Vladimir, I realized that my fix:

> Also one corner case was not well handled by the interpreter/c1 assembly code. For x^y, when x < 0, y has to be an integer and we need to test whether it's odd or even. The previous code would fallback to the runtime code if y was too large for a 32 bit integer. So for instance Math.pow(x,y) with x < 0 and a large y would be computed by the C code but Math.pow(-x, y) which should be +/- Math.pow(x, y) depending on y's parity would be computed by the assembly code and results would differ. So on the x < 0 code path, I now check for y+1 == y which is true for very large numbers that are all even and if the test fails I use a 64 bit rather than a 32 bit integer which is guaranteed to not overflow for numbers where y+1 != y.

was correct but could be simpler. So here is one more webrev:

http://cr.openjdk.java.net/~roland/7133857/webrev.03/

In the case where x < 0 and y is huge: y is even. y may not fit in a 64 bit integer and then we'll get the integer indefinite value which is even as well.

So either y fits in a 64 bit integer and we can test its parity or y doesn't fit in a 64 bit integer, y is even and we get an even indefinite value so the result of the parity test is correct.

So I turned the code that checks y+1 == y that I added in the 2nd webrev into debug code and the only change compared to the first webrev for non-debug code is that I use a 64 bit rather than a 32 bit integer in the x < 0 code path.

Roland.