47 lines
1.9 KiB
Plaintext
47 lines
1.9 KiB
Plaintext
These routines were produced by someone who is not a great authority on
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floating point, and may not be entirely correct. Where possible I tested
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the special cases for routines.
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The directory testieee contains test programs for IEEE-format machines.
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I took a stab at making them work on the vax, but gave up as dealing with
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exceptions (e.g. underflow, overflow, reserved operand) was just too tedious.
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Note: it is possible to build a library with MACHINE=ieee but a couple
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of warnings:
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Be careful when compiling floor.c. These routines rely on
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certain variables being only double precision. If these
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variables are placed in 68881 registers, they will be extended
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precision and the routines will produce incorrect results.
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Unless your compiler does its own register allocation, this
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is not likely to be a problem as none of the variables in
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question are declared "register". If you are using GCC
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you can specify -ffloat-store to avoid this problem.
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The C version of drem() in ieee/support.c appears to compute
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the incorrect results for drem(+-1, +-2). It yields 1 when
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it should be -1 and -1 when it should be 1. "should be" is
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based on what the VAX version yields and by cranking through
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the formula.
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If you do build using MACHINE=ieee and run the tests in testieee you
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will note that some routines return errors:
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floor/ceil/rint report that they got 0 when expecting -0.
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Don't really know which is correct, is floor(-0) == 0 or -0?
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For C it shouldn't really matter since 0 is the same as -0
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in comparisons.
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scalb(-1, -2100) returns 0 instead of -0. 2 ** -2100 is
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effectively 0 but -anything * 0 == -0 according to the 68881.
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Similarly for scalb(-pi, 2100). It returns INF instead
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of -INF. 2 ** 2100 is effectively INF but -anything * INF
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is -INF. What is correct?
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drem(+-1, +-2) fails as mentioned above. This is a real error.
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----
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Mike Hibler
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U of Utah CS Dept.
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mike@cs.utah.edu
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