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Revision tags: v7.0.0, v7.0.0-rc4, v7.0.0-rc3, v7.0.0-rc2, v7.0.0-rc1, v7.0.0-rc0, v6.1.1, v6.2.0, v6.2.0-rc4, v6.2.0-rc3, v6.2.0-rc2, v6.2.0-rc1, v6.2.0-rc0, v6.0.1, v6.1.0, v6.1.0-rc4, v6.1.0-rc3, v6.1.0-rc2, v6.1.0-rc1, v6.1.0-rc0, v6.0.0, v6.0.0-rc5, v6.0.0-rc4, v6.0.0-rc3, v6.0.0-rc2, v6.0.0-rc1, v6.0.0-rc0, v5.2.0, v5.2.0-rc4, v5.2.0-rc3, v5.2.0-rc2, v5.2.0-rc1, v5.2.0-rc0, v5.0.1, v5.1.0, v5.1.0-rc3, v5.1.0-rc2, v5.1.0-rc1, v5.1.0-rc0, v4.2.1 |
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eca30647 |
| 11-Jun-2020 |
Joseph Myers <joseph@codesourcery.com> |
target/i386: reimplement f2xm1 using floatx80 operations
The x87 f2xm1 emulation is currently based around conversion to
double. This is inherently unsuitable for a good emulation of any
floatx80 o
target/i386: reimplement f2xm1 using floatx80 operations
The x87 f2xm1 emulation is currently based around conversion to
double. This is inherently unsuitable for a good emulation of any
floatx80 operation, even before considering that it is a particularly
naive implementation using double (computing with pow and then
subtracting 1 rather than attempting a better emulation using expm1).
Reimplement using the soft-float operations, including additions and
multiplications with higher precision where appropriate to limit
accumulation of errors. I considered reusing some of the m68k code
for transcendental operations, but the instructions don't generally
correspond exactly to x87 operations (for example, m68k has 2^x and
e^x - 1, but not 2^x - 1); to avoid possible accumulation of errors
from applying multiple such operations each rounding to floatx80
precision, I wrote a direct implementation of 2^x - 1 instead. It
would be possible in principle to make the implementation more
efficient by doing the intermediate operations directly with
significands, signs and exponents and not packing / unpacking floatx80
format for each operation, but that would make it significantly more
complicated and it's not clear that's worthwhile; the m68k emulation
doesn't try to do that.
A test is included with many randomly generated inputs. The
assumption of the test is that the result in round-to-nearest mode
should always be one of the two closest floating-point numbers to the
mathematical value of 2^x - 1; the implementation aims to do somewhat
better than that (about 70 correct bits before rounding). I haven't
investigated how accurate hardware is.
Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.21.2006112341010.18393@digraph.polyomino.org.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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