[issue30104] Float rounding errors on AMD64 FreeBSD CURRENT Debug 3.x buildbot

[STINNER Victor]

STINNER Victor added the comment:

I created a test case: attached dtoa2.c.

Ok with GCC 6.3.1 (on Fedora 25) and clang 3.8 (on FreeBSD 11) with -O3:

haypo at selma$ gcc -O3 dtoa2.c -o x && ./x
text: 29865e170 -> float: 2.9865e+174
0x1.8265ea9f864bcp+579
{bc 64 f8 a9 5e 26 28 64}

[haypo at freebsd ~/prog/python/master]$ clang -O3 dtoa2.c -o x && ./x
text: 29865e170 -> float: 2.9865e+174
0x1.8265ea9f864bcp+579
{bc 64 f8 a9 5e 26 28 64}


Still ok with clang 3.8 with -O1:

[haypo at freebsd ~/prog/python/master]$ clang40 -O1 dtoa2.c -o x && ./x
text: 29865e170 -> float: 2.9865e+174
0x1.8265ea9f864bcp+579
{bc 64 f8 a9 5e 26 28 64}


Error with clang 4.0 using -O2 (on FreeBSD 11):

[haypo at freebsd ~/prog/python/master]$ clang40 -O2 dtoa2.c -o x && ./x
text: 29865e170 -> float: 2.9865e+174
0x1.8265ea9f864bdp+579    <~~~~~ HERE, bd instead of bc
{bd 64 f8 a9 5e 26 28 64} <~~~~~ HERE, bd instead of bc

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Added file: http://bugs.python.org/file46817/dtoa2.c

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<http://bugs.python.org/issue30104>
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