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Diffstat (limited to 'docs/users_guide/bugs.xml')
| -rw-r--r-- | docs/users_guide/bugs.xml | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/docs/users_guide/bugs.xml b/docs/users_guide/bugs.xml index 49d6a8dca6..ca0f65f981 100644 --- a/docs/users_guide/bugs.xml +++ b/docs/users_guide/bugs.xml @@ -523,6 +523,36 @@ checking for duplicates. The reason for this is efficiency, pure and simple. GHCi.</para> </listitem> + + <listitem> + <para>On 32-bit x86 platforms when using the native code + generator, the + <option>-fexcess-precision</option><indexterm><primary><option>-fexcess-precision</option></primary></indexterm> option + is always on. This means that floating-point calculations are + non-deterministic, because depending on how the program is + compiled (optimisation settings, for example), certain + calculations might be done at 80-bit precision instead of the + intended 32-bit or 64-bit precision. Floating-point results + may differ when optimisation is turned on. In the worst case, + referential transparency is violated, because for example + <literal>let x = E1 in E2</literal> can evaluate to a + different value than <literal>E2[E1/x]</literal>.</para> + + <para> + One workaround is to use the + <option>-msse2</option><indexterm><primary><option>-msse2</option></primary></indexterm> + option (see <xref linkend="options-platform" />, which + generates code to use the SSE2 instruction set instead of + the x87 instruction set. SSE2 code uses the correct + precision for all floating-point operations, and so gives + deterministic results. However, note that this only works + with processors that support SSE2 (Intel Pentium 4 or AMD + Athlon 64 and later), which is why the option is not enabled + by default. The libraries that come with GHC are probably + built without this option, unless you built GHC yourself. + </para> + </listitem> + </itemizedlist> </sect2> |