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Added Content and Edited for readability

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The example for signextend requested by @bshastry was added. Codeblock was moved before warning for readability.
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Vignesh Karthikeyan 2019-06-07 00:47:08 +05:30 committed by GitHub
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commit 1d9b6b5bbf
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1 changed files with 11 additions and 11 deletions
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docs/assembly.rst
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@ -394,17 +394,6 @@ use ``x_slot``, and to retrieve the byte-offset you use ``x_offset``.
Local Solidity variables are available for assignments, for example:
.. warning::
If you access variables of a type that spans less than 256 bits
(for example ``uint64``, ``address``, ``bytes16`` or ``byte``),
you cannot make any assumptions about bits not part of the
encoding of the type. Especially, do not assume them to be zero.
To be safe, always clear the data properly before you use it
in a context where this is important:
``uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ }``
To clean signed types, you can use the ``signextend`` opcode:
``assembly { signextend(0, x) }``
.. code::
pragma solidity >=0.4.11 <0.7.0;
@ -418,6 +407,17 @@ Local Solidity variables are available for assignments, for example:
}
}
.. warning::
If you access variables of a type that spans less than 256 bits
(for example ``uint64``, ``address``, ``bytes16`` or ``byte``),
you cannot make any assumptions about bits not part of the
encoding of the type. Especially, do not assume them to be zero.
To be safe, always clear the data properly before you use it
in a context where this is important:
``uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ }``
To clean signed types, you can use the ``signextend`` opcode:
``assembly { signextend(<bit_width_of_x>, x) }``
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