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Add return area clarification

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Chris Chinchilla 2020-01-21 14:18:05 +01:00 committed by chriseth
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docs/yul.rst
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@ -833,15 +833,19 @@ the ``dup`` and ``swap`` instructions as well as ``jump`` instructions, labels a
| insize, out, outsize) | | | providing g gas and v wei and output area |
| | | | mem[out...(out+outsize)) returning 0 on error (eg. out of gas) |
| | | | and 1 on success |
| | | | :ref:`See more <yul-call-return-area>` |
+-------------------------+-----+---+-----------------------------------------------------------------+
| callcode(g, a, v, in, | | F | identical to ``call`` but only use the code from a and stay |
| insize, out, outsize) | | | in the context of the current contract otherwise |
| | | | :ref:`See more <yul-call-return-area>` |
+-------------------------+-----+---+-----------------------------------------------------------------+
| delegatecall(g, a, in, | | H | identical to ``callcode`` but also keep ``caller`` |
| insize, out, outsize) | | | and ``callvalue`` |
| | | | :ref:`See more <yul-call-return-area>` |
+-------------------------+-----+---+-----------------------------------------------------------------+
| staticcall(g, a, in, | | B | identical to ``call(g, a, 0, in, insize, out, outsize)`` but do |
| insize, out, outsize) | | | not allow state modifications |
| | | | :ref:`See more <yul-call-return-area>` |
+-------------------------+-----+---+-----------------------------------------------------------------+
| return(p, s) | `-` | F | end execution, return data mem[p...(p+s)) |
+-------------------------+-----+---+-----------------------------------------------------------------+
@ -888,6 +892,17 @@ which are used to access other parts of a Yul object.
as arguments and return the size and offset in the data area, respectively.
For the EVM, the ``datacopy`` function is equivalent to ``codecopy``.
.. _yul-call-return-area:
.. note::
The ``call*`` instructions use the ``out`` and ``outsize`` parameters to define an area in memory where
the return data is placed. This area is written to depending on how many bytes the called contract returns.
If it returns more data, only the first ``outsize`` bytes are written. You can access the rest of the data
using the ``returndatacopy`` opcode. If it returns less data, then the remaining bytes are not touched at all.
You need to use the ``returndatasize`` opcode to check which part of this memory area contains the return data.
The remaining bytes will retain their values as of before the call. If the call fails (it returns ``0``),
nothing is written to that area, but you can still retrieve the failure data using ``returndatacopy``.
.. _yul-object:
Specification of Yul Object