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Document difference in evaluation order of arguments of addmod/mulmod by old and new code gens.

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Bhargava Shastry 2021-06-21 15:43:23 +02:00 committed by chriseth
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docs/ir/ir-breaking-changes.rst
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@ -145,7 +145,7 @@ The function ``preincr_u8(1)`` returns the following values:
.. index:: ! evaluation order; function arguments
On the other hand, function argument expressions are evaluated in the same order by both code generators.
On the other hand, function argument expressions are evaluated in the same order by both code generators with the exception of the global functions ``addmod`` and ``mulmod``.
For example:
.. code-block:: solidity
@ -165,6 +165,27 @@ The function ``g(1, 2)`` returns the following values:
- Old code generator: ``10`` (``add(2 + 3, 2 + 3)``) but the return value is unspecified in general
- New code generator: ``10`` but the return value is not guaranteed
The arguments to the global functions ``addmod`` and ``mulmod`` are evaluated right-to-left by the old code generator
and left-to-right by the new code generator.
For example:
::
// SPDX-License-Identifier: GPL-3.0
pragma solidity >0.8.0;
contract C {
function f() public pure returns (uint256 aMod, uint256 mMod) {
uint256 x = 3;
// Old code gen: add/mulmod(5, 4, 3)
// New code gen: add/mulmod(4, 5, 5)
aMod = addmod(++x, ++x, x);
mMod = mulmod(++x, ++x, x);
}
}
The function ``f()`` returns the following values:
- Old code generator: ``aMod = 0`` and ``mMod = 2``
- New code generator: ``aMod = 4`` and ``mMod = 0``
Internals
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