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Clarify stack height.

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chriseth 2017-02-22 18:57:31 +01:00
parent e963405a19
commit c85f9257f8
1 changed files with 14 additions and 5 deletions
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19
docs/assembly.rst
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@ -333,11 +333,12 @@ push their entry label (with virtual function resolution applied). The calling s
in solidity are:
- the caller pushes return label, arg1, arg2, ..., argn
- the call returns with ret1, ret2, ..., retn
- the call returns with ret1, ret2, ..., retm
This feature is still a bit cumbersome to use, because the stack offset essentially
changes during the call, and thus references to local variables will be wrong.
It is planned that the stack height changes can be specified in inline assembly.
The assumed stack height can be changed by using labels: ``funreturn[-2]:``
The number in brackets should be `m - n - 1`.
.. code::
@ -939,8 +940,16 @@ Pseudocode::
Opcode Stream Generation
------------------------
During opcode stream generation, we keep track of the current stack height,
so that accessing stack variables by name is possible.
During opcode stream generation, we keep track of the current stack height
in a counter,
so that accessing stack variables by name is possible. The stack height is modified with every opcode
that modifies the stack and with every label that is annotated with a stack
adjustment. Every time a new
local variable is introduced, it is registered together with the current
stack height. If a variable is accessed (either for copying its value or for
assignment), the appropriate DUP or SWAP instruction is selected depending
on the difference bitween the current stack height and the
stack height at the point the variable was introduced.
Pseudocode::
@ -980,7 +989,7 @@ Pseudocode::
POP
LabelDefinition(name [id1, ..., idn] :) ->
JUMPDEST
// register new variables id1, ..., idn and adjust the assumed stack
// register new local variables id1, ..., idn and adjust the stack
// height such that it matches the stack height at the beginning of
// the block plus all local variables including the just registered
// ones where idn is at the stack top. If n is zero, resets the stack