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[YulOpt] declare SSA var when control flow joins

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mingchuan
2019-09-17 13:22:02 +02:00
committed by chriseth
parent 5859a69c44
commit 520a3ccfff
17 changed files with 323 additions and 54 deletions
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libyul/optimiser/README.md
+22 -13
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@@ -119,8 +119,9 @@ so that other components can more easily work with it. The final representation
will be similar to a static-single-assignment (SSA) form, with the difference
that it does not make use of explicit "phi" functions which combines the values
from different branches of control flow because such a feature does not exist
in the Yul language. Instead, assignments to existing variables are
used.
in the Yul language. Instead, when control flow merges, if a variable is re-assigned
in one of the branches, a new SSA variable is declared to hold its current value,
so that the following expressions still only need to reference SSA variables.
An example transformation is the following:
@@ -139,21 +140,25 @@ as follows:
{
let _1 := 0
let a_1 := calldataload(_1)
let a_9 := calldataload(_1)
let a := a_9
let _2 := 0x20
let b_1 := calldataload(_2)
let b := b_1
let b_10 := calldataload(_2)
let b := b_10
let _3 := 0
let _4 := gt(a_1, _3)
if _4 {
let _4 := gt(a_9, _3)
if _4
{
let _5 := 0x20
let b_2 := mul(b_1, _5)
b := b_2
let b_11 := mul(b_10, _5)
b := b_11
}
let a_2 := add(a_1, 1)
let _6 := 0x20
let _7 := add(b, _6)
sstore(a_2, _7)
let b_12 := b
let _6 := 1
let a_13 := add(a_9, _6)
let _7 := 0x20
let _8 := add(b_12, _7)
sstore(a_13, _8)
}
Note that the only variable that is re-assigned in this snippet is ``b``.
@@ -240,6 +245,10 @@ reference to ``a`` by ``a_i``.
The current value mapping is cleared for a variable ``a`` at the end of each block
in which it was assigned to and at the end of the for loop init block if it is assigned
inside the for loop body or post block.
If a variable's value is cleared according to the rule above and the variable is declared outside
the block, a new SSA variable will be created at the location where control flow joins,
this includes the beginning of loop post/body block and the location right after
If/Switch/ForLoop/Block statement.
After this stage, the Redundant Assign Eliminator is recommended to remove the unnecessary
intermediate assignments.