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Mutable variables in dynamic branches prevent full constant folding in partial evaluation #2087

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idavis opened this issue Jan 2, 2025 · 0 comments · Fixed by #2089
Closed

Mutable variables in dynamic branches prevent full constant folding in partial evaluation #2087

idavis opened this issue Jan 2, 2025 · 0 comments · Fixed by #2089
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idavis commented Jan 2, 2025 •
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Given a mutable variable used in a quantum condition block, partial eval can't track that despite value being in a dynamic condition, it is statically constant.

@EntryPoint()
operation Test() : Unit {
    mutable value = 0;
    use q = Qubit();
    let cond = MResetZ(q);
    if cond == Zero {
        value -= 1 ;
    } else {
        value += 1;
    }
}

This leads to a variable being used in the CFG for the else block, which is converted into a constant during SSA transformation. This leaves SSA instructions performing mathematical computation on constant values.

This currently causes a panic in the compiler which assumes all constant operations were resolved during partial evaluation.

Actual:

Blocks:
Block 0:Block:
    Variable(0, Integer) = Store Integer(0)
    Call id(1), args( Qubit(0), Result(0), )
    Variable(1, Boolean) = Call id(2), args( Result(0), )
    Variable(2, Boolean) = Icmp Eq, Variable(1, Boolean), Bool(false)
    Branch Variable(2, Boolean), 2, 3
Block 1:Block:
    Call id(3), args( Integer(0), Pointer, )
    Return
Block 2:Block:
    Variable(0, Integer) = Store Integer(-1)
    Jump(1)
Block 3:Block:
    Variable(3, Integer) = Add Variable(0, Integer), Integer(1)
    Variable(0, Integer) = Store Variable(3, Integer)
    Jump(1)

In SSA processing:

Variable(3, Integer) = Add Variable(0, Integer), Integer(1)
Variable(0, Integer) = Store Variable(3, Integer)

Is turned into:

Variable(0, Integer) = Add Integer(0), Integer(1)

Which causes the panic. Given that partial evaluation can't fully perform constant folding, we most likely need to implement more SSA passes for constant folding, dead code elimination, at a minimum.

Workaround:
Save off the current mut value and use it on the rhs of the expressions in the conditional blocks:

operation Workaround() : Unit {
    mutable value = 0;
    use q = Qubit();
    let cond = MResetZ(q);
    let orig = value;
    if cond == Zero {
        value = orig - 1 ;
    } else {
        value = orig + 1;
    }
}
@idavis idavis added bug Something isn't working needs triage labels Jan 2, 2025
@swernli swernli self-assigned this Jan 4, 2025
swernli added a commit that referenced this issue Jan 4, 2025
@swernli
Mutable variables in dynamic branches prevent full constant folding i…
97a45f6 
…n partial evaluation

This fixes the bug by having partial evaluation more explicitly track different variable mappings to literals across branches and recombining those mappings that match (ie: are constant) when all branches are done. This also includes partial eval and RIR SSA pass fixes to correctly support immutable and mutable copies of dynamic variables. New test cases for several combinations of constant folding at partial eval are included, as well as a new test case confirming RIR SSA fix.

Fixes #2087
swernli added a commit that referenced this issue Jan 4, 2025
@swernli
Mutable variables in dynamic branches prevent full constant folding i…
4210dd2 
…n partial evaluation

This fixes the bug by having partial evaluation more explicitly track different variable mappings to literals across branches and recombining those mappings that match (ie: are constant) when all branches are done. This also includes partial eval and RIR SSA pass fixes to correctly support immutable and mutable copies of dynamic variables. New test cases for several combinations of constant folding at partial eval are included, as well as a new test case confirming RIR SSA fix.

Fixes #2087
@swernli swernli mentioned this issue Jan 6, 2025
Merged
This was referenced Jan 7, 2025
Open
Merged
github-merge-queue bot pushed a commit that referenced this issue Jan 9, 2025
@swernli
Mutable variables in dynamic branches prevent full constant folding i…
d7f8962 
…n partial evaluation (#2089)

This fixes the bug by having partial evaluation more explicitly track
different variable mappings to literals across branches and recombining
those mappings that match (ie: are constant) when all branches are done.
This also includes partial eval and RIR SSA pass fixes to correctly
support immutable and mutable copies of dynamic variables. New test
cases for several combinations of constant folding at partial eval are
included, as well as a new test case confirming RIR SSA fix.

Fixes #2087
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