Qiskit / qiskit / 15864966871

Pull #14665

github

Use Arc instead of Box for internal expression nodes in SymbolExpr (#14660)

This commit updates the internal construction of the SymbolExpr struct
use a reference counting pointer instead of a regular pointer to a heap
allocated object. The way the binary tree gets constructed internally
using a normal pointer results in a lot of a copies of elements on the
tree. This creates a large performance overhead for the symbol expr as
we end up copying and freeing elements on the binary tree a large
amount. This was the root cause of the regression reported in #14653
as the expression that gets generated by the transpiler internally ends
up adding a lot of elements to global phase expression and that results
in a lot of memory allocation and deallocation overhead. By using a
reference counting pointer instead of creating and freeing copies of
the expressions on the tree we instead only keep a single copy and
just increment or decrement the reference count.

Fixes #14653

(cherry picked from commit 3b2600202)

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