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I've implemented and validated a patch that does this (attached). Pretty straightforward, and it seems to work.

Nofib results are:

            Min          +0.0%     -4.1%     +0.0%     +0.5%     +0.0%
            Max          +0.1%     +0.0%     +2.3%   +274.5%     +0.0%
 Geometric Mean          +0.0%     -0.1%     +0.4%    +14.3%     -0.0%

So a small but solid reduction in allocations (ignore the runtime results, I was doing something else with the machine at the time so they can't be trusted).