gh-125196: Use PyUnicodeWriter for repr(dict)

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• Issue: Use the public PyUnicodeWriter API #125196

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Benchmark:

importpyperfrunner=pyperf.Runner() runner.bench_func('repr({})', repr,{}) runner.bench_func("repr({'a': 'A'})", repr,{'a': 'A'}) runner.bench_func("repr({'a': 'A', ...})", repr,{'a': 'A', 'b': 'B', 'c': 'C', 'd': 'D', 'e': 'E'}) runner.bench_func("repr({'a': 1})", repr,{'a': 1}) runner.bench_func("repr({'a': 1, ...})", repr,{'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5})

Results, Python built with gcc -O3, CPU isolation, --rigorous:

+-----------------------+--------+----------------------+ | Benchmark | ref | change | +=======================+========+======================+ | repr({}) | 106 ns | 107 ns: 1.01x slower | +1 ns +-----------------------+--------+----------------------+ | repr({'a': 'A'}) | 260 ns | 255 ns: 1.02x faster | -5 ns +-----------------------+--------+----------------------+ | repr({'a': 'A', ...}) | 717 ns | 728 ns: 1.01x slower | +11 ns +-----------------------+--------+----------------------+ | repr({'a': 1, ...}) | 802 ns | 755 ns: 1.06x faster | -47 ns +-----------------------+--------+----------------------+ | Geometric mean | (ref) | 1.01x faster | +-----------------------+--------+----------------------+ Benchmark hidden because not significant (1): repr({'a': 1}) 

There is basically no significan effect on performance. Only repr({'a': 1, ...}) is a little bit faster thanks to the integer fast-path.

UPDATE: I reran benchmarks with --rigorous.

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Merged, thanks for the review @methane.