This was only an experiment. Improvement was marginal: only +3% on AMD64
(an Athlon 64 3200+). However, it may be interesting to test it on EM64T
(e.g. newer P4s) since an older P3/800, hence in 32-bit mode, got a +15%
improvement in Speedometer 4 benchmarks.
Rationale: lahf/seto sequences avoid load/stores to the stack (push/pop)
and it was thus hoped to be faster.
Anyhow, SAHF_SETO_PROFITABLE can only be enabled manually at this time.
Edit your generated Makefile for testing, but first make sure your CPU
supports lahf in 64-bit mode (lahf_lm flag in /proc/cpuinfo).
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