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Executing rustc -C help shows (among other things):
-C opt-level=val -- optimize with possible levels 0-3, s, or z The levels 0 to 3 are fairly intuitive, I think: the higher the level, the more aggressive optimizations will be performed. However, I have no clue what the s and z options are doing and I couldn't find Rust-related information about them.
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-O1 (optimize minimally) -O2 (optimize more) -O3 (optimize even more) -Ofast (optimize very aggressively to the point of breaking standard compliance)
6.4 Optimization levels. In order to control compilation-time and compiler memory usage, and the trade-offs between speed and space for the resulting executable, GCC provides a range of general optimization levels, numbered from 0--3, as well as individual options for specific types of optimization.
The degree to which the compiler will optimize the code it generates is controlled by the -O flag. No optimization. In the absence of any version of the -O flag, the compiler generates straightforward code with no instruction reordering or other attempt at performance improvement. -O or -O2.
It seems like you are not the only one confused, as described in a Rust issue. It seems to follow the same pattern as Clang:
Os For optimising the size when compiling. Oz For even more size optimisation.
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Looking at these and these lines in Rust's source code, I can say that s means optimize for size, and z means optimize for size some more.
All optimizations seem to be performed by the LLVM code-generation engine.
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