Flag for handling inlining of strcmp/memcmp on i386

[Martin Thuresson <martint at google dot com>]

Gcc currently inlines memcmp and strcmp to repz cmpsb during
optimization. ?Since the library call has optimizations, such as
reading full, aligned words, it turns out that byte-by-byte
comparison is usually slower than calling the library functions.

The diagrams show performance numbers for the library
call and the inlined version. The numbers are from a
microbenchmark that compare buffers, (both equal and not equal
buffers), of various lengths.

http://www.ce.chalmers.se/~martin/foo/amd_opteron_call_repz.png
http://www.ce.chalmers.se/~martin/foo/intel_core_call_repz.png

The performance impact can be large for programs handling large
strings that are expected to be equal, though I did not see any
performance change on Spec2006 (less than 1% difference).

This patch introduces the flag -minline-compares that controls
the inlining.

Thanks,
Martin

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