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Provides versions, on average, are about 34 times longer that Requires
versions. More precisely, if we consider all rpmsetcmp calls for
"apt-shell <<<unmet" command, then sum(c1)/sum(c2)=33.88. This means
that we can save some time and instructions by skipping intermediate
bytes - in other words, by stepping a few bytes at a time. Of course,
after all the bytes are skipped, we must recheck a few final bytes and
possibly step back. Also, this requires more than one sentinel for
proper boundary checking.
This change implements two such "steppers" - 4-byte stepper for c1/c2
ratio below 16 and 8-byte stepper which is used otherwise. When
stepping back, both steppers use bisecting. Note that replacing last
two bisecting steps with a simple loop might be actually more efficient
with respect to branch prediction and CPU's BTB. It is very hard to
measure any user time improvement, though, even in a series of 100 runs.
The improvement is next to none, at least on older AMD CPUs. And so I
choose to keep bisecting.
callgrind annotations for "apt-shell <<<unmet", previous commit:
2,279,520,414 PROGRAM TOTALS
646,107,201 lib/set.c:decode_base62_golomb
502,438,804 lib/set.c:rpmsetcmp
98,243,148 sysdeps/x86_64/memcmp.S:bcmp
93,038,752 sysdeps/x86_64/strcmp.S:__GI_strcmp
callgrind annotations for "apt-shell <<<unmet", this commit:
2,000,254,692 PROGRAM TOTALS
642,039,009 lib/set.c:decode_base62_golomb
227,036,590 lib/set.c:rpmsetcmp
98,247,798 sysdeps/x86_64/memcmp.S:bcmp
93,047,422 sysdeps/x86_64/strcmp.S:__GI_strcmp
798ce0db28