Unroll pkgCache::sHash 8 time, break up dependency

Unroll pkgCache::sHash 8 times and break up the dependency between
the iterations by expanding the calculation
    H(n) = 33 * H(n-1) + c
8 times rather than performing it 8 times. This seems to yield about
a 0.4% performance improvement.

I tried unrolling 4 and 2 bytes as well, those only having 3 ifs at
the end rather than 1 small loop; but that was actually slower -
potentially the code got to large and the cache went bonkers.

I also tried unrolling 4 times instead of 8, thinking that smaller
code might yield better results overall then, but that was slower as
well.

1 files changed, 16 insertions, 2 deletions

diff --git a/apt-pkg/pkgcache.cc b/apt-pkg/pkgcache.cc
index 458860b0a..f7f3537aa 100644
--- a/apt-pkg/pkgcache.cc
+++ b/apt-pkg/pkgcache.cc
@@ -209,8 +209,22 @@ bool pkgCache::ReMap(bool const &Errorchecks)
 map_id_t pkgCache::sHash(StringView Str) const
 {
    uint32_t Hash = 5381;
-   for (auto I = Str.begin(); I != Str.end(); ++I)
-      Hash = 33 * Hash + tolower_ascii_unsafe(*I);
+   auto I = Str.begin();
+   auto End = Str.end();
+   for (; I + 7 < End; I += 8)
+   {
+      Hash = (33u * 33u * 33u * 33u * 33u * 33u * 33u * 33u * Hash +
+	      33u * 33u * 33u * 33u * 33u * 33u * 33u * tolower_ascii_unsafe(I[0]) +
+	      33u * 33u * 33u * 33u * 33u * 33u * tolower_ascii_unsafe(I[1]) +
+	      33u * 33u * 33u * 33u * 33u * tolower_ascii_unsafe(I[2]) +
+	      33u * 33u * 33u * 33u * tolower_ascii_unsafe(I[3]) +
+	      33u * 33u * 33u * tolower_ascii_unsafe(I[4]) +
+	      33u * 33u * tolower_ascii_unsafe(I[5]) +
+	      33u * tolower_ascii_unsafe(I[6]) +
+	      tolower_ascii_unsafe(I[7]));
+   }
+   for (; I != End; ++I)
+      Hash = 33u * Hash + tolower_ascii_unsafe(*I);
    return Hash % HeaderP->GetHashTableSize();
 }
 uint32_t pkgCache::CacheHash()