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<p>><span style="color: rgb(33, 33, 33); font-size: 12pt;">I don't believe that spending time on "tuning" shift2 value is a good idea because I doubt that that is theoretically meaningful. </span></p>
<p><span style="color: rgb(33, 33, 33); font-size: 12pt;">> If my understanding is correct, changing that value doesn't have any significant effects.</span></p>
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<div>It have effect. Setting to values >= 11 gives boost for llvm-check. Even given that results are varie a bit, </div>
<div>T<span style="font-size: 12pt;">i</span><span style="font-size: 12pt;">me(Shift2=6) is </span><span style="font-size: 12pt;">always noticable worse than Time(Shift2=11).</span></div>
<div><span style="font-size: 12pt;">It is not a calculation error, difference is stable.</span></div>
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<div>I am not much in favor of tuning it too much, because anyways in linker we do not have information about all symbols that loader<br>
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<div>will try to lookup. So I believe it is just impossible to find ideal .gnu_hash. But I would hardcode any value in [11,20] instead of 6 probably,<br>
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<div>it is a free boost.<br>
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<div>>The bloom filter for the .gnu.hash table is a two-bit bloom filter. To choose two bit locations, we compute one hash value H and create</div>
<div>>two hash values H1 and H2 in the following manner :Assume a 64-bit machine. Bits H[0,5] are always used for computing the</div>
<div>>first hash H1. Bits H[Shift2, Shift2 + 5] are used for computing the second hash H2.<br>
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<div>>As long as the hash function produces evenly distributed hash values, choosing bits [6, 11] isn't different from choosing other bits, say [15, 20]. </div>
<div>>No Shift2 greater than 5 should be better than any other Shift2 values in theory.</div>
<div>> </div>
<div>>Am I missing something?</div>
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I think there is a mistake here. See, we have following code:<br>
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<div>Val |= uint64_t(1) << (Sym.Hash % C);</div>
<div>Val |= uint64_t(1) << ((Sym.Hash >> getShift2()) % C);</div>
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<div>C == 64 for 64 bits, ot 2^6. It is simplified to:<br>
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<div>Val |= uint64_t(1) << (Sym.Hash >> 6);</div>
<div>Val |= uint64_t(1) << (Sym.Hash >> (Shift2 + 6));</div>
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<div>Or I believe bits H[6, 63] are used to find Bit 1 and H[6 + Shift2, 63] for Bit 2.<br>
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<div>George.<br>
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