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2015-02-26overhaul optimized i386 memset asmRich Felker-32/+61
on most cpu models, "rep stosl" has high overhead that makes it undesirable for small memset sizes. the new code extends the minimal-branch fast path for short memsets from size 15 up to size 62, and shrink-wraps this code path. in addition, "rep stosl" is very sensitive to misalignment. the cost varies with size and with cpu model, but it has been observed performing 1.5 to 4 times slower when the destination address is not aligned mod 16. the new code thus ensures alignment mod 16, but also preserves any existing additional alignment, in case there are cpu models where it is beneficial. this version is based in part on changes to the x86_64 memset asm proposed by Denys Vlasenko.
2013-08-01optimized memset asm for i386 and x86_64Rich Felker-0/+47
the concept of both versions is the same; they differ only in details. for long runs, they use "rep movsl" or "rep movsq", and for small runs, they use a trick, writing from both ends towards the middle, that reduces the number of branches needed. in addition, if memset is called multiple times with the same length, all branches will be predicted; there are no loops. for larger runs, there are likely faster approaches than "rep", at least on some cpu models. for 32-bit, it's unlikely that there is any faster approach that does not require non-baseline instructions; doing anything fancier would require inspecting cpu capabilities. for 64-bit, there may very well be faster versions that work on all models; further optimization could be explored in the future. with these changes, memset is anywhere between 50% faster and 6 times faster, depending on the cpu model and the length and alignment of the destination buffer.