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Add GPU-compatible upper bound and lower bound algorithms to AMReX_Algorithm #2958

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merged 5 commits into from
Sep 23, 2022
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Add GPU-compatible upper bound and lower bound algorithms to AMReX_Algorithm #2958

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e33a02d
add upper and lower bound algorithms to AMReX_Algorithm
lucafedeli88 Sep 22, 2022
f56a1df
use AMREX_GPU_HOST_DEVICE
lucafedeli88 Sep 22, 2022
28be07d
use std::ptrdiff_t
lucafedeli88 Sep 22, 2022
acac786
change implementation
lucafedeli88 Sep 23, 2022
92ae490
use only <
lucafedeli88 Sep 23, 2022
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51 changes: 51 additions & 0 deletions Src/Base/AMReX_Algorithm.H
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Original file line number Diff line number Diff line change
Expand Up @@ -157,6 +157,57 @@ namespace amrex
return hi;
}

template<typename ItType, typename ValType>
AMREX_GPU_HOST_DEVICE
ItType upper_bound (ItType first, ItType last, const ValType& val)
{
#if AMREX_DEVICE_COMPILE
std::ptrdiff_t count = last-first;
while(count>0){
auto it = first;
const auto step = count/2;
it += step;
if (!(val<*it)){
first = ++it;
count -= step + 1;
}
else{
count = step;
}
}

return first;
#else
return std::upper_bound(first, last, val);
#endif
}

template<typename ItType, typename ValType>
AMREX_GPU_HOST_DEVICE
ItType lower_bound (ItType first, ItType last, const ValType& val)
{
#ifdef AMREX_DEVICE_COMPILE
std::ptrdiff_t count = last-first;
while(count>0)
{
auto it = first;
const auto step = count/2;
it += step;
if (!(val<=*it)){
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I think this should be *it < value, because the algorithm is only supposed to use <, not <=.

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I think that we actually need <= with this formulation. Otherwise with the vector [0,1,2,3,4,5,5,6,6,7] passing 0 as value gives 1 instead of 0 and passing 7 gives END instead of 7.

However, it could be reformulated as if (val > *it ) . I can do that if you think that it would be better

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I am confused. Is if (val > *it) mathematically equivalent to if (*it <val) (which I suggested) for your example? Maybe you didn't remove !?

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@WeiqunZhang WeiqunZhang Sep 23, 2022 •
edited
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We should only use <, not >, because a data type with < but without > works with std::lower_bound.

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Oh sorry! My fault... I misread your comment: I thought that you meant !(val<*it) !
You are totally right!

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It won't make difference for arithmetic types. But the user will expect this to work too.

struct foo {
    int i;
    bool operator< (foo const& rhs) const {
        return i < rhs.i;
    }
    // No operator> defined
};

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I'll do the correction right now

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Sorry again, I've implemented your suggestion now

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No problems. Thanks for your hard work!

first = ++it;
count -= step + 1;
}
else{
count = step;
}
}

return first;
#else
return std::lower_bound(first, last, val);
#endif
}

namespace detail {

struct clzll_tag {};
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