The compiler can still see the max value being passed in and your example block has a fixed extent. By using first<> you get another span of fixed extent. This loop in general would benefit from being changed to something that leverages the fixed extent even more. There are different approaches, but ideally you want to iterate in a stride, which relies on the fixed extent and always returned subspans of fixed extents each step, so you dispense all the arithmetic access and subspan calls. On Fri, 6 Dec 2024 at 18:56, Andrey Semashev via Boost < boost@lists.boost.org> wrote:
On 12/6/24 21:46, Claudio DeSouza via Boost wrote:
detail::read32le( block.subspan(i * 4).first<4>());
So you also get a sized span, and avoid checks.
I think, first() would need to check the dynamic span size anyway.
PS: Please, don't top-post.
Assuming you had code like this:
void transform( span
block ) { for( int i = 0; i < 16; ++i ) { x[ i ] = detail::read32le( block.subspan(i * 4, 4) ); } } I would presume, the subspan call would have a runtime check for the offset and size. Then the converting constructor from span<const unsigned char> returned from subspan to span
accepted by read32le would have to check the size again. Yes, the compiler could optimize those checks away, if it's smart enough. Or it could not, e.g. if optimizations are disabled or it fails to propagate constants for some reason. But none of these checks are actually necessary, except maybe one at the entry, to enforce the size of the entire block. Maybe not even that, if the callers of transform are guaranteed to always pass the whole blocks. _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
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On Fri, 6 Dec 2024 at 17:42, Andrey Semashev via Boost < boost@lists.boost.org> wrote:
On 12/6/24 20:30, Peter Dimov via Boost wrote:
You can see practical manifestations of these examples in e.g
https://github.com/pdimov/hash2/blob/develop/include/boost/hash2/md5.hpp
where at
https://github.com/pdimov/hash2/blob/26b16023dc639ae354e246458719e4b569bc1d9...
you have
BOOST_CXX14_CONSTEXPR void transform( unsigned char const block[ 64
] )
with the implicit precondition that `block` points at at least 64 valid
bytes,
and then at
https://github.com/pdimov/hash2/blob/26b16023dc639ae354e246458719e4b569bc1d9...
you have
for( int i = 0; i < 16; ++i ) { x[ i ] = detail::read32le( block + i * 4 ); }
which relies on this implicit precondition to read 16 subspans of size
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