Boost C++ Libraries: Ticket #9231: Suboptimal assignment

component changed; owner set

Domagoj Šarić — Fri, 11 Oct 2013 13:05:37 GMT

owner set to John Maddock
component None → multiprecision

status changed; resolution set

John Maddock — Sat, 12 Oct 2013 08:19:31 GMT

status new → closed
resolution → fixed

(In [86258]) Optimize copying of allocator-free cpp_int's via memcpy. Fix consistency of checks for exponents < 0 in powm. Fixes #9231. Refs #9236.

anonymous — Sat, 12 Oct 2013 08:37:26 GMT

Thanks... ps. did you check why this->resize(other.size(), other.size()); does not evaluate to a nop (I forgot to add that this happens in powm when it copy-constructs the double_type)_?

description changed

Domagoj Šarić — Sat, 12 Oct 2013 08:38:53 GMT

description modified (diff)

John Maddock — Sat, 12 Oct 2013 10:02:41 GMT

Thanks... ps. did you check why this->resize(other.size(), other.size()); does not evaluate to a nop (I forgot to add that this happens in powm when it copy-constructs the double_type)_?

Sigh.... this is harder to optimize away - it goes via a general N bit to M bit conversion routine (which is already split many different ways, I really don't want to split it up any more or it becomes unmaintainable) so there is a check that the new size requested doesn't exceed the space available. This situation can also happen in calls to resize() from addition/subtraction/multiplication/shift routines, as it has to handle overflow in these cases.

Domagoj Šarić — Sat, 12 Oct 2013 11:56:13 GMT

MSVC is good at propagating constants (which need not be actual ICEs, but can be resolved as such at LTCG time) so that resize() is receiving two constants there...the fact that the compiler wasn't able to fully eliminate the code (the function call isn't there, it was inlined but not fully eliminated, I could see some branching) smells of something 'weird' being done in the resize() function...don't have time currently to examine the matter further...

John Maddock — Sat, 12 Oct 2013 12:02:12 GMT

(In [86262]) Use memcpy in more places. Add optimized bitwise operations for unsigned integers. Fixes #9243. Refs #9231.