[PATCH] Use Rvalue refs in APInt

[Pete Cooper via llvm-commits]

Hi David, Chandler

I’ve been measuring the number of allocations in LLVM and see millions of APInt’s in the trace.

The majority of the traces come from things like ConstantRange where its common to see expressions like 'this_max * Other_max + 1’ (from ConstantRange::multiply).

This patch attempts to reduce the number of allocations with r-value refs on expressions such as these.  Using the bitcode for verify-uselistorder, this reduces the total number of allocations with ‘opt -O2 verify-uselistorder’ from 19.5m to 19.1m.  

There are also compiler time wins to be had.  Before I started reducing the allocations, ConstantRange::multiply was 6.5% of ‘opt -O2’ time on this use case, and after a series of patches to reduce its allocations from 26m to 16m, it drops to about 2.5% of compile time.

I’ve only covered the case where the LHS is an APInt&& and RHS is a uint64_t.  I could add in all the possible cases, i.e.:
- LHS is APInt&&, RHS is APInt&
- LHS is APInt&, RHS is APInt&&
- LHS is APInt&&, RHS is APInt&&

I’m happy to do this, but it may eventually become unwieldy.

What do you think of this particular patch, and then what do you think about going forwards whether its a good idea to cover the other cases with r-value refs?

Cheers,
Pete

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