[llvm-dev] Choosing the appropriate iterator for the job

Martin J. O'Riordan via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 30 03:11:00 PST 2017

At the moment I am in the process of getting our out-of-tree compiler up to
date with respect to the v4.0 branch, and thankfully all is going well.
However, one of the most common changes I have had to make has to do with
iterations over machine instructions and basic blocks.


These are changes I have had to make incrementally each time we catch up
with a release of LLVM, and basically the issue is that I have to replace
something like:


void foo(MachineInstruction* x);

MachineBasicBlock::iterator index = ...;






This pattern of having to make this edit (adding the '&*') looks really
scary to me, I am always nervous of expressions that look like this, and
over the past three or so releases we have had to make similar changes
(usually, but not always to various MI/MBB iterators).


So my guess is that we are using the wrong iterator, or an inappropriate
iterator; but there are so many iterators now that I wonder if there is
guidance on which iterator is most appropriate to a particular context or


The main types of iteration we need are over the instructions in a
basic-block, sometimes including the MI contents of bundles, and sometimes
over the bundle (treating them as a single MI).  And of course the 'const'
versus editable iterations.


Does anybody have pointers to existing developer documentation advising
which iterators to use, when and how?  Or have advice they can give here in
this forum if such documentation does not exist (I haven't found it if it
does exist)?


I think it would be really useful information in general, and since our code
is originally derived from LLVM v2.7, as LLVM modernises, we often have to
make tweaks to code built a long time ago to get it working again.  The
trick of adding '&*' works, but is deeply inelegant and error-prone, and I'm
pretty sure that it can be made a lot cleaner if we use the right iterator
for the job.


Thanks for your advice,




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