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

Matthias Braun via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 30 11:24:37 PST 2017

> On Jan 30, 2017, at 3:11 AM, Martin J. O'Riordan via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 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 = ...;
> foo(index);
> to:
> foo(&*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 usage.
> 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 don't think there is documentation or broader consensus/patterns throughout the CodeGen library.
As for myself: I try to use MachineInstr* as much as possible: Using range based for esp. with make_range let's you mostly avoid MachineBasicBlock::iterator. I tend to only use the iterator when I want to indicate a position inside a basic block (for inserting instructions before/behind it).

- Matthias
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