[llvm-dev] [GlobalISel] Questions about selection regions

[Ahmed Bougacha via llvm-dev]

On Tue, Jan 31, 2017 at 7:12 AM, Bekket McClane via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi,
> I've been studying the global instruction selector introduced recently. One
> of the properties of global instruction selectors is that they select
> instructions across basic blocks such that they can get more information in
> order to choose optimal patterns.
> However, the current global isel implementation still iterates over BBs
> within functions, which is same as the original SelectionDAG approach, and
> select individual instruction without "peeking" the information of other
> basic blocks. IMOO, neither do the overall design nor the individual
> instruction selector routines take the "global information" into concern.
> Are my observations correct?

It iterates over instructions (within blocks within functions) because
instructions are the "roots" of the selector logic.  When examining a
root, the selector is free to examine the instructions defining the
operands of the root; these might be in different basic blocks.

Put another way, the difference between SelectionDAG and GlobalISel is
that the SelectionDAG selector logic can only access instructions
contained in the parent of the root instruction (because at any given
time, that's the only block that we created a DAG for).
In GlobalISel, the entire function is accessible, so the selector can
consume instructions even if they are not in the same block as the
root instruction. It's in that sense that GlobalISel is "global".

> Also, are there any examples which show that the current global isel take
> information across multiple BBs within a function?

I don't think any of the targets use this feature right now.  The
first use will probably be folding of G_CONSTANT nodes (that we
currently all emit in the entry block) into immediate operands.

HTH,
-Ahmed

> Best Regards,
> --
> Bekket McClane
> Department of Computer Science,
> National Tsing Hua University
>
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