[llvm-dev] Disable optimization on basic block level

[David Jones via llvm-dev]

How do you disable optimization for a function?

I ask because my application often compiles machine-generated code that
results in pathological structures that take a long time to optimize, for
little benefit. As an example, if a basic block has over a million
instructions in it, then DSE can take a while, as it is O(n^2) in the
number of instructions in the block. In my application (at least), this
block is typically executed only once at run time, and is not part of a
loop, so I really don't care about code quality for that block.

I am able to identify some of these conditions. I would like to tell LLVM
to "not try so hard" on the pathological blocks, while keeping
optimizations active for ordinary cases.


On Mon, Apr 24, 2017 at 1:17 PM, Matthias Braun via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> +CC llvm-dev again!
>
> > On Apr 24, 2017, at 12:46 AM, Max Muster <olimex5812 at gmail.com> wrote:
> >
> > Hi Matthias,
> >
> > thanks for your answer.
> >
> > The reason it would be useful for me is that I am inserting a basic
> block at IR level as part of a function pass.
> > However, LLVM's optimization passes are clever enough to optimize my
> inserted code at IR level.
> Of course a compiler is only concerned about the observable effects of a
> program. Whether there is an extra block or not is not considered part of
> that. You should probably reconsider what you actually want here. We also
> cannot really help you with the information given so far, I am pretty sure
> you have higher goals than just keeping a basic block (otherwise you could
> just stop the compiler after your pass and the block would stay :)
>
> >
> > Are there other possibilities to avoid optimization at certain parts of
> IR code?
> Add some instructions triggering observable effects. Typical examples for
> "tricking" optimizers would be volatile loads or volatile asm blocks
> (possibly without actual instructions inside).
>
> - Matthias
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