[llvm-dev] My own codegen is 2.5x slower than llc?

[Bruce Hoult via llvm-dev]

What percentage of performance advantage do you expect to get from having a
basic block with 14000 instructions, rather than breaking it up a bit?

On Wed, May 30, 2018 at 12:02 AM, David Jones via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> My back-end code generator uses LLVM 5.0.1 to optimize and generate code
> for x86_64.
>
> If I run it on a given sample of IR, it takes almost 5 minutes to generate
> object code.  95%+ of this time is spent in MergeConsecutiveStores().  (One
> function has a basic block with 14000 instructions, which is a pathological
> case for MergeConsecutiveStores.)
>
> If, instead, I dump out the LLVM IR, and manually run both opt and llc on
> it with -O2, the whole affair takes only 2 minutes.
>
> I am using a dynamically linked LLVM library.  I have verified using GDB
> that both my code generator and llc are invoking the shared library (i.e.
> the exact same code) so I would not expect to see a 2.5x performance
> difference.
>
> What could explain this?
>
>
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