[llvm-dev] Why is printing an Instruction so slow?

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 5 09:36:32 PDT 2015

Instruction printing is usually slow because it uses SlotNumbering, which
is a whole function analysis to determine what numbers to use for unnamed

If printing is part of your critical path, you can use
'Value::print(raw_ostream&, ModuleSlotTracker&)' instead of operator<<. I
believe it will help you do the analysis once and print many instructions.

On Mon, Oct 5, 2015 at 8:17 AM, Stephen Thomas via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
> When writing my custom passes, I often emit log messages to llvm::errs()
> like so:
> llvm::errs() << "Could not handle instruction: " << *Inst << std::endl;
> where Inst is a pointer to an Instruction.
> I've profiled my code and found that this way of printing an Instruction
> takes a very long time: indeed, it is the bottleneck of my pass.
> Of course, I could always switch these log messages off for production
> runs, but during development and debugging, I'd like to have them on to
> make debugging much easier.
> Is there a faster way to print an Instruction?
> Thanks,
> Steve
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