[LLVMdev] llc -O# / opt -O# differences

[Dimitri Tcaciuc]

Great, thanks for the info!

So to extrapolate, (referring to LLVM C bindings) running PassManager
+ populating PassManagerBuilder at it's own OptLevel actually takes
care of different category of optimizations and will not step on what
Target machine CodeGenLevel argument + TargetMachineEmitToFile are
working on?

Dimitri.

On Sat, Jun 30, 2012 at 1:28 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> Dimitri Tcaciuc wrote:
>>
>> Is the llc/opt difference expected?
>
>
> Yes. "opt" runs the optimizers, which take LLVM IR as input and produce LLVM
> IR as output. "opt -O2 -debug-pass=Arguments" will show you a list of the
> individual optimizations (and analyses) that opt -O2 performs. It's possible
> to run them individually (opt -scalarrepl -instcombine) to create a list
> that's better for your own compiler, but -O2 has the ones we think are good
> for a C/C++ compiler.
>
> "llc" is the code generator, which takes LLVM IR in and produces machine
> code. There are some places in the code generator where it has the choice
> between spending compile time to produce good code, or getting the code out
> quickly, and the -O flag to llc specifies that choice. For example, you can
> do register allocation by trying to figure out the most efficient registers
> that minimize the number of spills, or you can just pick the registers
> starting from one, and spill it if it's already used. Any optimizations llc
> does are things that can't possibly happen in an IR-to-IR pass (since the IR
> is SSA form, we can't do register allocation there).
>
> If you want optimized code, you'd run the IR optimizers and ask the code
> generator to spend time producing good code. Or if you want unoptimized
> code, don't run any IR optimizers and ask the code generator to produce code
> as quickly as it can. You can of course choose some other combination by
> running opt and llc yourself, as you noticed.
>
> Nick