[LLVMdev] Optimization Levels - Need The Details

[Vikram Adve]

Reid,

I have one substantial change to suggest to this.  I think the 
distinction between module-level and cross-module optimization is 
artificial and unnecessary in LLVM because transparent link-time 
optimization makes intra-module and cross-module optimizations 
indistiguishable.  It *is* important to distinguish between fast and 
slow optimizations.  Because of this, I would suggest a simpler scheme:

	-O0 and -O1  -- unchanged
	-O2                -- fast function-, module- and program-level 
optimizations
	-O3		   -- aggressive (potentially slow) function-, module- and 
program-level optimizations

A second important distinction that doesn't exist in LLVM today is 
between "always-safe" and "sometimes-unsafe optimizations."  The latter 
class includes reordering optimizations on floating point code.  We 
need to identify these and provide a flag to disable them all (in 
addition to the individual optimization on/off flags).

In the near future, I think we should plan another flag for runtime 
optimization.  It should *not* be just another level in the above list 
(like -O4) because it should be orthogonal to the above set of choices, 
i.e., you can combine runtime opt. with any level of development-time 
opt:
	-Oruntime	-- transparent run-time optimization (or -Odynamic)

--Vikram
http://www.cs.uiuc.edu/~vadve
http://llvm.cs.uiuc.edu/

On Aug 14, 2004, at 7:25 PM, Reid Spencer wrote:

> Folks,
>
> I'm at the point in developing llvmc (Compiler Driver) where I need to
> get the details on the specific optimization arguments that the -O
> family of options should (by default) issue to "opt". I'm soliciting
> your feedback on this so I can start testing optimization. Hopefully 
> you
> can provide it by early this coming week.
>
> For clarity, the -O options are currently defined as:
>
> -O0 - synonym for -O1
> -O1 - options to speed up compilation, not execution
> -O2 - simple/fast module/function optimizations
> -O3 - aggressive module/function optimizations (as well as -O2)
> -O4 - simple/fast link time optimizations (as well as -O3)
> -O5 - aggressive link time optimizations (as well as -O4)
>
> What I'm looking for is a list of equivalent "opt" options for each of
> the five optimization levels defined above. This supports language 
> front
> ends that don't do their own optimization (e.g. Stacker).
>
> Thanks,
>
> Reid.
> _______________________________________________
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