[LLVMdev] Alternate instruction sequences

[Devang Patel]

On Nov 9, 2011, at 12:52 AM, cafxx wrote:

> I was wondering, is there any way to express in the IR that an 
> instruction/instruction sequence/basic 
> block/region/function/module/whatever is an alternate version of 
> another?

There is not a way to represent --- instruction I1 is an alternative for instruction I2 --- in LLVM IR.
-
Devang


> e.g. let's keep things simple and say that I have an 
> instruction I. An optimization pass reads it and could be able to 
> produce one or more functionally-equivalent instructions I1, ..., In 
> without being really able to ascertain which one is "best" w.r.t. the 
> optimization goal. While this could be true even for a single, isolated, 
> pass it's all the more true for chains of optimizations (i.e. 'which one 
> of the alternate instructions will be the "best" after all other 
> optimizations have run?'). In this case, wouldn't it be interesting to 
> be able to insert into the IR all (or some of) the alternate versions, 
> mark them as such and defer the decision about which one will be used 
> after all optimizations have run?
> B.r.,
> Carlo Alberto
> 
> 
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