[LLVMdev] Proposal to disable some of DAG combine optimizations

[Dan Gohman]

Disabling this optimization in the DAG combiner isn't going to
eliminate the problem; instcombine, GVN, and maybe even others also
happen to perform this optimization. You may find it more effective
to look for ways for codegen to recover in these kinds of situations.

Dan

On Mar 19, 2009, at 10:38 AM, Alireza.Moshtaghi at microchip.com wrote:

> Some of the optimizations that the first DAG combine performs is  
> counter
> productive for our 8-bit target. For example in:
>
> // I dropped the types because they are irrelevant.
> // Excuse me for changing the syntax...
> store %tmp1, %var
> %tmp2 = load %var
> %tmp4 = add %tmp3, %tmp2
>
> Since load is the only user of var and since var has just be stored  
> to,
> it assumes that %tmp1 is alive and it goes ahead and removes the load
> and does:
>
> store %tmp1, var
> tmp4 = add %tmp3 , %tmp1
>
> This is great for architectures that have more than one registers
> because it is likely that value of %tmp1 is already in a physical
> register, hence saving an instruction. However for our 8-bit
> architecture with only one register, this kind of assumptions will  
> just
> result in extra overhead because "add" operates only on memory, so we
> have to generate more instructions to store tmp1 to memory and then  
> use
> that memory location for "add". But without the optimizations, we  
> could
> just use var and everything would work out just fine.
>
> So I propose to add a bit mask and a method to TargetLowering class so
> targets can individually select some of the optimizations to be turned
> off.
>
> Thoughts?
>
> Alireza Moshtaghi
> Senior Software Engineer
> Development Systems, Microchip Technology
>
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