[llvm-dev] Why ISel Shifts operations can only be expanded for Value type vector ?

[Friedman, Eli via llvm-dev]

On 3/3/2017 9:21 AM, vivek pandya via llvm-dev wrote:
> Hello LLVM Devs,
>
> I am working on a target on which no SHL instruction is available. So 
> wanted to expand it through MUL. But currently it is only possible to 
> expand SHL for vector types.
>
> One possible reason I can think is because LLVM tries to optimize MUL 
> to SHL in certain cases and that can make compiler co in loop or may 
> end up generating wrong code.
>
> But I think SHL should be able to expanded to MUL and to prevent 
> looping between MUL and SHL we can put a condition that only optimize 
> MUL to SHL when SHL is not expanded operation. The similar logic can 
> be applied to DIV and SRA.
>
> If there is any other reasons for not doing this, kindly explain.

There isn't anything fundamental; it just hasn't come up for any in-tree 
target.

-Eli

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