[llvm-dev] How to specify the RegisterClass of an IMPLICIT_DEF?

[Krzysztof Parzyszek via llvm-dev]

On 4/12/2018 8:01 AM, Dominique Torette via llvm-dev wrote:
> 
> But there is one small issue in the inference of RegisterClass of the 
> implicitly defined register.
> 
> As shown below, the %vreg6<def> is implicitly defined as FPUabRegisterClass.
> 
> This register class accepts the v2f32 type, but for others addressing 
> mode context this register should be FPUabOffsetClass.
> 
> Is there a mechanism, an option to inforce/control the RegisterClass of 
> registers defined by IMPLICIT_DEF ?

A virtual register has a specific register class assigned to it, and is 
implicitly a member of any superclass of that class. If you want to use 
the register in a place that requires another class, you will need to 
create a new virtual register for the new class and copy the value into it.

In instruction selection, it's usually the type of the value that 
determines the register class of the register holding it. Also, the 
register classes of instruction operands can restrict it further. If you 
want to put a certain value into a register from a specific class, you 
can use COPY_TO_REGCLASS (just like the code you quoted does):
   Dst = COPY_TO_REGCLASS Src, RegClass

-Krzysztof

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