llvm dev - May 2015 - [LLVMdev] can we define a physical register for both integer and floating point

Hi,

My question sounds kind of silly, but can we define a physical register for
both integer and floating point in LLVM backend?

In the section "Defining a Register Class" in tutorial "Writing
an LLVM
Backend", when we define a Register Class, we need to define the regType,
which makes sense to me. But if both integer and floating point are 32
bits, why cannot we use a physical register to represent different types of
numbers accordingly.

And what if my backend does not have floating point physical register, but
my IR has floating point virtual register?

Thanks

Xiangyang
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Hi Xiangyang,
> On May 8, 2015, at 1:02 PM, Xiangyang Guo <xguo6 at ncsu.edu> wrote:
> 
> Hi,
> 
> My question sounds kind of silly, but can we define a physical register for
both integer and floating point in LLVM backend?
> 
> In the section "Defining a Register Class" in tutorial
"Writing an LLVM Backend", when we define a Register Class, we need to
define the regType, which makes sense to me. But if both integer and floating
point are 32 bits, why cannot we use a physical register to represent different
types of numbers accordingly.
I’m not sure what you mean here.
For a register class to handle different types is perfectly fine, e.g., on
AArch64 both i32 and f32 can live in the same class:
def FPR32 : RegisterClass<"AArch64", [f32, i32], 32,(sequence
"S%u", 0, 31)>;

> 
> And what if my backend does not have floating point physical register, but
my IR has floating point virtual register?
You need to use a soft-float library.

Cheers,
Q.
> 
> Thanks
> 
> Xiangyang 
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