[llvm-dev] [GlobalISel] Legalize generic instructions that also depend on type of scalar, not only scalar size

[Daniel Sanders via llvm-dev]

For G_LOAD/G_STORE it should be ok to use double load/store instructions to load/store i64 values. You just end up with a copy between fprs and gprs if you need to perform an operation that the fpu can't do.

Could you elaborate on the issue with G_SELECT, G_EXTRACT, and G_INSERT?

> On 21 Sep 2018, at 02:28, Petar Avramovic <Petar.Avramovic at rt-rk.com> wrote:
> 
> Hi,
> 
> Mips32 has 64 bit floating point instructions, while i64 instructions have to be emulated with i32 instructions. This means that G_LOAD should be custom legalized for s64 integer value, and be legal for s64 floating point value. There are also other generic instructions with the same problem: G_STORE, G_SELECT, G_EXTRACT, and G_INSERT.
> 
> There are also other configurations where integer and floating point instructions of the same size are not simultaneously available. This problem was already addressed here http://lists.llvm.org/pipermail/llvm-dev/2017-July/114978.html for G_LOAD/G_STORE.
> 
> Legality of an instruction should not depend on surrounding instructions. Because of that, approach from regbankselect that iterates through uses of G_LOAD def register and checks if some of the uses was an generic floating point instruction should not be an option for legalizer.
> 
> Since GlobalISel Legalizer cannot distinguish between them using only LLT, only other option that I can see at this moment is having "F" variant of the generic instruction (like this is the case with G_ADD and G_FADD).
> 
> Is this change possible? Or are there some other approaches for dealing with this problem?
> 
> Petar
> 
>