[llvm-dev] Question about legal integer type

[Tim Northover via llvm-dev]

On 26 June 2017 at 06:11, 陳韋任 via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> In other words, we will load i8 into a 32-bit register, then apply
> 32-bit operation on it. In this case, is i8 a legal
> or illegal type?

This is extremely typical and in all similar backends I know of i8 is
illegal despite that.

> Currently, we treats it as *illegal*, but I find some
> optimizations will be skipped. For example,
> in InstCombiner::visitTrunc [1], it calls shouldChangeType, which
> checks both Src and Dest are legal type,
> then do optimization if shouldChangeType returns true.

Typically truncs and extensions are handled (and become free) during
ISel. You end up with zextload, sextload and truncstore nodes which
just do the obvious thing.

I haven't thought about that particular InstCombine, but given that
most LLVM targets have this behaviour the chances are it actually
improves the code in your case too.

Cheers.

Tim.