[llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode

[Adrian Prantl via llvm-dev]

> On Sep 21, 2015, at 11:16 AM, Pieb, Wolfgang via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello!
>  
> At Sony we've seen some serious customer interest in having the 'this' pointer visible throughout an entire function during
> debugging. However, optimizations may eliminate it after its last use, so we've been looking for a way to artificially extend its
> liverange to the end of the function.
>  
> So far, the most compelling way we can think of, and one we have used successfully in the past in at least one other compiler,
> is to create a 'fake use' of the 'this' pointer at the end of the function, compelling the rest of the compiler to not optimize it away.
>  
> At the moment there doesn't seem to be a good way to create such a fake use in LLVM (please enlighten us if you know of one), so we are
> proposing to introduce a new intrinsic (e.g. llvm.fake_use), which would take a single value argument, representing a use of that value.
> The intrinsic would be lowered to a new invariant TargetOpcode (e.g. FAKE_USE), which serves the same purpose at the MI level.
> Code emission would simply ignore the new opcode.
>  
> Frontends could use the intrinsic to extend liveranges of variables as desired. As a first use case, clang would accept a new option
> (e.g. -fkeep-this-ptr) which would cause a fake use of 'this' to be inserted at the end of a function, making it available for inspection
> throughout the entire function body.

>  
> One important note is that since such an option would affect code generation, it cannot be automatically enabled by -g. However, should there be
> eventually support for a -Og mode (optimize for debugging), that mode could enable it.
>  
> Any comments or alternative ideas are appreciated.

The clang frontend creates all variables in stack slots and relies on LLVM to lower them into registers if applicable. Would the new intrinsic refer to the stack slot, would it describe a load of the value?

What happens if the function is inlined, will the new intrinsic prevent other optimizations?

— adrian

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