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

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 21 11:23:03 PDT 2015

Oops, forgot to reply-all (& adding a few other people to the 'to' line)
---------- Forwarded message ----------
From: David Blaikie <dblaikie at gmail.com>
Date: Mon, Sep 21, 2015 at 11:22 AM
Subject: Re: [llvm-dev] extending liveness of 'this' pointer via FAKE_USE
To: "Pieb, Wolfgang" <Wolfgang_Pieb at playstation.sony.com>

On Mon, 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.

How much have you observed the actual (usually a register, yes?) register
being reused, versus the debug info just not successfully describing its
location? LLVM's optimized debug info still does a pretty bad job tracking
optimized values (especially anything that gets promoted out of an alloca
into a register)

>  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.

Is there some more general heuristic we should consider rather than special
casing the 'this' pointer? (something that might help other cases of
parameters being optimized away entirely that would still be valuable to
the user? Should we try to preserve all parameter values? All variables?)

>  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.

Do you have much in the way of performance numbers to get a sense of the
impact of this feature? (& possibly the impact with the sliding scale of
"this pointer or all parameters or all variables" sort of thing)

>  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.

Is it worth considering adding -Og now/for this feature?

> Any comments or alternative ideas are appreciated.
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