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

[Pieb, Wolfgang via llvm-dev]

We thought about putting the 'this' pointer into memory but that would mean that even small member functions would acquire a stack frame (on architectures where leaf routines can get away without one), which may degrade performance considerably. You could apply some heuristics and determine when a store is unnecessary, but inlining may complicate things. A fake_use operation would be inlined like any other instruction.

We performed some internal evaluations of location coverage (at -O1 on x86), and about half of the member functions already had 100% location coverage for the this pointer, mostly because they are small. For these methods extending the live range to the end of the function would probably make no difference in code generation anyway. For the longer routines, a fake-use at the end of the function would probably cause a spill early on, similar to the effect of an explicit store.

Another consideration is that we'd like to extend the concept to other variables as well.  This 'this' pointer Is invariant, but other parameters and locals are generally not, so we believe extending the live range would be a more general solution.

-- wolfgang


From: Smith, Kevin B [mailto:kevin.b.smith at intel.com]
Sent: Monday, September 21, 2015 7:12 PM
To: Pieb, Wolfgang
Cc: llvm-dev at lists.llvm.org
Subject: RE: extending liveness of 'this' pointer via FAKE_USE opcode

Why extend the live-range?

If it isn't already in memory (and for many architectures, it is already in memory), put the this pointer into memory, and change the
debug information so that the location-expression of the this parameter is marked to be that memory?

That has two nice properties:

1 - If you do this early (in the function IR, not necessarily in the pass ordering), it really only costs a single store, and doesn't otherwise really affect register allocation.  Mark the memory itself in such a way that it cannot be deleted.

2 - This is only required for architectures/calling conventions where the this ptr isn't already in memory, and where this extra debugging is required/desired.

Kevin Smith

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Pieb, Wolfgang via llvm-dev
Sent: Monday, September 21, 2015 11:17 AM
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: [llvm-dev] extending liveness of 'this' pointer via FAKE_USE opcode

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.
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