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Gmane
From: Peter Collingbourne <peter <at> pcc.me.uk>
Subject: Proposal: Loads/stores with deterministic trap/unwind behavior
Newsgroups: gmane.comp.compilers.llvm.devel
Date: Tuesday 1st April 2014 01:58:03 UTC (over 3 years ago)
Hi,

I wanted to propose an IR extension that would allow us to support
zero-cost
exception handling for non-call operations that may trap. I wanted to start
with loads and stores through a null pointer, and later we might extend
this to
div/rem/mod zero. This feature is obviously useful for implementing
languages
such as Java and Go which deterministically translate such operations into
exceptions which may be caught by the user.

There are a couple of somewhat orthogonal features that this would entail:

 1) Deterministic handling for loads and stores through a null pointer.
 2) Ability to unwind a load/store to a specific basic block, like invoke.

At the moment, we do not exactly have 1), as the optimizer considers
non-volatile loads/stores through a null pointer to have undefined
behavior. Volatile loads/stores are closer, but they come with their own
set of baggage that can inhibit optimization. (For example, if we can prove
that a load would always succeed, 'volatile' prevents us from reordering
the load or deleting it if it is dead.) So I propose to add an attribute to
'load' and 'store', which we can call, say, 'nullcheck', with the following
additional semantics:

 - If the memory address is between zero and a target-defined value (i.e.
the
   size of the zero page) the instruction is guaranteed to trap in a
   target-defined manner.

 - The optimizer may only delete or reorder nullcheck instructions if the
   program cannot observe such a transformation by installing a signal
handler
   for the trap.  Therefore, the optimizer would be able to drop the
attribute
   if it can prove that the address will always be non-null.

To support 2), I propose a couple of new instructions. I haven't come up
with
great names for these instructions, but:

 - 'iload' is to 'load' as 'invoke' is to 'call'. That is, the instruction
is
   a terminator and has normal and unwind destinations. e.g.

   %v = iload i8* %ptr to label %try.cont unwind label %lpad

 - Similarly, 'istore' is to 'store' as 'invoke' is to 'call'.

   istore i8 %v, i8* %ptr to label %try.cont unwind label %lpad

These instructions always have 'nullcheck' semantics, plus:

 - If the instruction traps and the program has installed a signal handler
   for the trap which unwinds, the unwind is guaranteed to land at the
   landing pad.

I've been working on an implementation of 'iload' and 'istore' which are
in the attached patches, if you are interested. (They aren't ready to go
in yet.) I have asm parsing/printing for both, and code generation for
'iload'. Would be interested in getting feedback on code generation as this
is my first serious foray into the backend -- I haven't tried running the
generated code yet and the DAG builder is a mashup of the DAG builders for
'invoke' and 'load', but I've eyeballed the asm it generates (e.g. llc
produces
iload-exception.s for the attached iload-exception.ll) and it looks
reasonable.

Thanks,
-- 
Peter
 
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