[llvm-dev] [RFC] Refinement of convergent semantics

Owen Anderson via llvm-dev
2015-09-04T20:25:37Z

Hi all,

In light of recent discussions regarding updating passes to respect convergent
semantics, and whether or not it is sufficient for barriers, I would like to
propose a change in convergent semantics that should resolve a lot of the
identified problems regarding loop unrolling, loop unswitching, etc.  Credit to
John McCall for talking this over with me and seeding the core ideas.

Today, convergent operations may only be moved into control-equivalent
locations, or, in layman’s terms, a convergent operation may neither be sunk
into nor hoisted out of, a condition.  This causes problems for full loop
unrolling, as the control dependence on the loop counter is eliminated, but our
intuition indicates that this dependence was somehow trivial.  More concretely,
all know uses of convergent are OK with full unrolling, making this semantic
undesirable.  Related problems arise in loop unswitching as well.

The proposed change is to split the semantics of convergent into two
annotations:
	convergent - this operation may not be made control dependent on any additional
values (aka may not be sunk into a condition)
	nospeculate - this operation may not be added to any program trace on which it
was not previously executed (same as notrap?)

Most of today’s convergent operations (barriers, arithmetic gradients) would
continue to be marked only as convergent.  The new semantics would allow full
loop unrolling, and provide clarity on which loop unswitching operations are
allowed, examples below.

The one case where nospeculate would also be needed is in the case of texture
fetches that compute implicit gradients.  Because the computed gradient forms
part of the addressing mode, gibberish gradients here can cause invalid memory
dereferences.

—Owen

——————————————————

Loop Unswitching Examples

ALLOWED:
for (…) {
  if (c) { convergent(); }
}

DISALLOWED:
for (…) {
  if (c) { … }
  convergent();
}