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Fix broken loop partitioning due to recent changes. #4243

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Nov 15, 2019
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Fix broken loop partitioning due to recent changes. #4243

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52 changes: 28 additions & 24 deletions src/pass/loop_partition.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -513,17 +513,19 @@ Stmt LoopPartitioner::TryPartition(const Node* node,
bool pre_stmt_recurse = true;
if (middle_interval_i->HasLowerBound()) {
body_begin = ir::Simplify(middle_interval.min());
Expr cond = (body_begin - min >= 0);
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@umangyadav umangyadav Nov 1, 2019 •
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@kimishpatel
we can also think about changing this condition to body_begin - min > 0 instead of using >= that can also prevent recursing on zero extent loops.

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@umangyadav, we can do this one, however we still generate zero extent loop. Just that we dont recursively partition anything generated for zero extent loop.
I still prefer to generate cleaner output that has no such zero extent loops that need fixing afterwards, unless somehow we fix it in the loop partition itself. This can be done as you suggested via RemoveNoOp however it seems little convoluted way to go about this. I would rather have loop partition generate clean IR with valid loop iterations.
Is there a reason why you would prefer to generate zero extent loop?

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@kimishpatel There is no specific reason or preference for generating the zero extent loops.

My point is that there is always possibility of having the zero extent loops.

We need to make sure not to recurse on those loops by having some mechanism e.g either guard conditions, RemoveNoOp etc.

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@umangyadav, I understand. Do you have specific example where we can still generate zero extent loops despite this PR? Then it will be easier to see what would be a better fix.

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@umangyadav I agree we might need a cleanup pass. But if we can avoid generating zero extent loops, it's desirable.

if (!analyzer_.CanProve(cond)) {
LOG(WARNING) << "Cannot prove: " << cond
<< ", when generating the pre doubt loop";
body_begin = Max::make(body_begin, min);
// stop recursing on this interval if we can't prove it has non-negative length
pre_stmt_recurse = false;
}
if (!partition_thread_scope) {
Stmt pre_body = Substitute(body, {{Var{var}, var + min}});
pre_stmt = MakeFor(node, body_begin - min, pre_body);
if (!analyzer_.CanProve(body_begin == min)) {
Expr cond = (body_begin - min >= 0);
if (!analyzer_.CanProve(cond)) {
LOG(WARNING) << "Cannot prove: " << cond
<< ", when generating the pre doubt loop";
body_begin = Max::make(body_begin, min);
// stop recursing on this interval if we can't prove it has non-negative length
pre_stmt_recurse = false;
}
if (!partition_thread_scope) {
Stmt pre_body = Substitute(body, {{Var{var}, var + min}});
pre_stmt = MakeFor(node, body_begin - min, pre_body);
}
}
} else {
body_begin = min;
Expand All @@ -536,19 +538,21 @@ Stmt LoopPartitioner::TryPartition(const Node* node,
bool post_stmt_recurse = true;
if (middle_interval_i->HasUpperBound()) {
post_doubt_begin = ir::Simplify(middle_interval.max() + 1);
// require the extent to be non-negative
Expr cond = (max - post_doubt_begin + 1 >= 0);
if (!analyzer_.CanProve(cond)) {
LOG(WARNING) << "Cannot prove: " << cond
<< ", when generating the post doubt loop";
post_doubt_begin = Min::make(post_doubt_begin, max+1);
// stop recursing on this interval if we can't prove it has non-negative length
post_stmt_recurse = false;
}
if (!partition_thread_scope) {
Stmt post_body =
Substitute(body, {{Var{var}, var + post_doubt_begin}});
post_stmt = MakeFor(node, max - post_doubt_begin + 1, post_body);
if (!analyzer_.CanProve(middle_interval.max() == max)) {
// require the extent to be non-negative
Expr cond = (max - post_doubt_begin + 1 >= 0);
if (!analyzer_.CanProve(cond)) {
LOG(WARNING) << "Cannot prove: " << cond
<< ", when generating the post doubt loop";
post_doubt_begin = Min::make(post_doubt_begin, max+1);
// stop recursing on this interval if we can't prove it has non-negative length
post_stmt_recurse = false;
}
if (!partition_thread_scope) {
Stmt post_body =
Substitute(body, {{Var{var}, var + post_doubt_begin}});
post_stmt = MakeFor(node, max - post_doubt_begin + 1, post_body);
}
}
} else {
post_doubt_begin = max + 1;
Expand Down
22 changes: 22 additions & 0 deletions tests/python/unittest/test_pass_loop_partition.py
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Original file line number Diff line number Diff line change
Expand Up @@ -365,6 +365,27 @@ def test_conv_tiling():
stmt = tvm.ir_pass.Simplify(stmt)
assert(not any(collect_visit(stmt, lambda x: isinstance(x, tvm.stmt.IfThenElse))))


def test_multilevel_splitting_with_indivisble_factors():
import topi
A = tvm.placeholder((130,), dtype="float32")
B = topi.nn.relu(A)
s = tvm.create_schedule(B.op)
(y,) = s[B].op.axis
(yo, yi) = s[B].split(y, factor=8)
(yoo, yoi) = s[B].split(yo, factor=16)
s[B].reorder(yoo, yoi, yi)
s[B].unroll(yi)

## But this does the right thing.
with tvm.build_config(partition_const_loop=True):
lowered_body = tvm.lower(s, [A, B]).body
def visit_stmt(op):
return(isinstance(op, tvm.expr.Max))
num_max = collect_visit(lowered_body, visit_stmt)
assert num_max.count(True) == 10


def test_double_splitting_with_indivisible_factors():
m = 48
dtype="float32"
Expand Down Expand Up @@ -443,4 +464,5 @@ def test_simple_rfactor():
test_cce_loop_3()
test_conv_tiling()
test_double_splitting_with_indivisible_factors()
test_multilevel_splitting_with_indivisble_factors()
test_simple_rfactor()