Revision a0518f338c6c26b2464fa6d19d8707b567e4dded authored by Steven Johnson on 31 October 2022, 20:08:04 UTC, committed by Steven Johnson on 31 October 2022, 20:08:04 UTC
ApplySplit.cpp
#include "ApplySplit.h"
#include "IR.h"
#include "IROperator.h"
#include "Simplify.h"
#include "Substitute.h"
namespace Halide {
namespace Internal {
using std::map;
using std::string;
using std::vector;
vector<ApplySplitResult> apply_split(const Split &split, bool is_update, const string &prefix,
map<string, Expr> &dim_extent_alignment) {
vector<ApplySplitResult> result;
Expr outer = Variable::make(Int(32), prefix + split.outer);
Expr outer_max = Variable::make(Int(32), prefix + split.outer + ".loop_max");
if (split.is_split()) {
Expr inner = Variable::make(Int(32), prefix + split.inner);
Expr old_max = Variable::make(Int(32), prefix + split.old_var + ".loop_max");
Expr old_min = Variable::make(Int(32), prefix + split.old_var + ".loop_min");
Expr old_extent = Variable::make(Int(32), prefix + split.old_var + ".loop_extent");
dim_extent_alignment[split.inner] = split.factor;
Expr base = outer * split.factor + old_min;
string base_name = prefix + split.inner + ".base";
Expr base_var = Variable::make(Int(32), base_name);
string old_var_name = prefix + split.old_var;
Expr old_var = Variable::make(Int(32), old_var_name);
map<string, Expr>::iterator iter = dim_extent_alignment.find(split.old_var);
TailStrategy tail = split.tail;
internal_assert(tail != TailStrategy::Auto)
<< "An explicit tail strategy should exist at this point\n";
if ((iter != dim_extent_alignment.end()) &&
is_const_zero(simplify(iter->second % split.factor))) {
// We have proved that the split factor divides the
// old extent. No need to adjust the base or add an if
// statement.
dim_extent_alignment[split.outer] = iter->second / split.factor;
} else if (is_negative_const(split.factor) || is_const_zero(split.factor)) {
user_error << "Can't split " << split.old_var << " by " << split.factor
<< ". Split factors must be strictly positive\n";
} else if (is_const_one(split.factor)) {
// The split factor trivially divides the old extent,
// but we know nothing new about the outer dimension.
} else if (tail == TailStrategy::GuardWithIf ||
tail == TailStrategy::Predicate ||
tail == TailStrategy::PredicateLoads ||
tail == TailStrategy::PredicateStores) {
// It's an exact split but we failed to prove that the
// extent divides the factor. Use predication to guard
// the calls and/or provides.
// Bounds inference has trouble exploiting an if
// condition. We'll directly tell it that the loop
// variable is bounded above by the original loop max by
// replacing the variable with a promise-clamped version
// of it. We don't also use the original loop min because
// it needlessly complicates the expressions and doesn't
// actually communicate anything new.
Expr guarded = promise_clamped(old_var, old_var, old_max);
string guarded_var_name = prefix + split.old_var + ".guarded";
Expr guarded_var = Variable::make(Int(32), guarded_var_name);
ApplySplitResult::Type predicate_type, substitution_type;
switch (tail) {
case TailStrategy::GuardWithIf:
substitution_type = ApplySplitResult::Substitution;
predicate_type = ApplySplitResult::Predicate;
break;
case TailStrategy::Predicate:
// This is identical to GuardWithIf, but maybe it makes
// sense to keep it anyways?
substitution_type = ApplySplitResult::Substitution;
predicate_type = ApplySplitResult::Predicate;
break;
case TailStrategy::PredicateLoads:
substitution_type = ApplySplitResult::SubstitutionInCalls;
predicate_type = ApplySplitResult::PredicateCalls;
break;
case TailStrategy::PredicateStores:
substitution_type = ApplySplitResult::SubstitutionInProvides;
predicate_type = ApplySplitResult::PredicateProvides;
break;
default:
break;
}
// Inject the if condition *after* doing the substitution
// for the guarded version.
result.emplace_back(prefix + split.old_var, guarded_var, substitution_type);
result.emplace_back(guarded_var_name, guarded, ApplySplitResult::LetStmt);
result.emplace_back(likely(old_var <= old_max), predicate_type);
} else if (tail == TailStrategy::ShiftInwards) {
// Adjust the base downwards to not compute off the
// end of the realization.
// We'll only mark the base as likely (triggering a loop
// partition) if we're at or inside the innermost
// non-trivial loop.
base = likely_if_innermost(base);
base = Min::make(base, old_max + (1 - split.factor));
} else {
internal_assert(tail == TailStrategy::RoundUp);
}
// Define the original variable as the base value computed above plus the inner loop variable.
result.emplace_back(old_var_name, base_var + inner, ApplySplitResult::LetStmt);
result.emplace_back(base_name, base, ApplySplitResult::LetStmt);
} else if (split.is_fuse()) {
// Define the inner and outer in terms of the fused var
Expr fused = Variable::make(Int(32), prefix + split.old_var);
Expr inner_min = Variable::make(Int(32), prefix + split.inner + ".loop_min");
Expr outer_min = Variable::make(Int(32), prefix + split.outer + ".loop_min");
Expr inner_extent = Variable::make(Int(32), prefix + split.inner + ".loop_extent");
const Expr &factor = inner_extent;
Expr inner = fused % factor + inner_min;
Expr outer = fused / factor + outer_min;
result.emplace_back(prefix + split.inner, inner, ApplySplitResult::Substitution);
result.emplace_back(prefix + split.outer, outer, ApplySplitResult::Substitution);
result.emplace_back(prefix + split.inner, inner, ApplySplitResult::LetStmt);
result.emplace_back(prefix + split.outer, outer, ApplySplitResult::LetStmt);
// Maintain the known size of the fused dim if
// possible. This is important for possible later splits.
map<string, Expr>::iterator inner_dim = dim_extent_alignment.find(split.inner);
map<string, Expr>::iterator outer_dim = dim_extent_alignment.find(split.outer);
if (inner_dim != dim_extent_alignment.end() &&
outer_dim != dim_extent_alignment.end()) {
dim_extent_alignment[split.old_var] = inner_dim->second * outer_dim->second;
}
} else {
// rename or purify
result.emplace_back(prefix + split.old_var, outer, ApplySplitResult::Substitution);
result.emplace_back(prefix + split.old_var, outer, ApplySplitResult::LetStmt);
}
return result;
}
vector<std::pair<string, Expr>> compute_loop_bounds_after_split(const Split &split, const string &prefix) {
// Define the bounds on the split dimensions using the bounds
// on the function args. If it is a purify, we should use the bounds
// from the dims instead.
vector<std::pair<string, Expr>> let_stmts;
Expr old_var_extent = Variable::make(Int(32), prefix + split.old_var + ".loop_extent");
Expr old_var_max = Variable::make(Int(32), prefix + split.old_var + ".loop_max");
Expr old_var_min = Variable::make(Int(32), prefix + split.old_var + ".loop_min");
if (split.is_split()) {
Expr inner_extent = split.factor;
Expr outer_extent = (old_var_max - old_var_min + split.factor) / split.factor;
let_stmts.emplace_back(prefix + split.inner + ".loop_min", 0);
let_stmts.emplace_back(prefix + split.inner + ".loop_max", inner_extent - 1);
let_stmts.emplace_back(prefix + split.inner + ".loop_extent", inner_extent);
let_stmts.emplace_back(prefix + split.outer + ".loop_min", 0);
let_stmts.emplace_back(prefix + split.outer + ".loop_max", outer_extent - 1);
let_stmts.emplace_back(prefix + split.outer + ".loop_extent", outer_extent);
} else if (split.is_fuse()) {
// Define bounds on the fused var using the bounds on the inner and outer
Expr inner_extent = Variable::make(Int(32), prefix + split.inner + ".loop_extent");
Expr outer_extent = Variable::make(Int(32), prefix + split.outer + ".loop_extent");
Expr fused_extent = inner_extent * outer_extent;
let_stmts.emplace_back(prefix + split.old_var + ".loop_min", 0);
let_stmts.emplace_back(prefix + split.old_var + ".loop_max", fused_extent - 1);
let_stmts.emplace_back(prefix + split.old_var + ".loop_extent", fused_extent);
} else if (split.is_rename()) {
let_stmts.emplace_back(prefix + split.outer + ".loop_min", old_var_min);
let_stmts.emplace_back(prefix + split.outer + ".loop_max", old_var_max);
let_stmts.emplace_back(prefix + split.outer + ".loop_extent", old_var_extent);
}
// Do nothing for purify
return let_stmts;
}
} // namespace Internal
} // namespace Halide
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