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//
// Copyright (c) 2016 The ANGLE Project Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// BreakVariableAliasingInInnerLoops.h: To optimize simple assignments, the HLSL compiler frontend
// may record a variable as aliasing another. Sometimes the alias information gets garbled
// so we work around this issue by breaking the aliasing chain in inner loops.
#include "BreakVariableAliasingInInnerLoops.h"
#include "compiler/translator/IntermNode.h"
// A HLSL compiler developer gave us more details on the root cause and the workaround needed:
// The root problem is that if the HLSL compiler is applying aliasing information even on
// incomplete simulations (in this case, a single pass). The bug is triggered by an assignment
// that comes from a series of assignments, possibly with swizzled or ternary operators with
// known conditionals, where the source is before the loop.
// So, a workaround is to add a +0 term to variables the first time they are assigned to in
// an inner loop (if they are declared in an outside scope, otherwise there is no need).
// This will break the aliasing chain.
// For simplicity here we add a +0 to any assignment that is in at least two nested loops. Because
// the bug only shows up with swizzles, and ternary assignment, whole array or whole structure
// assignment don't need a workaround.
namespace sh
{
namespace
{
class AliasingBreaker : public TIntermTraverser
{
public:
AliasingBreaker() : TIntermTraverser(true, false, true) {}
protected:
bool visitBinary(Visit visit, TIntermBinary *binary)
{
if (visit != PreVisit)
{
return false;
}
if (mLoopLevel < 2 || !binary->isAssignment())
{
return true;
}
TIntermTyped *B = binary->getRight();
TType type = B->getType();
if (!type.isScalar() && !type.isVector() && !type.isMatrix())
{
return true;
}
if (type.isArray() || IsSampler(type.getBasicType()))
{
return true;
}
// We have a scalar / vector / matrix assignment with loop depth 2.
// Transform it from
// A = B
// to
// A = (B + typeof<B>(0));
TIntermBinary *bPlusZero = new TIntermBinary(EOpAdd, B, TIntermTyped::CreateZero(type));
bPlusZero->setLine(B->getLine());
binary->replaceChildNode(B, bPlusZero);
return true;
}
bool visitLoop(Visit visit, TIntermLoop *loop)
{
if (visit == PreVisit)
{
mLoopLevel++;
}
else
{
ASSERT(mLoopLevel > 0);
mLoopLevel--;
}
return true;
}
private:
int mLoopLevel = 0;
};
} // anonymous namespace
void BreakVariableAliasingInInnerLoops(TIntermNode *root)
{
AliasingBreaker breaker;
root->traverse(&breaker);
}
} // namespace sh
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