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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