PruneNoOps.cpp

Branch :

Show log
Commit
Author : Shahbaz Youssefi
Date : 2021-08-05 23:49:23
Hash : 63248e7f
Message : Translator: Fix dead-code-elimination corner case The DCE code had a corner case bug where a switch case containing multiple DCE'ed `break`s followed by another case would cause some of the DCE'ed statements in the first case to be doubly pruned, failing on an assertion. This was due to the fact that visitBlock() was asking traversal to continue if a new case was visited while pruning nodes, but the traversal does not take into account that some statements need to be skipped and so would also visit the pruned statements. If the pruned statements contain a branch instruction, they get re-pruned. The visitBlock() function that does the pruning is reworked so that it more clearly traverses the statements. Bug: chromium:1237200 Change-Id: Ib078c2ea73ade756c7d7ef5a5c489fa53c39f352 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3077659 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/compiler/translator/tree_ops/PruneNoOps.cpp

//
// Copyright 2002 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.
//
// PruneNoOps.cpp: The PruneNoOps function prunes:
//   1. Empty declarations "int;". Empty declarators will be pruned as well, so for example:
//        int , a;
//      is turned into
//        int a;
//   2. Literal statements: "1.0;". The ESSL output doesn't define a default precision for float,
//      so float literal statements would end up with no precision which is invalid ESSL.
//   3. Statements after discard, return, break and continue.

#include "compiler/translator/tree_ops/PruneNoOps.h"

#include "compiler/translator/Symbol.h"
#include "compiler/translator/tree_util/IntermTraverse.h"

namespace sh
{

namespace
{

bool IsNoOp(TIntermNode *node)
{
    if (node->getAsConstantUnion() != nullptr)
    {
        return true;
    }
    bool isEmptyDeclaration = node->getAsDeclarationNode() != nullptr &&
                              node->getAsDeclarationNode()->getSequence()->empty();
    if (isEmptyDeclaration)
    {
        return true;
    }
    return false;
}

class PruneNoOpsTraverser : private TIntermTraverser
{
  public:
    ANGLE_NO_DISCARD static bool apply(TCompiler *compiler,
                                       TIntermBlock *root,
                                       TSymbolTable *symbolTable);

  private:
    PruneNoOpsTraverser(TSymbolTable *symbolTable);
    bool visitDeclaration(Visit, TIntermDeclaration *node) override;
    bool visitBlock(Visit visit, TIntermBlock *node) override;
    bool visitLoop(Visit visit, TIntermLoop *loop) override;
    bool visitBranch(Visit visit, TIntermBranch *node) override;

    bool mIsBranchVisited = false;
};

bool PruneNoOpsTraverser::apply(TCompiler *compiler, TIntermBlock *root, TSymbolTable *symbolTable)
{
    PruneNoOpsTraverser prune(symbolTable);
    root->traverse(&prune);
    return prune.updateTree(compiler, root);
}

PruneNoOpsTraverser::PruneNoOpsTraverser(TSymbolTable *symbolTable)
    : TIntermTraverser(true, true, true, symbolTable)
{}

bool PruneNoOpsTraverser::visitDeclaration(Visit visit, TIntermDeclaration *node)
{
    if (visit != PreVisit)
    {
        return true;
    }

    TIntermSequence *sequence = node->getSequence();
    if (sequence->size() >= 1)
    {
        TIntermSymbol *declaratorSymbol = sequence->front()->getAsSymbolNode();
        // Prune declarations without a variable name, unless it's an interface block declaration.
        if (declaratorSymbol != nullptr &&
            declaratorSymbol->variable().symbolType() == SymbolType::Empty &&
            !declaratorSymbol->isInterfaceBlock())
        {
            if (sequence->size() > 1)
            {
                // Generate a replacement that will remove the empty declarator in the beginning of
                // a declarator list. Example of a declaration that will be changed:
                // float, a;
                // will be changed to
                // float a;
                // This applies also to struct declarations.
                TIntermSequence emptyReplacement;
                mMultiReplacements.emplace_back(node, declaratorSymbol,
                                                std::move(emptyReplacement));
            }
            else if (declaratorSymbol->getBasicType() != EbtStruct)
            {
                // If there are entirely empty non-struct declarations, they result in
                // TIntermDeclaration nodes without any children in the parsing stage. These are
                // handled in visitBlock and visitLoop.
                UNREACHABLE();
            }
            else if (declaratorSymbol->getQualifier() != EvqGlobal &&
                     declaratorSymbol->getQualifier() != EvqTemporary)
            {
                // Single struct declarations may just declare the struct type and no variables, so
                // they should not be pruned. Here we handle an empty struct declaration with a
                // qualifier, for example like this:
                //   const struct a { int i; };
                // NVIDIA GL driver version 367.27 doesn't accept this kind of declarations, so we
                // convert the declaration to a regular struct declaration. This is okay, since ESSL
                // 1.00 spec section 4.1.8 says about structs that "The optional qualifiers only
                // apply to any declarators, and are not part of the type being defined for name."

                // Create a new variable to use in the declarator so that the variable and node
                // types are kept consistent.
                TType *type = new TType(declaratorSymbol->getType());
                if (mInGlobalScope)
                {
                    type->setQualifier(EvqGlobal);
                }
                else
                {
                    type->setQualifier(EvqTemporary);
                }
                TVariable *variable =
                    new TVariable(mSymbolTable, kEmptyImmutableString, type, SymbolType::Empty);
                queueReplacementWithParent(node, declaratorSymbol, new TIntermSymbol(variable),
                                           OriginalNode::IS_DROPPED);
            }
        }
    }
    return false;
}

bool PruneNoOpsTraverser::visitBlock(Visit visit, TIntermBlock *node)
{
    ASSERT(visit == PreVisit);

    TIntermSequence &statements = *node->getSequence();
    TIntermSequence emptyReplacement;

    // Visit each statement in the block one by one.  Once a branch is visited (break, continue,
    // return or discard), drop the rest of the statements.
    for (size_t statementIndex = 0; statementIndex < statements.size(); ++statementIndex)
    {
        TIntermNode *statement = statements[statementIndex];

        // If the statement is a switch case label, stop pruning and continue visiting the children.
        if (statement->getAsCaseNode() != nullptr)
        {
            mIsBranchVisited = false;
        }

        // If a branch is visited, prune the statement.  If the statement is a no-op, also prune it.
        if (mIsBranchVisited || IsNoOp(statement))
        {
            mMultiReplacements.emplace_back(node, statement, std::move(emptyReplacement));
            continue;
        }

        // Visit the statement if not pruned.
        statement->traverse(this);
    }

    // If the parent is a block and mIsBranchVisited is set, this is a nested block without any
    // condition (like if, loop or switch), so the rest of the parent block should also be pruned.
    // Otherwise the parent block should be unaffected.
    if (mIsBranchVisited && getParentNode()->getAsBlock() == nullptr)
    {
        mIsBranchVisited = false;
    }

    return false;
}

bool PruneNoOpsTraverser::visitLoop(Visit visit, TIntermLoop *loop)
{
    if (visit != PreVisit)
    {
        return true;
    }

    TIntermTyped *expr = loop->getExpression();
    if (expr != nullptr && IsNoOp(expr))
    {
        loop->setExpression(nullptr);
    }
    TIntermNode *init = loop->getInit();
    if (init != nullptr && IsNoOp(init))
    {
        loop->setInit(nullptr);
    }

    return true;
}

bool PruneNoOpsTraverser::visitBranch(Visit visit, TIntermBranch *node)
{
    ASSERT(visit == PreVisit);

    mIsBranchVisited = true;

    // Only possible child is the value of a return statement, which has nothing to prune.
    return false;
}
}  // namespace

bool PruneNoOps(TCompiler *compiler, TIntermBlock *root, TSymbolTable *symbolTable)
{
    return PruneNoOpsTraverser::apply(compiler, root, symbolTable);
}

}  // namespace sh

kc3-lang/angle/src/compiler/translator/tree_ops/PruneNoOps.cpp

Commit

kc3-lang/angle /src/compiler/translator/tree_ops/PruneNoOps.cpp