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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
#include "jit/BranchHinting.h"
#include "jit/IonAnalysis.h"
#include "jit/JitSpewer.h"
#include "jit/MIRGenerator.h"
#include "jit/MIRGraph.h"
using namespace js;
using namespace js::jit;
// Implementation of the branch hinting proposal
// Some control instructions (if and br_if) can have a hint of the form
// Likely or unlikely. That means a specific branch will be likely/unlikely
// to be executed at runtime.
// In a first pass, we tag the basic blocks if we have a hint.
// In a Mir to Mir transformation, we read the hints and do something with it:
// - Unlikely blocks are pushed to the end of the function.
// Because of Ion's structure, we don't do that for blocks inside a loop.
// - TODO: do something for likely blocks.
// - TODO: register allocator can be tuned depending on the hints.
bool jit::BranchHinting(MIRGenerator* mir, MIRGraph& graph) {
JitSpew(JitSpew_BranchHint, "Beginning BranchHinting pass");
// Push to end all blocks marked as unlikely
mozilla::Vector<MBasicBlock*, 0> toBeMoved;
for (MBasicBlock* block : graph) {
// Collect the unlikely blocks
// Several possibilities:
// - The unlikely block is in the middle of a loop: keep it there to not
// break Ion assumptions.
// - The unlikely block is not in a loop: push it to the end of the
// function.
if (block->branchHintingUnlikely() && block->loopDepth() == 0) {
if (!toBeMoved.append(block)) {
return false;
}
}
}
for (MBasicBlock* block : toBeMoved) {
#ifdef JS_JITSPEW
JitSpew(JitSpew_BranchHint, "Pushing block%u to the end", block->id());
#endif
graph.moveBlockToEnd(block);
}
if (!toBeMoved.empty()) {
// Renumber blocks after moving them around.
RenumberBlocks(graph);
}
return true;
}