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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
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "jit/LICM.h"
+
+#include "jit/IonAnalysis.h"
+#include "jit/JitSpewer.h"
+#include "jit/MIRGenerator.h"
+#include "jit/MIRGraph.h"
+
+using namespace js;
+using namespace js::jit;
+
+// There are two constants which control whether a loop is LICM'd or is left
+// unchanged. For rationale see comment in jit::LICM() below.
+//
+// A bit of quick profiling with the wasm Embenchen suite on x64 shows that
+// the threshold pair (100,25) has either no effect or gives a small net
+// reduction in memory traffic, compared to unconstrained LICMing. Halving
+// them to (50,12) gives a small overall increase in memory traffic,
+// suggesting it excludes too many loops from LICM. Doubling them to (200,50)
+// gives a win that is even smaller than (100,25), hence (100,25) seems the
+// best choice.
+//
+// If a loop has more than this number of basic blocks in its body, it won't
+// be LICM'd.
+static constexpr size_t LargestAllowedLoop = 100;
+
+// If a loop contains an MTableSwitch instruction that has more than this many
+// successors, it won't be LICM'd.
+static constexpr size_t LargestAllowedTableSwitch = 25;
+
+// Test whether any instruction in the loop possiblyCalls().
+static bool LoopContainsPossibleCall(MIRGraph& graph, MBasicBlock* header,
+ MBasicBlock* backedge) {
+ for (auto i(graph.rpoBegin(header));; ++i) {
+ MOZ_ASSERT(i != graph.rpoEnd(),
+ "Reached end of graph searching for blocks in loop");
+ MBasicBlock* block = *i;
+ if (!block->isMarked()) {
+ continue;
+ }
+
+ for (auto insIter(block->begin()), insEnd(block->end()); insIter != insEnd;
+ ++insIter) {
+ MInstruction* ins = *insIter;
+ if (ins->possiblyCalls()) {
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM, " Possible call found at %s%u", ins->opName(),
+ ins->id());
+#endif
+ return true;
+ }
+ }
+
+ if (block == backedge) {
+ break;
+ }
+ }
+ return false;
+}
+
+// Tests whether any instruction in the loop is a table-switch with more than
+// `LargestAllowedTableSwitch` successors. If it returns true, it also
+// returns the actual number of successors of the instruction in question,
+// although that is used only for statistics/debug printing.
+static bool LoopContainsBigTableSwitch(MIRGraph& graph, MBasicBlock* header,
+ /*OUT*/ size_t* numSuccessors) {
+ MBasicBlock* backedge = header->backedge();
+
+ for (auto i(graph.rpoBegin(header));; ++i) {
+ MOZ_ASSERT(i != graph.rpoEnd(),
+ "Reached end of graph searching for blocks in loop");
+ MBasicBlock* block = *i;
+ if (!block->isMarked()) {
+ continue;
+ }
+
+ for (auto insIter(block->begin()), insEnd(block->end()); insIter != insEnd;
+ ++insIter) {
+ MInstruction* ins = *insIter;
+ if (ins->isTableSwitch() &&
+ ins->toTableSwitch()->numSuccessors() > LargestAllowedTableSwitch) {
+ *numSuccessors = ins->toTableSwitch()->numSuccessors();
+ return true;
+ }
+ }
+
+ if (block == backedge) {
+ break;
+ }
+ }
+ return false;
+}
+
+// When a nested loop has no exits back into what would be its parent loop,
+// MarkLoopBlocks on the parent loop doesn't mark the blocks of the nested
+// loop, since they technically aren't part of the loop. However, AliasAnalysis
+// currently does consider such nested loops to be part of their parent
+// loops. Consequently, we can't use IsInLoop on dependency() values; we must
+// test whether a dependency() is *before* the loop, even if it is not
+// technically in the loop.
+static bool IsBeforeLoop(MDefinition* ins, MBasicBlock* header) {
+ return ins->block()->id() < header->id();
+}
+
+// Test whether the given instruction is inside the loop (and thus not
+// loop-invariant).
+static bool IsInLoop(MDefinition* ins) { return ins->block()->isMarked(); }
+
+// Test whether the given instruction is cheap and not worth hoisting unless
+// one of its users will be hoisted as well.
+static bool RequiresHoistedUse(const MDefinition* ins, bool hasCalls) {
+ if (ins->isBox()) {
+ MOZ_ASSERT(!ins->toBox()->input()->isBox(),
+ "Box of a box could lead to unbounded recursion");
+ return true;
+ }
+
+ // Integer constants are usually cheap and aren't worth hoisting on their
+ // own, in general. Floating-point constants typically are worth hoisting,
+ // unless they'll end up being spilled (eg. due to a call).
+ if (ins->isConstant() && (!IsFloatingPointType(ins->type()) || hasCalls)) {
+ return true;
+ }
+
+ return false;
+}
+
+// Test whether the given instruction has any operands defined within the loop.
+static bool HasOperandInLoop(MInstruction* ins, bool hasCalls) {
+ // An instruction is only loop invariant if it and all of its operands can
+ // be safely hoisted into the loop preheader.
+ for (size_t i = 0, e = ins->numOperands(); i != e; ++i) {
+ MDefinition* op = ins->getOperand(i);
+
+ if (!IsInLoop(op)) {
+ continue;
+ }
+
+ if (RequiresHoistedUse(op, hasCalls)) {
+ // Recursively test for loop invariance. Note that the recursion is
+ // bounded because we require RequiresHoistedUse to be set at each
+ // level.
+ if (!HasOperandInLoop(op->toInstruction(), hasCalls)) {
+ continue;
+ }
+ }
+
+ return true;
+ }
+ return false;
+}
+
+// Test whether the given instruction is hoistable, ignoring memory
+// dependencies.
+static bool IsHoistableIgnoringDependency(MInstruction* ins, bool hasCalls) {
+ return ins->isMovable() && !ins->isEffectful() &&
+ !HasOperandInLoop(ins, hasCalls);
+}
+
+// Test whether the given instruction has a memory dependency inside the loop.
+static bool HasDependencyInLoop(MInstruction* ins, MBasicBlock* header) {
+ // Don't hoist if this instruction depends on a store inside the loop.
+ if (MDefinition* dep = ins->dependency()) {
+ return !IsBeforeLoop(dep, header);
+ }
+ return false;
+}
+
+// Test whether the given instruction is hoistable.
+static bool IsHoistable(MInstruction* ins, MBasicBlock* header, bool hasCalls) {
+ return IsHoistableIgnoringDependency(ins, hasCalls) &&
+ !HasDependencyInLoop(ins, header);
+}
+
+// In preparation for hoisting an instruction, hoist any of its operands which
+// were too cheap to hoist on their own.
+static void MoveDeferredOperands(MInstruction* ins, MInstruction* hoistPoint,
+ bool hasCalls) {
+ // If any of our operands were waiting for a user to be hoisted, make a note
+ // to hoist them.
+ for (size_t i = 0, e = ins->numOperands(); i != e; ++i) {
+ MDefinition* op = ins->getOperand(i);
+ if (!IsInLoop(op)) {
+ continue;
+ }
+ MOZ_ASSERT(RequiresHoistedUse(op, hasCalls),
+ "Deferred loop-invariant operand is not cheap");
+ MInstruction* opIns = op->toInstruction();
+
+ // Recursively move the operands. Note that the recursion is bounded
+ // because we require RequiresHoistedUse to be set at each level.
+ MoveDeferredOperands(opIns, hoistPoint, hasCalls);
+
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM,
+ " Hoisting %s%u (now that a user will be hoisted)",
+ opIns->opName(), opIns->id());
+#endif
+
+ opIns->block()->moveBefore(hoistPoint, opIns);
+ opIns->setBailoutKind(BailoutKind::LICM);
+ }
+}
+
+static void VisitLoopBlock(MBasicBlock* block, MBasicBlock* header,
+ MInstruction* hoistPoint, bool hasCalls) {
+ for (auto insIter(block->begin()), insEnd(block->end()); insIter != insEnd;) {
+ MInstruction* ins = *insIter++;
+
+ if (!IsHoistable(ins, header, hasCalls)) {
+#ifdef JS_JITSPEW
+ if (IsHoistableIgnoringDependency(ins, hasCalls)) {
+ JitSpew(JitSpew_LICM,
+ " %s%u isn't hoistable due to dependency on %s%u",
+ ins->opName(), ins->id(), ins->dependency()->opName(),
+ ins->dependency()->id());
+ }
+#endif
+ continue;
+ }
+
+ // Don't hoist a cheap constant if it doesn't enable us to hoist one of
+ // its uses. We want those instructions as close as possible to their
+ // use, to minimize register pressure.
+ if (RequiresHoistedUse(ins, hasCalls)) {
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM, " %s%u will be hoisted only if its users are",
+ ins->opName(), ins->id());
+#endif
+ continue;
+ }
+
+ // Hoist operands which were too cheap to hoist on their own.
+ MoveDeferredOperands(ins, hoistPoint, hasCalls);
+
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM, " Hoisting %s%u", ins->opName(), ins->id());
+#endif
+
+ // Move the instruction to the hoistPoint.
+ block->moveBefore(hoistPoint, ins);
+ ins->setBailoutKind(BailoutKind::LICM);
+ }
+}
+
+static void VisitLoop(MIRGraph& graph, MBasicBlock* header) {
+ MInstruction* hoistPoint = header->loopPredecessor()->lastIns();
+
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM, " Visiting loop with header block%u, hoisting to %s%u",
+ header->id(), hoistPoint->opName(), hoistPoint->id());
+#endif
+
+ MBasicBlock* backedge = header->backedge();
+
+ // This indicates whether the loop contains calls or other things which
+ // clobber most or all floating-point registers. In such loops,
+ // floating-point constants should not be hoisted unless it enables further
+ // hoisting.
+ bool hasCalls = LoopContainsPossibleCall(graph, header, backedge);
+
+ for (auto i(graph.rpoBegin(header));; ++i) {
+ MOZ_ASSERT(i != graph.rpoEnd(),
+ "Reached end of graph searching for blocks in loop");
+ MBasicBlock* block = *i;
+ if (!block->isMarked()) {
+ continue;
+ }
+
+#ifdef JS_JITSPEW
+ JitSpew(JitSpew_LICM, " Visiting block%u", block->id());
+#endif
+
+ VisitLoopBlock(block, header, hoistPoint, hasCalls);
+
+ if (block == backedge) {
+ break;
+ }
+ }
+}
+
+bool jit::LICM(MIRGenerator* mir, MIRGraph& graph) {
+ JitSpew(JitSpew_LICM, "Beginning LICM pass");
+
+ // Iterate in RPO to visit outer loops before inner loops. We'd hoist the
+ // same things either way, but outer first means we do a little less work.
+ for (auto i(graph.rpoBegin()), e(graph.rpoEnd()); i != e; ++i) {
+ MBasicBlock* header = *i;
+ if (!header->isLoopHeader()) {
+ continue;
+ }
+
+ bool canOsr;
+ size_t numBlocks = MarkLoopBlocks(graph, header, &canOsr);
+
+ if (numBlocks == 0) {
+ JitSpew(JitSpew_LICM,
+ " Skipping loop with header block%u -- contains zero blocks",
+ header->id());
+ continue;
+ }
+
+ // There are various reasons why we might choose not to LICM a given loop:
+ //
+ // (a) Hoisting out of a loop that has an entry from the OSR block in
+ // addition to its normal entry is tricky. In theory we could clone
+ // the instruction and insert phis. In practice we don't bother.
+ //
+ // (b) If the loop contains a large number of blocks, we play safe and
+ // punt, in order to reduce the risk of creating excessive register
+ // pressure by hoisting lots of values out of the loop. In a larger
+ // loop there's more likely to be duplication of invariant expressions
+ // within the loop body, and that duplication will be GVN'd but only
+ // within the scope of the loop body, so there's less loss from not
+ // lifting them out of the loop entirely.
+ //
+ // (c) If the loop contains a multiway switch with many successors, there
+ // could be paths with low probabilities, from which LICMing will be a
+ // net loss, especially if a large number of values are hoisted out.
+ // See bug 1708381 for a spectacular example and bug 1712078 for
+ // further discussion.
+ //
+ // It's preferable to perform test (c) only if (a) and (b) pass since (c)
+ // is more expensive to determine -- requiring a visit to all the MIR
+ // nodes -- than (a) or (b), which only involve visiting all blocks.
+
+ bool doVisit = true;
+ if (canOsr) {
+ JitSpew(JitSpew_LICM, " Skipping loop with header block%u due to OSR",
+ header->id());
+ doVisit = false;
+ } else if (numBlocks > LargestAllowedLoop) {
+ JitSpew(JitSpew_LICM,
+ " Skipping loop with header block%u "
+ "due to too many blocks (%u > thresh %u)",
+ header->id(), (uint32_t)numBlocks, (uint32_t)LargestAllowedLoop);
+ doVisit = false;
+ } else {
+ size_t switchSize = 0;
+ if (LoopContainsBigTableSwitch(graph, header, &switchSize)) {
+ JitSpew(JitSpew_LICM,
+ " Skipping loop with header block%u "
+ "due to oversize tableswitch (%u > thresh %u)",
+ header->id(), (uint32_t)switchSize,
+ (uint32_t)LargestAllowedTableSwitch);
+ doVisit = false;
+ }
+ }
+
+ if (doVisit) {
+ VisitLoop(graph, header);
+ }
+
+ UnmarkLoopBlocks(graph, header);
+
+ if (mir->shouldCancel("LICM (main loop)")) {
+ return false;
+ }
+ }
+
+ return true;
+}