| /* |
| * Copyright (C) 2013, 2015 Apple Inc. All rights reserved. |
| * |
| * Redistribution and use in source and binary forms, with or without |
| * modification, are permitted provided that the following conditions |
| * are met: |
| * 1. Redistributions of source code must retain the above copyright |
| * notice, this list of conditions and the following disclaimer. |
| * 2. Redistributions in binary form must reproduce the above copyright |
| * notice, this list of conditions and the following disclaimer in the |
| * documentation and/or other materials provided with the distribution. |
| * |
| * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY |
| * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR |
| * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR |
| * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, |
| * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, |
| * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR |
| * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY |
| * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE |
| * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| */ |
| |
| #include "config.h" |
| #include "BinarySwitch.h" |
| |
| #if ENABLE(JIT) |
| |
| #include "JSCInlines.h" |
| #include <wtf/ListDump.h> |
| |
| namespace JSC { |
| |
| namespace BinarySwitchInternal { |
| static const bool verbose = false; |
| } |
| |
| static unsigned globalCounter; // We use a different seed every time we are invoked. |
| |
| BinarySwitch::BinarySwitch(GPRReg value, const Vector<int64_t>& cases, Type type) |
| : m_value(value) |
| , m_weakRandom(globalCounter++) |
| , m_index(0) |
| , m_caseIndex(UINT_MAX) |
| , m_type(type) |
| { |
| if (cases.isEmpty()) |
| return; |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("Original cases: ", listDump(cases), "\n"); |
| |
| for (unsigned i = 0; i < cases.size(); ++i) |
| m_cases.append(Case(cases[i], i)); |
| |
| std::sort(m_cases.begin(), m_cases.end()); |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("Sorted cases: ", listDump(m_cases), "\n"); |
| |
| for (unsigned i = 1; i < m_cases.size(); ++i) |
| RELEASE_ASSERT(m_cases[i - 1] < m_cases[i]); |
| |
| build(0, false, m_cases.size()); |
| } |
| |
| BinarySwitch::~BinarySwitch() |
| { |
| } |
| |
| bool BinarySwitch::advance(MacroAssembler& jit) |
| { |
| if (m_cases.isEmpty()) { |
| m_fallThrough.append(jit.jump()); |
| return false; |
| } |
| |
| if (m_index == m_branches.size()) { |
| RELEASE_ASSERT(m_jumpStack.isEmpty()); |
| return false; |
| } |
| |
| for (;;) { |
| const BranchCode& code = m_branches[m_index++]; |
| switch (code.kind) { |
| case NotEqualToFallThrough: |
| switch (m_type) { |
| case Int32: |
| m_fallThrough.append(jit.branch32( |
| MacroAssembler::NotEqual, m_value, |
| MacroAssembler::Imm32(static_cast<int32_t>(m_cases[code.index].value)))); |
| break; |
| case IntPtr: |
| m_fallThrough.append(jit.branchPtr( |
| MacroAssembler::NotEqual, m_value, |
| MacroAssembler::ImmPtr(bitwise_cast<const void*>(static_cast<intptr_t>(m_cases[code.index].value))))); |
| break; |
| } |
| break; |
| case NotEqualToPush: |
| switch (m_type) { |
| case Int32: |
| m_jumpStack.append(jit.branch32( |
| MacroAssembler::NotEqual, m_value, |
| MacroAssembler::Imm32(static_cast<int32_t>(m_cases[code.index].value)))); |
| break; |
| case IntPtr: |
| m_jumpStack.append(jit.branchPtr( |
| MacroAssembler::NotEqual, m_value, |
| MacroAssembler::ImmPtr(bitwise_cast<const void*>(static_cast<intptr_t>(m_cases[code.index].value))))); |
| break; |
| } |
| break; |
| case LessThanToPush: |
| switch (m_type) { |
| case Int32: |
| m_jumpStack.append(jit.branch32( |
| MacroAssembler::LessThan, m_value, |
| MacroAssembler::Imm32(static_cast<int32_t>(m_cases[code.index].value)))); |
| break; |
| case IntPtr: |
| m_jumpStack.append(jit.branchPtr( |
| MacroAssembler::LessThan, m_value, |
| MacroAssembler::ImmPtr(bitwise_cast<const void*>(static_cast<intptr_t>(m_cases[code.index].value))))); |
| break; |
| } |
| break; |
| case Pop: |
| m_jumpStack.takeLast().link(&jit); |
| break; |
| case ExecuteCase: |
| m_caseIndex = code.index; |
| return true; |
| } |
| } |
| } |
| |
| void BinarySwitch::build(unsigned start, bool hardStart, unsigned end) |
| { |
| if (BinarySwitchInternal::verbose) |
| dataLog("Building with start = ", start, ", hardStart = ", hardStart, ", end = ", end, "\n"); |
| |
| auto append = [&] (const BranchCode& code) { |
| if (BinarySwitchInternal::verbose) |
| dataLog("==> ", code, "\n"); |
| m_branches.append(code); |
| }; |
| |
| unsigned size = end - start; |
| |
| RELEASE_ASSERT(size); |
| |
| // This code uses some random numbers to keep things balanced. It's important to keep in mind |
| // that this does not improve average-case throughput under the assumption that all cases fire |
| // with equal probability. It just ensures that there will not be some switch structure that |
| // when combined with some input will always produce pathologically good or pathologically bad |
| // performance. |
| |
| const unsigned leafThreshold = 3; |
| |
| if (size <= leafThreshold) { |
| if (BinarySwitchInternal::verbose) |
| dataLog("It's a leaf.\n"); |
| |
| // It turns out that for exactly three cases or less, it's better to just compare each |
| // case individually. This saves 1/6 of a branch on average, and up to 1/3 of a branch in |
| // extreme cases where the divide-and-conquer bottoms out in a lot of 3-case subswitches. |
| // |
| // This assumes that we care about the cost of hitting some case more than we care about |
| // bottoming out in a default case. I believe that in most places where we use switch |
| // statements, we are more likely to hit one of the cases than we are to fall through to |
| // default. Intuitively, if we wanted to improve the performance of default, we would |
| // reduce the value of leafThreshold to 2 or even to 1. See below for a deeper discussion. |
| |
| bool allConsecutive = false; |
| |
| if ((hardStart || (start && m_cases[start - 1].value == m_cases[start].value - 1)) |
| && start + size < m_cases.size() |
| && m_cases[start + size - 1].value == m_cases[start + size].value - 1) { |
| allConsecutive = true; |
| for (unsigned i = 0; i < size - 1; ++i) { |
| if (m_cases[start + i].value + 1 != m_cases[start + i + 1].value) { |
| allConsecutive = false; |
| break; |
| } |
| } |
| } |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("allConsecutive = ", allConsecutive, "\n"); |
| |
| Vector<unsigned, 3> localCaseIndices; |
| for (unsigned i = 0; i < size; ++i) |
| localCaseIndices.append(start + i); |
| |
| std::random_shuffle( |
| localCaseIndices.begin(), localCaseIndices.end(), |
| [this] (unsigned n) { |
| // We use modulo to get a random number in the range we want fully knowing that |
| // this introduces a tiny amount of bias, but we're fine with such tiny bias. |
| return m_weakRandom.getUint32() % n; |
| }); |
| |
| for (unsigned i = 0; i < size - 1; ++i) { |
| append(BranchCode(NotEqualToPush, localCaseIndices[i])); |
| append(BranchCode(ExecuteCase, localCaseIndices[i])); |
| append(BranchCode(Pop)); |
| } |
| |
| if (!allConsecutive) |
| append(BranchCode(NotEqualToFallThrough, localCaseIndices.last())); |
| |
| append(BranchCode(ExecuteCase, localCaseIndices.last())); |
| return; |
| } |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("It's not a leaf.\n"); |
| |
| // There are two different strategies we could consider here: |
| // |
| // Isolate median and split: pick a median and check if the comparison value is equal to it; |
| // if so, execute the median case. Otherwise check if the value is less than the median, and |
| // recurse left or right based on this. This has two subvariants: we could either first test |
| // equality for the median and then do the less-than, or we could first do the less-than and |
| // then check equality on the not-less-than path. |
| // |
| // Ignore median and split: do a less-than comparison on a value that splits the cases in two |
| // equal-sized halves. Recurse left or right based on the comparison. Do not test for equality |
| // against the median (or anything else); let the recursion handle those equality comparisons |
| // once we bottom out in a list that case 3 cases or less (see above). |
| // |
| // I'll refer to these strategies as Isolate and Ignore. I initially believed that Isolate |
| // would be faster since it leads to less branching for some lucky cases. It turns out that |
| // Isolate is almost a total fail in the average, assuming all cases are equally likely. How |
| // bad Isolate is depends on whether you believe that doing two consecutive branches based on |
| // the same comparison is cheaper than doing the compare/branches separately. This is |
| // difficult to evaluate. For small immediates that aren't blinded, we just care about |
| // avoiding a second compare instruction. For large immediates or when blinding is in play, we |
| // also care about the instructions used to materialize the immediate a second time. Isolate |
| // can help with both costs since it involves first doing a < compare+branch on some value, |
| // followed by a == compare+branch on the same exact value (or vice-versa). Ignore will do a < |
| // compare+branch on some value, and then the == compare+branch on that same value will happen |
| // much later. |
| // |
| // To evaluate these costs, I wrote the recurrence relation for Isolate and Ignore, assuming |
| // that ComparisonCost is the cost of a compare+branch and ChainedComparisonCost is the cost |
| // of a compare+branch on some value that you've just done another compare+branch for. These |
| // recurrence relations compute the total cost incurred if you executed the switch statement |
| // on each matching value. So the average cost of hitting some case can be computed as |
| // Isolate[n]/n or Ignore[n]/n, respectively for the two relations. |
| // |
| // Isolate[1] = ComparisonCost |
| // Isolate[2] = (2 + 1) * ComparisonCost |
| // Isolate[3] = (3 + 2 + 1) * ComparisonCost |
| // Isolate[n_] := With[ |
| // {medianIndex = Floor[n/2] + If[EvenQ[n], RandomInteger[], 1]}, |
| // ComparisonCost + ChainedComparisonCost + |
| // (ComparisonCost * (medianIndex - 1) + Isolate[medianIndex - 1]) + |
| // (2 * ComparisonCost * (n - medianIndex) + Isolate[n - medianIndex])] |
| // |
| // Ignore[1] = ComparisonCost |
| // Ignore[2] = (2 + 1) * ComparisonCost |
| // Ignore[3] = (3 + 2 + 1) * ComparisonCost |
| // Ignore[n_] := With[ |
| // {medianIndex = If[EvenQ[n], n/2, Floor[n/2] + RandomInteger[]]}, |
| // (medianIndex * ComparisonCost + Ignore[medianIndex]) + |
| // ((n - medianIndex) * ComparisonCost + Ignore[n - medianIndex])] |
| // |
| // This does not account for the average cost of hitting the default case. See further below |
| // for a discussion of that. |
| // |
| // It turns out that for ComparisonCost = 1 and ChainedComparisonCost = 1, Ignore is always |
| // better than Isolate. If we assume that ChainedComparisonCost = 0, then Isolate wins for |
| // switch statements that have 20 cases or fewer, though the margin of victory is never large |
| // - it might sometimes save an average of 0.3 ComparisonCost. For larger switch statements, |
| // we see divergence between the two with Ignore winning. This is of course rather |
| // unrealistic since the chained comparison is never free. For ChainedComparisonCost = 0.5, we |
| // see Isolate winning for 10 cases or fewer, by maybe 0.2 ComparisonCost. Again we see |
| // divergence for large switches with Ignore winning, for example if a switch statement has |
| // 100 cases then Ignore saves one branch on average. |
| // |
| // Our current JIT backends don't provide for optimization for chained comparisons, except for |
| // reducing the code for materializing the immediate if the immediates are large or blinding |
| // comes into play. Probably our JIT backends live somewhere north of |
| // ChainedComparisonCost = 0.5. |
| // |
| // This implies that using the Ignore strategy is likely better. If we wanted to incorporate |
| // the Isolate strategy, we'd want to determine the switch size threshold at which the two |
| // cross over and then use Isolate for switches that are smaller than that size. |
| // |
| // The average cost of hitting the default case is similar, but involves a different cost for |
| // the base cases: you have to assume that you will always fail each branch. For the Ignore |
| // strategy we would get this recurrence relation; the same kind of thing happens to the |
| // Isolate strategy: |
| // |
| // Ignore[1] = ComparisonCost |
| // Ignore[2] = (2 + 2) * ComparisonCost |
| // Ignore[3] = (3 + 3 + 3) * ComparisonCost |
| // Ignore[n_] := With[ |
| // {medianIndex = If[EvenQ[n], n/2, Floor[n/2] + RandomInteger[]]}, |
| // (medianIndex * ComparisonCost + Ignore[medianIndex]) + |
| // ((n - medianIndex) * ComparisonCost + Ignore[n - medianIndex])] |
| // |
| // This means that if we cared about the default case more, we would likely reduce |
| // leafThreshold. Reducing it to 2 would reduce the average cost of the default case by 1/3 |
| // in the most extreme cases (num switch cases = 3, 6, 12, 24, ...). But it would also |
| // increase the average cost of taking one of the non-default cases by 1/3. Typically the |
| // difference is 1/6 in either direction. This makes it a very simple trade-off: if we believe |
| // that the default case is more important then we would want leafThreshold to be 2, and the |
| // default case would become 1/6 faster on average. But we believe that most switch statements |
| // are more likely to take one of the cases than the default, so we use leafThreshold = 3 |
| // and get a 1/6 speed-up on average for taking an explicit case. |
| |
| unsigned medianIndex = (start + end) / 2; |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("medianIndex = ", medianIndex, "\n"); |
| |
| // We want medianIndex to point to the thing we will do a less-than compare against. We want |
| // this less-than compare to split the current sublist into equal-sized sublists, or |
| // nearly-equal-sized with some randomness if we're in the odd case. With the above |
| // calculation, in the odd case we will have medianIndex pointing at either the element we |
| // want or the element to the left of the one we want. Consider the case of five elements: |
| // |
| // 0 1 2 3 4 |
| // |
| // start will be 0, end will be 5. The average is 2.5, which rounds down to 2. If we do |
| // value < 2, then we will split the list into 2 elements on the left and three on the right. |
| // That's pretty good, but in this odd case we'd like to at random choose 3 instead to ensure |
| // that we don't become unbalanced on the right. This does not improve throughput since one |
| // side will always get shafted, and that side might still be odd, in which case it will also |
| // have two sides and one of them will get shafted - and so on. We just want to avoid |
| // deterministic pathologies. |
| // |
| // In the even case, we will always end up pointing at the element we want: |
| // |
| // 0 1 2 3 |
| // |
| // start will be 0, end will be 4. So, the average is 2, which is what we'd like. |
| if (size & 1) { |
| RELEASE_ASSERT(medianIndex - start + 1 == end - medianIndex); |
| medianIndex += m_weakRandom.getUint32() & 1; |
| } else |
| RELEASE_ASSERT(medianIndex - start == end - medianIndex); |
| |
| RELEASE_ASSERT(medianIndex > start); |
| RELEASE_ASSERT(medianIndex + 1 < end); |
| |
| if (BinarySwitchInternal::verbose) |
| dataLog("fixed medianIndex = ", medianIndex, "\n"); |
| |
| append(BranchCode(LessThanToPush, medianIndex)); |
| build(medianIndex, true, end); |
| append(BranchCode(Pop)); |
| build(start, hardStart, medianIndex); |
| } |
| |
| void BinarySwitch::Case::dump(PrintStream& out) const |
| { |
| out.print("<value: " , value, ", index: ", index, ">"); |
| } |
| |
| void BinarySwitch::BranchCode::dump(PrintStream& out) const |
| { |
| switch (kind) { |
| case NotEqualToFallThrough: |
| out.print("NotEqualToFallThrough"); |
| break; |
| case NotEqualToPush: |
| out.print("NotEqualToPush"); |
| break; |
| case LessThanToPush: |
| out.print("LessThanToPush"); |
| break; |
| case Pop: |
| out.print("Pop"); |
| break; |
| case ExecuteCase: |
| out.print("ExecuteCase"); |
| break; |
| } |
| |
| if (index != UINT_MAX) |
| out.print("(", index, ")"); |
| } |
| |
| } // namespace JSC |
| |
| #endif // ENABLE(JIT) |
| |
