[llvm] r185184 - Add a division operator to BlockFrequency.
Benjamin Kramer
benny.kra at gmail.com
Fri Jun 28 12:09:21 PDT 2013
On 28.06.2013, at 20:23, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
> Author: stoklund
> Date: Fri Jun 28 13:23:42 2013
> New Revision: 185184
>
> URL: http://llvm.org/viewvc/llvm-project?rev=185184&view=rev
> Log:
> Add a division operator to BlockFrequency.
>
> Allow a BlockFrequency to be divided by a non-zero BranchProbability
> with saturating arithmetic. This will be used to compute the frequency
> of a loop header given the probability of leaving the loop.
>
> Our long division algorithm already saturates on overflow, so that was a
> freebie.
>
> Modified:
> llvm/trunk/include/llvm/Support/BlockFrequency.h
> llvm/trunk/lib/Support/BlockFrequency.cpp
> llvm/trunk/unittests/Support/BlockFrequencyTest.cpp
>
> Modified: llvm/trunk/include/llvm/Support/BlockFrequency.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/BlockFrequency.h?rev=185184&r1=185183&r2=185184&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Support/BlockFrequency.h (original)
> +++ llvm/trunk/include/llvm/Support/BlockFrequency.h Fri Jun 28 13:23:42 2013
> @@ -27,6 +27,9 @@ class BlockFrequency {
> uint64_t Frequency;
> static const int64_t ENTRY_FREQ = 1 << 14;
>
> + // Scale frequency by N/D, saturating on overflow.
> + void scale(uint32_t N, uint32_t D);
> +
> public:
> BlockFrequency(uint64_t Freq = 0) : Frequency(Freq) { }
>
> @@ -42,6 +45,11 @@ public:
> BlockFrequency &operator*=(const BranchProbability &Prob);
> const BlockFrequency operator*(const BranchProbability &Prob) const;
>
> + /// \brief Divide by a non-zero branch probability using saturating
> + /// arithmetic.
> + BlockFrequency &operator/=(const BranchProbability &Prob);
> + BlockFrequency operator/(const BranchProbability &Prob) const;
> +
> /// \brief Adds another block frequency using saturating arithmetic.
> BlockFrequency &operator+=(const BlockFrequency &Freq);
> const BlockFrequency operator+(const BlockFrequency &Freq) const;
>
> Modified: llvm/trunk/lib/Support/BlockFrequency.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/BlockFrequency.cpp?rev=185184&r1=185183&r2=185184&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Support/BlockFrequency.cpp (original)
> +++ llvm/trunk/lib/Support/BlockFrequency.cpp Fri Jun 28 13:23:42 2013
> @@ -42,12 +42,14 @@ void mult96bit(uint64_t freq, uint32_t N
> }
>
>
> -/// div96bit - Divide 96-bit value stored in W array by D. Return 64-bit frequency.
> +/// div96bit - Divide 96-bit value stored in W array by D.
> +/// Return 64-bit quotient, saturated to UINT64_MAX on overflow.
> uint64_t div96bit(uint64_t W[2], uint32_t D) {
> uint64_t y = W[0];
> uint64_t x = W[1];
> int i;
>
> + // This long division algorithm automatically saturates on overflow.
> for (i = 1; i <= 64 && x; ++i) {
> uint32_t t = (int)x >> 31;
> x = (x << 1) | (y >> 63);
> @@ -63,31 +65,30 @@ uint64_t div96bit(uint64_t W[2], uint32_
>
> }
>
> +void BlockFrequency::scale(uint32_t N, uint32_t D) {
> + assert(D != 0 && "Division by zero");
>
> -BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
> - uint32_t n = Prob.getNumerator();
> - uint32_t d = Prob.getDenominator();
> -
> - assert(n <= d && "Probability must be less or equal to 1.");
> -
> - // Calculate Frequency * n.
> - uint64_t mulLo = (Frequency & UINT32_MAX) * n;
> - uint64_t mulHi = (Frequency >> 32) * n;
> - uint64_t mulRes = (mulHi << 32) + mulLo;
> -
> - // If there was overflow use 96-bit operations.
> - if (mulHi > UINT32_MAX || mulRes < mulLo) {
> - // 96-bit value represented as W[1]:W[0].
> - uint64_t W[2];
> -
> - // Probability is less or equal to 1 which means that results must fit
> - // 64-bit.
> - mult96bit(Frequency, n, W);
> - Frequency = div96bit(W, d);
> - return *this;
> + // Calculate Frequency * N.
> + uint64_t MulLo = (Frequency & UINT32_MAX) * N;
> + uint64_t MulHi = (Frequency >> 32) * N;
> + uint64_t MulRes = (MulHi << 32) + MulLo;
> +
> + // If the product fits in 64 bits, just use built-in division.
> + if (MulHi <= UINT32_MAX && MulRes <= MulLo) {
The right side checks that the addition didn't overflow, shouldn't it be MulRes >= MulLo?
- Ben
> + Frequency = MulRes / D;
> + return;
> }
>
> - Frequency = mulRes / d;
> + // Product overflowed, use 96-bit operations.
> + // 96-bit value represented as W[1]:W[0].
> + uint64_t W[2];
> + mult96bit(Frequency, N, W);
> + Frequency = div96bit(W, D);
> + return;
> +}
> +
> +BlockFrequency &BlockFrequency::operator*=(const BranchProbability &Prob) {
> + scale(Prob.getNumerator(), Prob.getDenominator());
> return *this;
> }
>
> @@ -98,6 +99,17 @@ BlockFrequency::operator*(const BranchPr
> return Freq;
> }
>
> +BlockFrequency &BlockFrequency::operator/=(const BranchProbability &Prob) {
> + scale(Prob.getDenominator(), Prob.getNumerator());
> + return *this;
> +}
> +
> +BlockFrequency BlockFrequency::operator/(const BranchProbability &Prob) const {
> + BlockFrequency Freq(Frequency);
> + Freq /= Prob;
> + return Freq;
> +}
> +
> BlockFrequency &BlockFrequency::operator+=(const BlockFrequency &Freq) {
> uint64_t Before = Freq.Frequency;
> Frequency += Freq.Frequency;
>
> Modified: llvm/trunk/unittests/Support/BlockFrequencyTest.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Support/BlockFrequencyTest.cpp?rev=185184&r1=185183&r2=185184&view=diff
> ==============================================================================
> --- llvm/trunk/unittests/Support/BlockFrequencyTest.cpp (original)
> +++ llvm/trunk/unittests/Support/BlockFrequencyTest.cpp Fri Jun 28 13:23:42 2013
> @@ -52,6 +52,24 @@ TEST(BlockFrequencyTest, MaxToMax) {
> EXPECT_EQ(Freq.getFrequency(), UINT64_MAX);
> }
>
> +TEST(BlockFrequency, Divide) {
> + BlockFrequency Freq(0x3333333333333333ULL);
> + Freq /= BranchProbability(1, 2);
> + EXPECT_EQ(Freq.getFrequency(), 0x6666666666666666ULL);
> +}
> +
> +TEST(BlockFrequencyTest, Saturate) {
> + BlockFrequency Freq(0x3333333333333333ULL);
> + Freq /= BranchProbability(100, 300);
> + EXPECT_EQ(Freq.getFrequency(), 0x9999999999999999ULL);
> + Freq /= BranchProbability(1, 2);
> + EXPECT_EQ(Freq.getFrequency(), UINT64_MAX);
> +
> + Freq = 0x1000000000000000ULL;
> + Freq /= BranchProbability(10000, 160000);
> + EXPECT_EQ(Freq.getFrequency(), UINT64_MAX);
> +}
> +
> TEST(BlockFrequencyTest, ProbabilityCompare) {
> BranchProbability A(4, 5);
> BranchProbability B(4U << 29, 5U << 29);
>
>
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