1 //===-------------- lib/Support/BranchProbability.cpp -----------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements Branch Probability class.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Support/BranchProbability.h"
15 #include "llvm/Config/llvm-config.h"
16 #include "llvm/Support/Debug.h"
17 #include "llvm/Support/Format.h"
18 #include "llvm/Support/raw_ostream.h"
19 #include <cassert>
20 
21 using namespace llvm;
22 
23 const uint32_t BranchProbability::D;
24 
25 raw_ostream &BranchProbability::print(raw_ostream &OS) const {
26   if (isUnknown())
27     return OS << "?%";
28 
29   // Get a percentage rounded to two decimal digits. This avoids
30   // implementation-defined rounding inside printf.
31   double Percent = rint(((double)N / D) * 100.0 * 100.0) / 100.0;
32   return OS << format("0x%08" PRIx32 " / 0x%08" PRIx32 " = %.2f%%", N, D,
33                       Percent);
34 }
35 
36 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
37 LLVM_DUMP_METHOD void BranchProbability::dump() const { print(dbgs()) << '\n'; }
38 #endif
39 
40 BranchProbability::BranchProbability(uint32_t Numerator, uint32_t Denominator) {
41   assert(Denominator > 0 && "Denominator cannot be 0!");
42   assert(Numerator <= Denominator && "Probability cannot be bigger than 1!");
43   if (Denominator == D)
44     N = Numerator;
45   else {
46     uint64_t Prob64 =
47         (Numerator * static_cast<uint64_t>(D) + Denominator / 2) / Denominator;
48     N = static_cast<uint32_t>(Prob64);
49   }
50 }
51 
52 BranchProbability
53 BranchProbability::getBranchProbability(uint64_t Numerator,
54                                         uint64_t Denominator) {
55   assert(Numerator <= Denominator && "Probability cannot be bigger than 1!");
56   // Scale down Denominator to fit in a 32-bit integer.
57   int Scale = 0;
58   while (Denominator > UINT32_MAX) {
59     Denominator >>= 1;
60     Scale++;
61   }
62   return BranchProbability(Numerator >> Scale, Denominator);
63 }
64 
65 // If ConstD is not zero, then replace D by ConstD so that division and modulo
66 // operations by D can be optimized, in case this function is not inlined by the
67 // compiler.
68 template <uint32_t ConstD>
69 static uint64_t scale(uint64_t Num, uint32_t N, uint32_t D) {
70   if (ConstD > 0)
71     D = ConstD;
72 
73   assert(D && "divide by 0");
74 
75   // Fast path for multiplying by 1.0.
76   if (!Num || D == N)
77     return Num;
78 
79   // Split Num into upper and lower parts to multiply, then recombine.
80   uint64_t ProductHigh = (Num >> 32) * N;
81   uint64_t ProductLow = (Num & UINT32_MAX) * N;
82 
83   // Split into 32-bit digits.
84   uint32_t Upper32 = ProductHigh >> 32;
85   uint32_t Lower32 = ProductLow & UINT32_MAX;
86   uint32_t Mid32Partial = ProductHigh & UINT32_MAX;
87   uint32_t Mid32 = Mid32Partial + (ProductLow >> 32);
88 
89   // Carry.
90   Upper32 += Mid32 < Mid32Partial;
91 
92   // Check for overflow.
93   if (Upper32 >= D)
94     return UINT64_MAX;
95 
96   uint64_t Rem = (uint64_t(Upper32) << 32) | Mid32;
97   uint64_t UpperQ = Rem / D;
98 
99   // Check for overflow.
100   if (UpperQ > UINT32_MAX)
101     return UINT64_MAX;
102 
103   Rem = ((Rem % D) << 32) | Lower32;
104   uint64_t LowerQ = Rem / D;
105   uint64_t Q = (UpperQ << 32) + LowerQ;
106 
107   // Check for overflow.
108   return Q < LowerQ ? UINT64_MAX : Q;
109 }
110 
111 uint64_t BranchProbability::scale(uint64_t Num) const {
112   return ::scale<D>(Num, N, D);
113 }
114 
115 uint64_t BranchProbability::scaleByInverse(uint64_t Num) const {
116   return ::scale<0>(Num, D, N);
117 }
118