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