1
2/*--------------------------------------------------------------------*/
3/*--- Branch predictor simulation                  cg_branchpred.c ---*/
4/*--------------------------------------------------------------------*/
5
6/*
7   This file is part of Cachegrind, a Valgrind tool for cache
8   profiling programs.
9
10   Copyright (C) 2002-2017 Nicholas Nethercote
11      njn@valgrind.org
12
13   This program is free software; you can redistribute it and/or
14   modify it under the terms of the GNU General Public License as
15   published by the Free Software Foundation; either version 2 of the
16   License, or (at your option) any later version.
17
18   This program is distributed in the hope that it will be useful, but
19   WITHOUT ANY WARRANTY; without even the implied warranty of
20   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
21   General Public License for more details.
22
23   You should have received a copy of the GNU General Public License
24   along with this program; if not, write to the Free Software
25   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
26   02111-1307, USA.
27
28   The GNU General Public License is contained in the file COPYING.
29*/
30
31
32/* This file contains the actual branch predictor simulator and its
33   associated state.  As with cg_sim.c it is #included directly into
34   cg_main.c.  It provides:
35
36   - a taken/not-taken predictor for conditional branches
37   - a branch target address predictor for indirect branches
38
39   Function return-address prediction is not modelled, on the basis
40   that return stack predictors almost always predict correctly, and
41   also that it is difficult for Valgrind to robustly identify
42   function calls and returns.
43*/
44
45/* How many bits at the bottom of an instruction address are
46   guaranteed to be zero? */
47#if defined(VGA_ppc32) || defined(VGA_ppc64be)  || defined(VGA_ppc64le) \
48    || defined(VGA_mips32) || defined(VGA_mips64) || defined(VGA_arm64)
49#  define N_IADDR_LO_ZERO_BITS 2
50#elif defined(VGA_x86) || defined(VGA_amd64)
51#  define N_IADDR_LO_ZERO_BITS 0
52#elif defined(VGA_s390x) || defined(VGA_arm)
53#  define N_IADDR_LO_ZERO_BITS 1
54#else
55#  error "Unsupported architecture"
56#endif
57
58
59/* Get a taken/not-taken prediction for the instruction (presumably a
60   conditional branch) at instr_addr.  Once that's done, update the
61   predictor state based on whether or not it was actually taken, as
62   indicated by 'taken'.  Finally, return 1 for a mispredict and 0 for
63   a successful predict.
64
65   The predictor is an array of 16k (== 2^14) 2-bit saturating
66   counters.  Given the address of the branch instruction, the array
67   index to use is computed both from the low order bits of the branch
68   instruction's address, and the global history - that is, from the
69   taken/not-taken behaviour of the most recent few branches.  This
70   makes the predictor able to correlate this branch's behaviour with
71   that of other branches.
72
73   TODO: use predictor written by someone who understands this stuff.
74   Perhaps it would be better to move to a standard GShare predictor
75   and/or tournament predictor.
76*/
77/* The index is composed of N_HIST bits at the top and N_IADD bits at
78   the bottom.  These numbers chosen somewhat arbitrarily, but note
79   that making N_IADD_BITS too small (eg 4) can cause large amounts of
80   aliasing, and hence misprediction, particularly if the history bits
81   are mostly unchanging. */
82#define N_HIST_BITS 7
83#define N_IADD_BITS 7
84
85#define N_BITS     (N_HIST_BITS + N_IADD_BITS)
86#define N_COUNTERS (1 << N_BITS)
87
88static UWord shift_register = 0;   /* Contains global history */
89static UChar counters[N_COUNTERS]; /* Counter array; presumably auto-zeroed */
90
91
92static ULong do_cond_branch_predict ( Addr instr_addr, Word takenW )
93{
94   UWord indx;
95   Bool  predicted_taken, actually_taken, mispredict;
96
97   const UWord hist_mask = (1 << N_HIST_BITS) - 1;
98   const UWord iadd_mask = (1 << N_IADD_BITS) - 1;
99         UWord hist_bits = shift_register & hist_mask;
100         UWord iadd_bits = (instr_addr >> N_IADDR_LO_ZERO_BITS)
101                           & iadd_mask;
102
103   tl_assert(hist_bits <= hist_mask);
104   tl_assert(iadd_bits <= iadd_mask);
105   indx = (hist_bits << N_IADD_BITS) | iadd_bits;
106   tl_assert(indx < N_COUNTERS);
107   if (0) VG_(printf)("index = %d\n", (Int)indx);
108
109   tl_assert(takenW <= 1);
110   predicted_taken = counters[ indx ] >= 2;
111   actually_taken  = takenW > 0;
112
113   mispredict = (actually_taken && (!predicted_taken))
114                || ((!actually_taken) && predicted_taken);
115
116   shift_register <<= 1;
117   shift_register |= (actually_taken ? 1 : 0);
118
119   if (actually_taken) {
120      if (counters[indx] < 3)
121         counters[indx]++;
122   } else {
123      if (counters[indx] > 0)
124         counters[indx]--;
125   }
126
127   tl_assert(counters[indx] <= 3);
128
129   return mispredict ? 1 : 0;
130}
131
132
133/* A very simple indirect branch predictor.  Use the branch's address
134   to index a table which records the previous target address for this
135   branch (or whatever aliased with it) and use that as the
136   prediction. */
137#define N_BTAC_BITS 9
138#define N_BTAC      (1 << N_BTAC_BITS)
139static Addr btac[N_BTAC]; /* BTAC; presumably auto-zeroed */
140
141static ULong do_ind_branch_predict ( Addr instr_addr, Addr actual )
142{
143   Bool mispredict;
144   const UWord mask = (1 << N_BTAC_BITS) - 1;
145         UWord indx = (instr_addr >> N_IADDR_LO_ZERO_BITS)
146                      & mask;
147   tl_assert(indx < N_BTAC);
148   mispredict = btac[indx] != actual;
149   btac[indx] = actual;
150   return mispredict ? 1 : 0;
151}
152
153
154/*--------------------------------------------------------------------*/
155/*--- end                                          cg_branchpred.c ---*/
156/*--------------------------------------------------------------------*/
157
158