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95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
1 Wed Jun 25 15:13:51 CDT 2003
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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2
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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3 First-level instrumentation
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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4 ---------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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5
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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6 We use opt to do Bytecode-to-bytecode instrumentation. Look at
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
7 back-edges and insert llvm_first_trigger() function call which takes
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
8 no arguments and no return value. This instrumentation is designed to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
9 be easy to remove, for instance by writing a NOP over the function
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
10 call instruction.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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11
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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12 Keep count of every call to llvm_first_trigger(), and maintain
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
13 counters in a map indexed by return address. If the trigger count
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
14 exceeds a threshold, we identify a hot loop and perform second-level
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
15 instrumentation on the hot loop region (the instructions between the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
16 target of the back-edge and the branch that causes the back-edge). We
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
17 do not move code across basic-block boundaries.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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18
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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19
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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20 Second-level instrumentation
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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21 ---------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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22
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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23 We remove the first-level instrumentation by overwriting the CALL to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
24 llvm_first_trigger() with a NOP.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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25
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
26 The reoptimizer maintains a map between machine-code basic blocks and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
27 LLVM BasicBlock*s. We only keep track of paths that start at the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
28 first machine-code basic block of the hot loop region.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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29
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
30 How do we keep track of which edges to instrument, and which edges are
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
31 exits from the hot region? 3 step process.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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32
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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33 1) Do a DFS from the first machine-code basic block of the hot loop
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
34 region and mark reachable edges.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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35
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
36 2) Do a DFS from the last machine-code basic block of the hot loop
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
37 region IGNORING back edges, and mark the edges which are reachable in
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
38 1) and also in 2) (i.e., must be reachable from both the start BB and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
39 the end BB of the hot region).
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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40
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
41 3) Mark BBs which end in edges that exit the hot region; we need to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
42 instrument these differently.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
43
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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44 Assume that there is 1 free register. On SPARC we use %g1, which LLC
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
45 has agreed not to use. Shift a 1 into it at the beginning. At every
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
46 edge which corresponds to a conditional branch, we shift 0 for not
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
47 taken and 1 for taken into a register. This uniquely numbers the paths
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
48 through the hot region. Silently fail if we need more than 64 bits.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
49
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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50 At the end BB we call countPath and increment the counter based on %g1
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
51 and the return address of the countPath call. We keep track of the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
52 number of iterations and the number of paths. We only run this
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
53 version 30 or 40 times.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
54
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
55 Find the BBs that total 90% or more of execution, and aggregate them
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
56 together to form our trace. But we do not allow more than 5 paths; if
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
57 we have more than 5 we take the ones that are executed the most. We
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
58 verify our assumption that we picked a hot back-edge in first-level
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
59 instrumentation, by making sure that the number of times we took an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
60 exit edge from the hot trace is less than 10% of the number of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
61 iterations.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
62
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
63 LLC has been taught to recognize llvm_first_trigger() calls and NOT
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
64 generate saves and restores of caller-saved registers around these
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
65 calls.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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66
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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67
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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68 Phase behavior
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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69 --------------
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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70
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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71 We turn off llvm_first_trigger() calls with NOPs, but this would hide
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
72 phase behavior from us (when some funcs/traces stop being hot and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
73 others become hot.)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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74
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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75 We have a SIGALRM timer that counts time for us. Every time we get a
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
76 SIGALRM we look at our priority queue of locations where we have
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
77 removed llvm_first_trigger() calls. Each location is inserted along
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
78 with a time when we will next turn instrumentation back on for that
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
79 call site. If the time has arrived for a particular call site, we pop
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
80 that off the prio. queue and turn instrumentation back on for that
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
81 call site.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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82
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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83
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
84 Generating traces
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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85 -----------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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86
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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87 When we finally generate an optimized trace we first copy the code
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
88 into the trace cache. This leaves us with 3 copies of the code: the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
89 original code, the instrumented code, and the optimized trace. The
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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90 optimized trace does not have instrumentation. The original code and
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
91 the instrumented code are modified to have a branch to the trace
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
92 cache, where the optimized traces are kept.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
93
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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94 We copy the code from the original to the instrumentation version
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
95 by tracing the LLVM-to-Machine code basic block map and then copying
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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96 each machine code basic block we think is in the hot region into the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
97 trace cache. Then we instrument that code. The process is similar for
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
98 generating the final optimized trace; we copy the same basic blocks
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
99 because we might need to put in fixup code for exit BBs.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
100
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
101 LLVM basic blocks are not typically used in the Reoptimizer except
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
102 for the mapping information.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
103
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
104 We are restricted to using single instructions to branch between the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
105 original code, trace, and instrumented code. So we have to keep the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
106 code copies in memory near the original code (they can't be far enough
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
107 away that a single pc-relative branch would not work.) Malloc() or
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
108 data region space is too far away. this impacts the design of the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
109 trace cache.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
110
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
111 We use a dummy function that is full of a bunch of for loops which we
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
112 overwrite with trace-cache code. The trace manager keeps track of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
113 whether or not we have enough space in the trace cache, etc.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
114
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
115 The trace insertion routine takes an original start address, a vector
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
116 of machine instructions representing the trace, index of branches and
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
117 their corresponding absolute targets, and index of calls and their
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
118 corresponding absolute targets.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
119
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
120 The trace insertion routine is responsible for inserting branches from
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
121 the beginning of the original code to the beginning of the optimized
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
122 trace. This is because at some point the trace cache may run out of
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
123 space and it may have to evict a trace, at which point the branch to
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
124 the trace would also have to be removed. It uses a round-robin
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
125 replacement policy; we have found that this is almost as good as LRU
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
126 and better than random (especially because of problems fitting the new
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
127 trace in.)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
128
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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129 We cannot deal with discontiguous trace cache areas. The trace cache
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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130 is supposed to be cache-line-aligned, but it is not page-aligned.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
131
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132 We generate instrumentation traces and optimized traces into separate
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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133 trace caches. We keep the instrumented code around because you don't
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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134 want to delete a trace when you still might have to return to it
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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135 (i.e., return from an llvm_first_trigger() or countPath() call.)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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136
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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137