1 Wed Jun 25 15:13:51 CDT 2003 2 3 First-level instrumentation 4 --------------------------- 5 6 We use opt to do Bytecode-to-bytecode instrumentation. Look at 7 back-edges and insert llvm_first_trigger() function call which takes 8 no arguments and no return value. This instrumentation is designed to 9 be easy to remove, for instance by writing a NOP over the function 10 call instruction. 11 12 Keep count of every call to llvm_first_trigger(), and maintain 13 counters in a map indexed by return address. If the trigger count 14 exceeds a threshold, we identify a hot loop and perform second-level 15 instrumentation on the hot loop region (the instructions between the 16 target of the back-edge and the branch that causes the back-edge). We 17 do not move code across basic-block boundaries. 18 19 20 Second-level instrumentation 21 --------------------------- 22 23 We remove the first-level instrumentation by overwriting the CALL to 24 llvm_first_trigger() with a NOP. 25 26 The reoptimizer maintains a map between machine-code basic blocks and 27 LLVM BasicBlock*s. We only keep track of paths that start at the 28 first machine-code basic block of the hot loop region. 29 30 How do we keep track of which edges to instrument, and which edges are 31 exits from the hot region? 3 step process. 32 33 1) Do a DFS from the first machine-code basic block of the hot loop 34 region and mark reachable edges. 35 36 2) Do a DFS from the last machine-code basic block of the hot loop 37 region IGNORING back edges, and mark the edges which are reachable in 38 1) and also in 2) (i.e., must be reachable from both the start BB and 39 the end BB of the hot region). 40 41 3) Mark BBs which end in edges that exit the hot region; we need to 42 instrument these differently. 43 44 Assume that there is 1 free register. On SPARC we use %g1, which LLC 45 has agreed not to use. Shift a 1 into it at the beginning. At every 46 edge which corresponds to a conditional branch, we shift 0 for not 47 taken and 1 for taken into a register. This uniquely numbers the paths 48 through the hot region. Silently fail if we need more than 64 bits. 49 50 At the end BB we call countPath and increment the counter based on %g1 51 and the return address of the countPath call. We keep track of the 52 number of iterations and the number of paths. We only run this 53 version 30 or 40 times. 54 55 Find the BBs that total 90% or more of execution, and aggregate them 56 together to form our trace. But we do not allow more than 5 paths; if 57 we have more than 5 we take the ones that are executed the most. We 58 verify our assumption that we picked a hot back-edge in first-level 59 instrumentation, by making sure that the number of times we took an 60 exit edge from the hot trace is less than 10% of the number of 61 iterations. 62 63 LLC has been taught to recognize llvm_first_trigger() calls and NOT 64 generate saves and restores of caller-saved registers around these 65 calls. 66 67 68 Phase behavior 69 -------------- 70 71 We turn off llvm_first_trigger() calls with NOPs, but this would hide 72 phase behavior from us (when some funcs/traces stop being hot and 73 others become hot.) 74 75 We have a SIGALRM timer that counts time for us. Every time we get a 76 SIGALRM we look at our priority queue of locations where we have 77 removed llvm_first_trigger() calls. Each location is inserted along 78 with a time when we will next turn instrumentation back on for that 79 call site. If the time has arrived for a particular call site, we pop 80 that off the prio. queue and turn instrumentation back on for that 81 call site. 82 83 84 Generating traces 85 ----------------- 86 87 When we finally generate an optimized trace we first copy the code 88 into the trace cache. This leaves us with 3 copies of the code: the 89 original code, the instrumented code, and the optimized trace. The 90 optimized trace does not have instrumentation. The original code and 91 the instrumented code are modified to have a branch to the trace 92 cache, where the optimized traces are kept. 93 94 We copy the code from the original to the instrumentation version 95 by tracing the LLVM-to-Machine code basic block map and then copying 96 each machine code basic block we think is in the hot region into the 97 trace cache. Then we instrument that code. The process is similar for 98 generating the final optimized trace; we copy the same basic blocks 99 because we might need to put in fixup code for exit BBs. 100 101 LLVM basic blocks are not typically used in the Reoptimizer except 102 for the mapping information. 103 104 We are restricted to using single instructions to branch between the 105 original code, trace, and instrumented code. So we have to keep the 106 code copies in memory near the original code (they can't be far enough 107 away that a single pc-relative branch would not work.) Malloc() or 108 data region space is too far away. this impacts the design of the 109 trace cache. 110 111 We use a dummy function that is full of a bunch of for loops which we 112 overwrite with trace-cache code. The trace manager keeps track of 113 whether or not we have enough space in the trace cache, etc. 114 115 The trace insertion routine takes an original start address, a vector 116 of machine instructions representing the trace, index of branches and 117 their corresponding absolute targets, and index of calls and their 118 corresponding absolute targets. 119 120 The trace insertion routine is responsible for inserting branches from 121 the beginning of the original code to the beginning of the optimized 122 trace. This is because at some point the trace cache may run out of 123 space and it may have to evict a trace, at which point the branch to 124 the trace would also have to be removed. It uses a round-robin 125 replacement policy; we have found that this is almost as good as LRU 126 and better than random (especially because of problems fitting the new 127 trace in.) 128 129 We cannot deal with discontiguous trace cache areas. The trace cache 130 is supposed to be cache-line-aligned, but it is not page-aligned. 131 132 We generate instrumentation traces and optimized traces into separate 133 trace caches. We keep the instrumented code around because you don't 134 want to delete a trace when you still might have to return to it 135 (i.e., return from an llvm_first_trigger() or countPath() call.) 136 137 138
