Wed Jun 25 15:13:51 CDT 2003 First-level instrumentation --------------------------- We use opt to do Bytecode-to-bytecode instrumentation. Look at back-edges and insert llvm_first_trigger() function call which takes no arguments and no return value. This instrumentation is designed to be easy to remove, for instance by writing a NOP over the function call instruction. Keep count of every call to llvm_first_trigger(), and maintain counters in a map indexed by return address. If the trigger count exceeds a threshold, we identify a hot loop and perform second-level instrumentation on the hot loop region (the instructions between the target of the back-edge and the branch that causes the back-edge). We do not move code across basic-block boundaries. Second-level instrumentation --------------------------- We remove the first-level instrumentation by overwriting the CALL to llvm_first_trigger() with a NOP. The reoptimizer maintains a map between machine-code basic blocks and LLVM BasicBlock*s. We only keep track of paths that start at the first machine-code basic block of the hot loop region. How do we keep track of which edges to instrument, and which edges are exits from the hot region? 3 step process. 1) Do a DFS from the first machine-code basic block of the hot loop region and mark reachable edges. 2) Do a DFS from the last machine-code basic block of the hot loop region IGNORING back edges, and mark the edges which are reachable in 1) and also in 2) (i.e., must be reachable from both the start BB and the end BB of the hot region). 3) Mark BBs which end in edges that exit the hot region; we need to instrument these differently. Assume that there is 1 free register. On SPARC we use %g1, which LLC has agreed not to use. Shift a 1 into it at the beginning. At every edge which corresponds to a conditional branch, we shift 0 for not taken and 1 for taken into a register. This uniquely numbers the paths through the hot region. Silently fail if we need more than 64 bits. At the end BB we call countPath and increment the counter based on %g1 and the return address of the countPath call. We keep track of the number of iterations and the number of paths. We only run this version 30 or 40 times. Find the BBs that total 90% or more of execution, and aggregate them together to form our trace. But we do not allow more than 5 paths; if we have more than 5 we take the ones that are executed the most. We verify our assumption that we picked a hot back-edge in first-level instrumentation, by making sure that the number of times we took an exit edge from the hot trace is less than 10% of the number of iterations. LLC has been taught to recognize llvm_first_trigger() calls and NOT generate saves and restores of caller-saved registers around these calls. Phase behavior -------------- We turn off llvm_first_trigger() calls with NOPs, but this would hide phase behavior from us (when some funcs/traces stop being hot and others become hot.) We have a SIGALRM timer that counts time for us. Every time we get a SIGALRM we look at our priority queue of locations where we have removed llvm_first_trigger() calls. Each location is inserted along with a time when we will next turn instrumentation back on for that call site. If the time has arrived for a particular call site, we pop that off the prio. queue and turn instrumentation back on for that call site. Generating traces ----------------- When we finally generate an optimized trace we first copy the code into the trace cache. This leaves us with 3 copies of the code: the original code, the instrumented code, and the optimized trace. The optimized trace does not have instrumentation. The original code and the instrumented code are modified to have a branch to the trace cache, where the optimized traces are kept. We copy the code from the original to the instrumentation version by tracing the LLVM-to-Machine code basic block map and then copying each machine code basic block we think is in the hot region into the trace cache. Then we instrument that code. The process is similar for generating the final optimized trace; we copy the same basic blocks because we might need to put in fixup code for exit BBs. LLVM basic blocks are not typically used in the Reoptimizer except for the mapping information. We are restricted to using single instructions to branch between the original code, trace, and instrumented code. So we have to keep the code copies in memory near the original code (they can't be far enough away that a single pc-relative branch would not work.) Malloc() or data region space is too far away. this impacts the design of the trace cache. We use a dummy function that is full of a bunch of for loops which we overwrite with trace-cache code. The trace manager keeps track of whether or not we have enough space in the trace cache, etc. The trace insertion routine takes an original start address, a vector of machine instructions representing the trace, index of branches and their corresponding absolute targets, and index of calls and their corresponding absolute targets. The trace insertion routine is responsible for inserting branches from the beginning of the original code to the beginning of the optimized trace. This is because at some point the trace cache may run out of space and it may have to evict a trace, at which point the branch to the trace would also have to be removed. It uses a round-robin replacement policy; we have found that this is almost as good as LRU and better than random (especially because of problems fitting the new trace in.) We cannot deal with discontiguous trace cache areas. The trace cache is supposed to be cache-line-aligned, but it is not page-aligned. We generate instrumentation traces and optimized traces into separate trace caches. We keep the instrumented code around because you don't want to delete a trace when you still might have to return to it (i.e., return from a llvm_first_trigger() or countPath() call.)