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