This pass is a draft prototype!
For the moment, it provides an invariance degree for instructions, inner branches and loops (we call them chunks). It also peels (disabled by default) the previous chunks regarding to the degrees computed previously.
Build:
$ cd lqicm_pass
$ edit build.sh → give the right LLVM-DIR path to cmake
$ ./build.sh
Run:
$ edit run.sh → give the right LLVM_BUILD path
$ ./run.sh something.c/ll
Example:
$ ./run.sh test/files/Fact.c
↑ This is a basic same factorial computed n times. This pass detects
the invariance of the entire loop and can peel it (see also Fact4.c
which has the same factorial but of invariance degree equal to 2,
means that the loop can be peeled twice).
The pass is enabled by default when the libLQICMPass.so is loaded to
opt or directly into clang (using -load option).
Registered as EP_ModuleOptimizerEarly it can provides statistics
(with -mllvm -stats flags) on quasi-invariants detected before loop
optimizations as here for vim:
| 11m16,296s | vim -O3 ModuleOptimizerEarly |
|---|---|
| 3808 | number of loops |
| 32111 | number of instructions hoisted by LICM |
| 2266 | number of loops with several exit blocks |
| 125 | number of loops not well formed |
| 2391 | sum of loop not analyzed |
| 1417 | sum of loop analyzed by LQICM |
| 2480 | number of quasi-invariants detected |
| 333 | number of quasi-invariants Chunk detected |
| 24 | Number of chunks with deg >= 2 |
Note that there are some restrictions on the form of the loop: loops
with several exit blocks, latches or without header are not analyzed.
See that ~63% of the loops are not analyzed in the previous
statistics, this is why results are biased (it's probably be more) and
it's worse if LQICM is placed after the loop optimizations. We are
currently working for the flexibility of our analysis.