diff options
Diffstat (limited to 'llvm/lib/Transforms/Scalar/LICM.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/LICM.cpp | 28 |
1 files changed, 24 insertions, 4 deletions
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp index eb887af9cb5..8a29540a9eb 100644 --- a/llvm/lib/Transforms/Scalar/LICM.cpp +++ b/llvm/lib/Transforms/Scalar/LICM.cpp @@ -912,14 +912,23 @@ bool llvm::promoteLoopAccessesToScalars( // // If at least one store is guaranteed to execute, both properties are // satisfied, and promotion is legal. + // // This, however, is not a necessary condition. Even if no store/load is - // guaranteed to execute, we can still establish these properties: - // (p1) by proving that hoisting the load into the preheader is - // safe (i.e. proving dereferenceability on all paths through the loop). We + // guaranteed to execute, we can still establish these properties. + // We can establish (p1) by proving that hoisting the load into the preheader + // is safe (i.e. proving dereferenceability on all paths through the loop). We // can use any access within the alias set to prove dereferenceability, // since they're all must alias. - // (p2) by proving the memory is thread-local, so the memory model + // + // There are two ways establish (p2): + // a) Prove the location is thread-local. In this case the memory model // requirement does not apply, and stores are safe to insert. + // b) Prove a store dominates every exit block. In this case, if an exit + // blocks is reached, the original dynamic path would have taken us through + // the store, so inserting a store into the exit block is safe. Note that this + // is different from the store being guaranteed to execute. For instance, + // if an exception is thrown on the first iteration of the loop, the original + // store is never executed, but the exit blocks are not executed either. bool DereferenceableInPH = false; bool SafeToInsertStore = false; @@ -1001,6 +1010,17 @@ bool llvm::promoteLoopAccessesToScalars( } } + // If a store dominates all exit blocks, it is safe to sink. + // As explained above, if an exit block was executed, a dominating + // store must have been been executed at least once, so we are not + // introducing stores on paths that did not have them. + // Note that this only looks at explicit exit blocks. If we ever + // start sinking stores into unwind edges (see above), this will break. + if (!SafeToInsertStore) + SafeToInsertStore = llvm::all_of(ExitBlocks, [&](BasicBlock *Exit) { + return DT->dominates(Store->getParent(), Exit); + }); + // If the store is not guaranteed to execute, we may still get // deref info through it. if (!DereferenceableInPH) { |
