limit the number of instructions per block examined by dead store elimination

Summary: Dead store elimination gets very expensive when large numbers of instructions need to be analyzed. This patch limits the number of instructions analyzed per store to the value of the memdep-block-scan-limit parameter (which defaults to 100). This resulted in no observed difference in performance of the generated code, and no change in the statistics for the dead store elimination pass, but improved compilation time on some files by more than an order of magnitude.

Reviewers: dexonsmith, bruno, george.burgess.iv, dberlin, reames, davidxl

Subscribers: davide, chandlerc, dberlin, davidxl, eraman, tejohnson, mbodart, llvm-commits

Differential Revision: https://reviews.llvm.org/D15537

llvm-svn: 279833

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
index fbe2726bed7..4d4c3baef3f 100644
--- a/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -1050,6 +1050,13 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
     if (!Loc.Ptr)
       continue;
 
+    // Loop until we find a store we can eliminate or a load that
+    // invalidates the analysis. Without an upper bound on the number of
+    // instructions examined, this analysis can become very time-consuming.
+    // However, the potential gain diminishes as we process more instructions
+    // without eliminating any of them. Therefore, we limit the number of
+    // instructions we look at.
+    auto Limit = MD->getDefaultBlockScanLimit();
     while (InstDep.isDef() || InstDep.isClobber()) {
       // Get the memory clobbered by the instruction we depend on.  MemDep will
       // skip any instructions that 'Loc' clearly doesn't interact with.  If we
@@ -1138,8 +1145,9 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
       if (AA->getModRefInfo(DepWrite, Loc) & MRI_Ref)
         break;
 
-      InstDep = MD->getPointerDependencyFrom(Loc, false,
-                                             DepWrite->getIterator(), &BB);
+      InstDep = MD->getPointerDependencyFrom(Loc, /*isLoad=*/ false,
+                                             DepWrite->getIterator(), &BB,
+                                             /*QueryInst=*/ nullptr, &Limit);
     }
   }