[PATCH] D23028: GVN-hoist: limit the length of dependent instructions

[Sebastian Pop via llvm-commits]

sebpop created this revision.
sebpop added a reviewer: dberlin.
sebpop added a subscriber: llvm-commits.

Limit the number of times the while(1) loop is executed.
With this restriction the number of hoisted instructions does not change in a significant way on the test-suite.

https://reviews.llvm.org/D23028

Files:
  llvm/lib/Transforms/Scalar/GVNHoist.cpp

Index: llvm/lib/Transforms/Scalar/GVNHoist.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/GVNHoist.cpp
+++ llvm/lib/Transforms/Scalar/GVNHoist.cpp
@@ -53,6 +53,11 @@
     cl::desc("Hoist instructions from the beginning of the BB up to the "
              "maximum specified depth (default = 100, unlimited = -1)"));
 
+static cl::opt<int> MaxChainLength(
+    "gvn-hoist-max-chain-length", cl::Hidden, cl::init(10),
+    cl::desc("Maximum length of dependent chains to hoist "
+             "(default = 10, unlimited = -1)"));
+
 namespace {
 
 // Provides a sorting function based on the execution order of two instructions.
@@ -212,8 +217,13 @@
         DFSNumber[&Inst] = ++I;
     }
 
+    int ChainLength = 0;
+
     // FIXME: use lazy evaluation of VN to avoid the fix-point computation.
     while (1) {
+      if (MaxChainLength != -1 && ++ChainLength >= MaxChainLength)
+        return Res;
+
       auto HoistStat = hoistExpressions(F);
       if (HoistStat.first + HoistStat.second == 0)
         return Res;


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