[llvm] r354182 - [EarlyCSE & MSSA] Cap the clobbering calls in EarlyCSE.

Alina Sbirlea via llvm-commits llvm-commits at lists.llvm.org
Fri Feb 15 14:47:54 PST 2019

Author: asbirlea
Date: Fri Feb 15 14:47:54 2019
New Revision: 354182

URL: http://llvm.org/viewvc/llvm-project?rev=354182&view=rev
[EarlyCSE & MSSA] Cap the clobbering calls in EarlyCSE.

Unlimitted number of calls to getClobberingAccess can lead to high
compile times in pathological cases.
Limitting getClobberingAccess to a fairly high number. Can be adjusted
based on users/need.
Note: this is the only user of MemorySSA currently enabled by default.
The same handling exists in LICM (disabled atm). As MemorySSA gains more
users, this logic of capping will need to move inside MemorySSA.

Reviewers: george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D58248


Modified: llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp?rev=354182&r1=354181&r2=354182&view=diff
--- llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp Fri Feb 15 14:47:54 2019
@@ -75,6 +75,11 @@ STATISTIC(NumDSE,      "Number of trivia
 DEBUG_COUNTER(CSECounter, "early-cse",
               "Controls which instructions are removed");
+static cl::opt<unsigned> EarlyCSEMssaOptCap(
+    "earlycse-mssa-optimization-cap", cl::init(500), cl::Hidden,
+    cl::desc("Enable imprecision in EarlyCSE in pathological cases, in exchange "
+             "for faster compile. Caps the MemorySSA clobbering calls."));
 // SimpleValue
@@ -418,6 +423,7 @@ public:
   bool run();
+  unsigned ClobberCounter = 0;
   // Almost a POD, but needs to call the constructors for the scoped hash
   // tables so that a new scope gets pushed on. These are RAII so that the
   // scope gets popped when the NodeScope is destroyed.
@@ -662,8 +668,13 @@ bool EarlyCSE::isSameMemGeneration(unsig
   // LaterInst, if LaterDef dominates EarlierInst then it can't occur between
   // EarlierInst and LaterInst and neither can any other write that potentially
   // clobbers LaterInst.
-  MemoryAccess *LaterDef =
-      MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
+  MemoryAccess *LaterDef;
+  if (ClobberCounter < EarlyCSEMssaOptCap) {
+    LaterDef = MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
+    ClobberCounter++;
+  } else
+    LaterDef = LaterMA->getDefiningAccess();
   return MSSA->dominates(LaterDef, EarlierMA);

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