[PATCH] D58248: [EarlyCSE & MSSA] Cap the clobbering calls in EarlyCSE.

Alina Sbirlea via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Feb 14 12:50:34 PST 2019

asbirlea created this revision.
asbirlea added a reviewer: george.burgess.iv.
Herald added subscribers: jdoerfert, Prazek, jlebar, sanjoy.
Herald added a project: LLVM.

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.




Index: lib/Transforms/Scalar/EarlyCSE.cpp
--- lib/Transforms/Scalar/EarlyCSE.cpp
+++ lib/Transforms/Scalar/EarlyCSE.cpp
@@ -75,6 +75,11 @@
 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 @@
   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 @@
   // 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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