[PATCH] D57968: [LICM] Cap the clobbering calls in LICM.

[Alina Sbirlea via Phabricator via llvm-commits]

asbirlea created this revision.
asbirlea added reviewers: sanjoy, chandlerc, george.burgess.iv.
Herald added a subscriber: jlebar.
Herald added a project: LLVM.

Unlimitted number of calls to getClobberingAccess can lead to high
compile times in pathological cases.
Switching EnableLicmCap flag from bool to int, and enabling to default 100.
(tested to be appropriate for current bechmarks)
We can revisit this value when enabling MemorySSA.


Repository:
  rL LLVM

https://reviews.llvm.org/D57968

Files:
  lib/Transforms/Scalar/LICM.cpp


Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -106,17 +106,20 @@
 LICMN2Theshold("licm-n2-threshold", cl::Hidden, cl::init(0),
                cl::desc("How many instruction to cross product using AA"));
 
-// Experimental option to allow imprecision in LICM (use MemorySSA cap) in
-// pathological cases, in exchange for faster compile. This is to be removed
-// if MemorySSA starts to address the same issue. This flag applies only when
-// LICM uses MemorySSA instead on AliasSetTracker. When the flag is disabled
-// (default), LICM calls MemorySSAWalker's getClobberingMemoryAccess, which
-// gets perfect accuracy. When flag is enabled, LICM will call into MemorySSA's
-// getDefiningAccess, which may not be precise, since optimizeUses is capped.
-static cl::opt<bool> EnableLicmCap(
-    "enable-licm-cap", cl::init(false), cl::Hidden,
-    cl::desc("Enable imprecision in LICM (uses MemorySSA cap) in "
-             "pathological cases, in exchange for faster compile"));
+// Experimental option to allow imprecision in LICM in pathological cases, in
+// exchange for faster compile. This is to be removed if MemorySSA starts to
+// address the same issue. This flag applies only when LICM uses MemorySSA
+// instead on AliasSetTracker. LICM calls MemorySSAWalker's
+// getClobberingMemoryAccess, up to the value of the Cap, getting perfect
+// accuracy. Afterwards, LICM will call into MemorySSA's getDefiningAccess,
+// which may not be precise, since optimizeUses is capped. The result is
+// correct, but we may not get as "far up" as possible to get which access is
+// clobbering the one queried.
+static cl::opt<int> EnableLicmCap(
+    "enable-licm-cap", cl::init(100), cl::Hidden,
+    cl::desc("Enable imprecision in LICM in pathological cases, in exchange "
+             "for faster compile. Caps the MemorySSA clobbering calls."));
+static int EnableLicmCapCounter = 0;
 
 static bool LocalDisablePromotion = false;
 
@@ -309,6 +312,7 @@
   std::unique_ptr<AliasSetTracker> CurAST;
   std::unique_ptr<MemorySSAUpdater> MSSAU;
   LocalDisablePromotion = false;
+  EnableLicmCapCounter = 0;
 
   if (!MSSA) {
     LLVM_DEBUG(dbgs() << "LICM: Using Alias Set Tracker.\n");
@@ -1174,8 +1178,9 @@
     } else { // MSSAU
       if (isOnlyMemoryAccess(SI, CurLoop, MSSAU))
         return true;
-      if (!EnableLicmCap) {
+      if (EnableLicmCapCounter < EnableLicmCap) {
         auto *Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(SI);
+        EnableLicmCapCounter++;
         // If there are no clobbering Defs in the loop, we still need to check
         // for interfering Uses. If there are more accesses than the Promotion
         // cap, give up, we're not walking a list that long. Otherwise, walk the
@@ -2196,10 +2201,12 @@
                                              Loop *CurLoop) {
   MemoryAccess *Source;
   // See declaration of EnableLicmCap for usage details.
-  if (EnableLicmCap)
+  if (EnableLicmCapCounter >= EnableLicmCap)
     Source = MU->getDefiningAccess();
-  else
+  else {
     Source = MSSA->getSkipSelfWalker()->getClobberingMemoryAccess(MU);
+    EnableLicmCapCounter++;
+  }
   return !MSSA->isLiveOnEntryDef(Source) &&
          CurLoop->contains(Source->getBlock());
 }


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