[LICM] Cap the clobbering calls in LICM.

[asbirlea]

Summary

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.

[chandlerc]

LGTM

[asbirlea]

I disregarded the possiblity of threading in the initial diff.
Updated to pass the counter as an argument.
Kindly re-review.

[george.burgess.iv]

Thanks for this!

One currently-actionable nit from me

include/llvm/Transforms/Utils/LoopUtils.h

115 ↗

(On Diff #186060)

Nit: I'm OK with this as is, but if we grow another MSSA-related optimization limit/counter, please wrap the MSSA-related bits into some sort of options struct (a bit in spirit like llvm::ObjectSizeOpts, but we'd also be tracking the mutable EnableLicmCapCounter/etc in it)

lib/Transforms/Scalar/LICM.cpp

119

nit: enable sounds pretty bool-y for an int. Can we swap to licm-mssa-optimization-cap (or similar)?

[asbirlea]

Address comment (renamed the flag, and the counter to match it).

[asbirlea]

include/llvm/Transforms/Utils/LoopUtils.h

115 ↗

(On Diff #186060)

Noted, and I agree! I considered adding this in this patch, actually.
Additional refactoring may help somewhat, since the only user of sinkRegion and hoistRegion is LICM. We'd still needs these bits wrapped up for canSinkAndHoist which is used in LoopSink.
Leaving it as a separate cleanup for now.

[george.burgess.iv]

Deferring to Chandler's LGTM for the approach + first patch; changes since then LGTM.

Thanks again!

include/llvm/Transforms/Utils/LoopUtils.h

115 ↗

(On Diff #186060)

SGTM

lib/Transforms/Scalar/LICM.cpp

 
 // Default value of zero implies we use the regular alias set tracker mechanism
 // instead of the cross product using AA to identify aliasing of the memory
 // location we are interested in.
 static cl::opt<int>
 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;
 
 // Experimentally, memory promotion carries less importance than sinking and
 // hoisting. Limit when we do promotion when using MemorySSA, in order to save
 // compile time.
 static cl::opt<unsigned> AccessCapForMSSAPromotion(
     "max-acc-licm-promotion", cl::init(250), cl::Hidden,
bool LoopInvariantCodeMotion::runOnLoop(
     MemorySSA *MSSA, OptimizationRemarkEmitter *ORE, bool DeleteAST) {
   bool Changed = false;
 
   assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
 
   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");
     CurAST = collectAliasInfoForLoop(L, LI, AA);
   } else {
     LLVM_DEBUG(dbgs() << "LICM: Using MemorySSA.\n");
     MSSAU = make_unique<MemorySSAUpdater>(MSSA);
 
if (CurAST) {
       if (!UniqueI)
         // other memory op, give up
         return false;
       assert(UniqueI == SI && "AS must contain SI");
       return true;
     } 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
         // list, check each Use if it's optimized to an access outside the loop.
         // If yes, store is safe to hoist. This is fairly restrictive, but
         // conservatively correct.
         // TODO: Cache set of Uses on the first walk in runOnLoop, update when
         // moving accesses. Can also extend to dominating uses.
static bool pointerInvalidatedByLoop(MemoryLocation MemLoc,
   LLVM_DEBUG(dbgs() << "Aliasing okay for " << *(MemLoc.Ptr) << "\n");
   return false;
 }
 
 static bool pointerInvalidatedByLoopWithMSSA(MemorySSA *MSSA, MemoryUse *MU,
                                              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());
 }
 
 /// Little predicate that returns true if the specified basic block is in
 /// a subloop of the current one, not the current one itself.
 ///
 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI) {
   assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
   return LI->getLoopFor(BB) != CurLoop;
 }