[LICM] Report failing to hoist a load with an invariant address

[anemet]

Summary

These are interesting because lack of precision in alias information
could be standing in the way of this optimization.

An example is the case in the test suite that I showed in the DevMeeting
talk:

http://lab.llvm.org:8080/artifacts/opt-view_test-suite/build/MultiSource/Benchmarks/FreeBench/distray/CMakeFiles/distray.dir/html/_org_test-suite_MultiSource_Benchmarks_FreeBench_distray_distray.c.html#L236

canSinkOrHoistInst is also used from LoopSink, which does not use
opt-remarks so we need to take ORE as an optional argument.

[anemet]

Ping.

[hfinkel]

lib/Transforms/Scalar/LICM.cpp

477

Can we hoist a load without a loop-invariant address? It seems like we shouldn't be making aliasing queries for loads we can't possibly hoist, and so I don't understand why we specifically need the CurLoop->isLoopInvariant check here.

[anemet]

lib/Transforms/Scalar/LICM.cpp

477

No, you're right. The call to this is guarded with hasLoopInvariantOperands.

[anemet]

lib/Transforms/Scalar/LICM.cpp

477

Actually, I take this back. We need the check. See the new comment in the new version of the patch and also the new testcase for an example.

[anemet]

Add comment and testcase to explain why we need to check for the
loop-invariance of the pointer.

[hfinkel]

lib/Transforms/Scalar/LICM.cpp

477

Okay, but I wonder if this points to a larger problem. Maybe we just need a FIXME on this comment for now, but I'm thinking that this invariance check is cheap and the other checks (i.e. aliasing queries, dereferenceablility checks, etc.) are, at least in some cases, expensive. Thus, this guard really belongs elsewhere. We can sink something with a non-loop-invariant store either, can we?

[anemet]

lib/Transforms/Scalar/LICM.cpp

477

Yes but we *can* sink a load with a loop-variant address whose result is only used outside the loop (see testcase).

Loop-invariance is not tested in this case (because it shouldn't) however it is tested on other paths to this code (as it should) so I don't think there is a problem of wasting work.

[hfinkel]

lib/Transforms/Scalar/LICM.cpp

477

Ah, okay. That makes sense, thanks! LGTM.

include/llvm/Transforms/Utils/LoopUtils.h

 void getLoopAnalysisUsage(AnalysisUsage &AU);
 
 /// Returns true if the hoister and sinker can handle this instruction.
 /// If SafetyInfo is null, we are checking for sinking instructions from
 /// preheader to loop body (no speculation).
 /// If SafetyInfo is not null, we are checking for hoisting/sinking
 /// instructions from loop body to preheader/exit. Check if the instruction
 /// can execute speculatively.
-///
+/// If \p ORE is set use it to emit optimization remarks.
 bool canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                         Loop *CurLoop, AliasSetTracker *CurAST,
-                        LoopSafetyInfo *SafetyInfo);
+                        LoopSafetyInfo *SafetyInfo,
+                        OptimizationRemarkEmitter *ORE = nullptr);
 }
 
 #endif

lib/Transforms/Scalar/LICM.cpp

for (BasicBlock::iterator II = BB->end(); II != BB->begin();) {
     }
 
     // Check to see if we can sink this instruction to the exit blocks
     // of the loop.  We can do this if the all users of the instruction are
     // outside of the loop.  In this case, it doesn't even matter if the
     // operands of the instruction are loop invariant.
     //
     if (isNotUsedInLoop(I, CurLoop, SafetyInfo) &&
-        canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo)) {
+        canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE)) {
       ++II;
       Changed |= sink(I, LI, DT, CurLoop, CurAST, SafetyInfo, ORE);
     }
   }
   return Changed;
 }
 
 /// Walk the specified region of the CFG (defined by all blocks dominated by
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
         continue;
       }
 
       // Try hoisting the instruction out to the preheader.  We can only do this
       // if all of the operands of the instruction are loop invariant and if it
       // is safe to hoist the instruction.
       //
       if (CurLoop->hasLoopInvariantOperands(&I) &&
-          canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo) &&
+          canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
           isSafeToExecuteUnconditionally(
               I, DT, CurLoop, SafetyInfo,
               CurLoop->getLoopPreheader()->getTerminator()))
         Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
     }
 
   const std::vector<DomTreeNode *> &Children = N->getChildren();
   for (DomTreeNode *Child : Children)
void llvm::computeLoopSafetyInfo(LoopSafetyInfo *SafetyInfo, Loop *CurLoop) {
   if (Fn->hasPersonalityFn())
     if (Constant *PersonalityFn = Fn->getPersonalityFn())
       if (isFuncletEHPersonality(classifyEHPersonality(PersonalityFn)))
         SafetyInfo->BlockColors = colorEHFunclets(*Fn);
 }
 
 bool llvm::canSinkOrHoistInst(Instruction &I, AAResults *AA, DominatorTree *DT,
                               Loop *CurLoop, AliasSetTracker *CurAST,
-                              LoopSafetyInfo *SafetyInfo) {
+                              LoopSafetyInfo *SafetyInfo,
+                              OptimizationRemarkEmitter *ORE) {
   // Loads have extra constraints we have to verify before we can hoist them.
   if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
     if (!LI->isUnordered())
       return false; // Don't hoist volatile/atomic loads!
 
     // Loads from constant memory are always safe to move, even if they end up
     // in the same alias set as something that ends up being modified.
     if (AA->pointsToConstantMemory(LI->getOperand(0)))
       return true;
     if (LI->getMetadata(LLVMContext::MD_invariant_load))
       return true;
 
     // Don't hoist loads which have may-aliased stores in loop.
     uint64_t Size = 0;
     if (LI->getType()->isSized())
       Size = I.getModule()->getDataLayout().getTypeStoreSize(LI->getType());
 
     AAMDNodes AAInfo;
     LI->getAAMetadata(AAInfo);
 
-    return !pointerInvalidatedByLoop(LI->getOperand(0), Size, AAInfo, CurAST);
+    bool Invalidated =
+        pointerInvalidatedByLoop(LI->getOperand(0), Size, AAInfo, CurAST);
+    if (ORE && Invalidated && CurLoop->isLoopInvariant(LI->getPointerOperand()))
+      ORE->emit(OptimizationRemarkMissed(
+                    DEBUG_TYPE, "LoadWithLoopInvariantAddressInvalidated", LI)
+                << "failed to hoist load with loop-invariant address "
+                   "because the loop may invalidate its value");
+
+    return !Invalidated;
   } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
     // Don't sink or hoist dbg info; it's legal, but not useful.
     if (isa<DbgInfoIntrinsic>(I))
       return false;
 
     // Don't sink calls which can throw.
     if (CI->mayThrow())
       return false;

test/Transforms/LICM/opt-remarks-intervening-store.ll

+; RUN: opt < %s -licm -pass-remarks-missed=licm -o /dev/null 2>&1 | FileCheck %s
+; RUN: opt -aa-pipeline=basic-aa -passes='require<aa>,require<targetir>,require<scalar-evolution>,require<opt-remark-emit>,loop(licm)' %s -o /dev/null -pass-remarks-missed=licm 2>&1 | FileCheck %s
+target datalayout = "E-p:64:64:64-a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-v64:64:64-v128:128:128"
+
+; Without the noalias on %p, we can't optmize this and the remark should tell
+; us about it.
+
+define void @test(i32* %array, i32* %p) {
+Entry:
+  br label %Loop
+
+Loop:
+  %j = phi i32 [ 0, %Entry ], [ %Next, %Loop ]
+  %addr = getelementptr i32, i32* %array, i32 %j
+  %a = load i32, i32* %addr
+; CHECK: remark: /tmp/kk.c:2:20: failed to hoist load with loop-invariant address because the loop may invalidate its value
+  %b = load i32, i32* %p, !dbg !8
+  %a2 = add i32 %a, %b
+  store i32 %a2, i32* %addr
+  %Next = add i32 %j, 1
+  %cond = icmp eq i32 %Next, 0
+  br i1 %cond, label %Out, label %Loop
+
+Out:
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.9.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, enums: !2)
+!1 = !DIFile(filename: "/tmp/kk.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"PIC Level", i32 2}
+!5 = !{!"clang version 3.9.0 "}
+!6 = distinct !DISubprogram(name: "success", scope: !1, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 2, column: 20, scope: !6)