[GVN, OptDiag] Print the interesting instructions involved in missed load-elimination

[recommitting after the fix in r288307]

This includes the intervening store and the load/store that we're trying
to forward from in the optimization remark for the missed load
elimination.

This is hooked up under a new mode in ORE that allows for compile-time
budget for a bit more analysis to print more insightful messages.  This
mode is currently enabled for -fsave-optimization-record (-Rpass is
trickier since it is controlled in the front-end).

With this we can now print the red remark in http://lab.llvm.org:8080/artifacts/opt-view_test-suite/build/SingleSource/Benchmarks/Dhrystone/CMakeFiles/dry.dir/html/_org_test-suite_SingleSource_Benchmarks_Dhrystone_dry.c.html#L446

Differential Revision: https://reviews.llvm.org/D26490

llvm-svn: 288381

1 files changed, 36 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 9a14fcb07f9..81f273ff318 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1209,6 +1209,38 @@ static bool isLifetimeStart(const Instruction *Inst) {
   return false;
 }
 
+/// \brief Try to locate the three instruction involved in a missed
+/// load-elimination case that is due to an intervening store.
+static void reportMayClobberedLoad(LoadInst *LI, MemDepResult DepInfo,
+                                   DominatorTree *DT,
+                                   OptimizationRemarkEmitter *ORE) {
+  using namespace ore;
+  User *OtherAccess = nullptr;
+
+  OptimizationRemarkMissed R(DEBUG_TYPE, "LoadClobbered", LI);
+  R << "load of type " << NV("Type", LI->getType()) << " not eliminated"
+    << setExtraArgs();
+
+  for (auto *U : LI->getPointerOperand()->users())
+    if (U != LI && (isa<LoadInst>(U) || isa<StoreInst>(U)) &&
+        DT->dominates(cast<Instruction>(U), LI)) {
+      // FIXME: for now give up if there are multiple memory accesses that
+      // dominate the load.  We need further analysis to decide which one is
+      // that we're forwarding from.
+      if (OtherAccess)
+        OtherAccess = nullptr;
+      else
+        OtherAccess = U;
+    }
+
+  if (OtherAccess)
+    R << " in favor of " << NV("OtherAccess", OtherAccess);
+
+  R << " because it is clobbered by " << NV("ClobberedBy", DepInfo.getInst());
+
+  ORE->emit(R);
+}
+
 bool GVN::AnalyzeLoadAvailability(LoadInst *LI, MemDepResult DepInfo,
                                   Value *Address, AvailableValue &Res) {
 
@@ -1273,6 +1305,10 @@ bool GVN::AnalyzeLoadAvailability(LoadInst *LI, MemDepResult DepInfo,
       Instruction *I = DepInfo.getInst();
       dbgs() << " is clobbered by " << *I << '\n';
     );
+
+    if (ORE->allowExtraAnalysis())
+      reportMayClobberedLoad(LI, DepInfo, DT, ORE);
+
     return false;
   }
   assert(DepInfo.isDef() && "follows from above");