[Scalarizer] Centralize instruction DCE

As reported in https://reviews.llvm.org/D83101#2133062
the new visitInsertElementInst()/visitExtractElementInst() functionality
is causing miscompiles (previously-crashing test added)

It is due to the fact how the infra of Scalarizer is dealing with DCE,
it was not updated or was it ready for such scalar value forwarding.
It always assumed that the moment we "scalarized" something,
it can go away, and did so with prejudice.

But that is no longer safe/okay to do.

Instead, let's prevent it from ever shooting itself into foot,
and let's just accumulate the instructions-to-be-deleted
in a vector, and collectively cleanup (those that are *actually* dead)
them all at the end.

All existing tests are not reporting any new garbage leftovers,
but maybe it's test coverage issue.

lib/Transforms/Scalar/Scalarizer.cpp

@@ -22,8 +22,8 @@
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
-#include "llvm/IR/Dominators.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstVisitor.h"
@@ -41,6 +41,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include <cassert>
 #include <cstdint>
 #include <iterator>
@@ -222,6 +223,8 @@ private:
   ScatterMap Scattered;
   GatherList Gathered;
 
+  SmallVector<WeakTrackingVH, 32> PotentiallyDeadInstrs;
+
   unsigned ParallelLoopAccessMDKind;
 
   DominatorTree *DT;
@@ -383,11 +386,6 @@ Scatterer ScalarizerVisitor::scatter(Instruction *Point, Value *V) {
 // so that we can avoid creating the gathered form if all uses of Op are
 // replaced with uses of CV.
 void ScalarizerVisitor::gather(Instruction *Op, const ValueVector &CV) {
-  // Since we're not deleting Op yet, stub out its operands, so that it
-  // doesn't make anything live unnecessarily.
-  for (unsigned I = 0, E = Op->getNumOperands(); I != E; ++I)
-    Op->setOperand(I, UndefValue::get(Op->getOperand(I)->getType()));
-
   transferMetadataAndIRFlags(Op, CV);
 
   // If we already have a scattered form of Op (created from ExtractElements
@@ -402,7 +400,7 @@ void ScalarizerVisitor::gather(Instruction *Op, const ValueVector &CV) {
       Instruction *Old = cast<Instruction>(V);
       CV[I]->takeName(Old);
       Old->replaceAllUsesWith(CV[I]);
-      Old->eraseFromParent();
+      PotentiallyDeadInstrs.emplace_back(Old);
     }
   }
   SV = CV;
@@ -950,10 +948,13 @@ bool ScalarizerVisitor::finish() {
       Res->takeName(Op);
       Op->replaceAllUsesWith(Res);
     }
-    Op->eraseFromParent();
+    PotentiallyDeadInstrs.emplace_back(Op);
   }
   Gathered.clear();
   Scattered.clear();
+
+  RecursivelyDeleteTriviallyDeadInstructionsPermissive(PotentiallyDeadInstrs);
+
   return true;
 }
 

test/Transforms/Scalarizer/basic.ll

@@ -539,6 +539,19 @@ define <2 x float> @f22(<2 x float> %x, <2 x float> %y, <2 x float> %z) {
   ret <2 x float> %res
 }
 
+; See https://reviews.llvm.org/D83101#2133062
+define <2 x i32> @f23_crash(<2 x i32> %srcvec, i32 %v1) {
+; CHECK-LABEL: @f23_crash(
+; CHECK: %1 = extractelement <2 x i32> %srcvec, i32 0
+; CHECK: %t1.upto0 = insertelement <2 x i32> undef, i32 %1, i32 0
+; CHECK: %t1 = insertelement <2 x i32> %t1.upto0, i32 %v1, i32 1
+; CHECK: ret <2 x i32> %t1
+  %v0 = extractelement <2 x i32> %srcvec, i32 0
+  %t0 = insertelement <2 x i32> undef, i32 %v0, i32 0
+  %t1 = insertelement <2 x i32> %t0, i32 %v1, i32 1
+  ret <2 x i32> %t1
+}
+
 !0 = !{ !"root" }
 !1 = !{ !"set1", !0 }
 !2 = !{ !"set2", !0 }

test/Transforms/Scalarizer/crash-bug.ll

@@ -15,7 +15,6 @@ bb2:                                        ; preds = %bb1
 bb1:                                        ; preds = %bb2, %0
   %bb1_vec = phi <2 x i16> [ <i16 100, i16 200>, %0 ], [ %bb2_vec, %bb2 ]
 ;CHECK: bb1:
-;CHECK: %bb1_vec.i0 = phi i16 [ 100, %0 ], [ 0, %bb2 ]
 ;CHECK: %bb2_vec.i1 = phi i16 [ 200, %0 ], [ %bb2_vec.i1, %bb2 ]
   br i1 undef, label %bb3, label %bb2
 

test/Transforms/Scalarizer/phi-unreachable-pred.ll

@@ -11,11 +11,8 @@ define i16 @f1() {
 ; CHECK:       for.cond:
 ; CHECK-NEXT:    br i1 undef, label [[FOR_BODY:%.*]], label [[FOR_END]]
 ; CHECK:       for.end:
-; CHECK-NEXT:    [[PHI_I0:%.*]] = phi i16 [ 1, [[ENTRY:%.*]] ], [ undef, [[FOR_COND]] ]
-; CHECK-NEXT:    [[PHI_I1:%.*]] = phi i16 [ 1, [[ENTRY]] ], [ undef, [[FOR_COND]] ]
-; CHECK-NEXT:    [[PHI_I2:%.*]] = phi i16 [ 1, [[ENTRY]] ], [ undef, [[FOR_COND]] ]
-; CHECK-NEXT:    [[PHI_I3:%.*]] = phi i16 [ 1, [[ENTRY]] ], [ undef, [[FOR_COND]] ]
-; CHECK-NEXT:    ret i16 [[PHI_I0]]
+; CHECK-NEXT:    [[EXTRACT:%.*]] = phi i16 [ 1, [[ENTRY:%.*]] ], [ undef, [[FOR_COND]] ]
+; CHECK-NEXT:    ret i16 [[EXTRACT]]
 ;
 entry:
   br label %for.end