Revert "[InstCombine] avoid crash from deleting an instruction that still has uses (PR43723) (3rd try)"

This reverts commit 3db8a3e.
This caused a different memory-sanitizer failure than earlier attempts,
but it's still not right.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2340,21 +2340,13 @@ static bool isAllocSiteRemovable(Instruction *AI,
               return false;
             LLVM_FALLTHROUGH;
           }
+          case Intrinsic::invariant_start:
           case Intrinsic::invariant_end:
           case Intrinsic::lifetime_start:
           case Intrinsic::lifetime_end:
           case Intrinsic::objectsize:
             Users.emplace_back(I);
             continue;
-          case Intrinsic::invariant_start:
-            // Only delete this if it has no uses or a single 'end' use.
-            if (I->use_empty())
-              Users.emplace_back(I);
-            else if (I->hasOneUse() &&
-                     match(I->user_back(),
-                           m_Intrinsic<Intrinsic::invariant_end>()))
-              Users.emplace_back(I);
-            continue;
           }
         }
 
@@ -2402,27 +2394,21 @@ Instruction *InstCombiner::visitAllocSite(Instruction &MI) {
 
   if (isAllocSiteRemovable(&MI, Users, &TLI)) {
     for (unsigned i = 0, e = Users.size(); i != e; ++i) {
+      // Lowering all @llvm.objectsize calls first because they may
+      // use a bitcast/GEP of the alloca we are removing.
       if (!Users[i])
        continue;
 
       Instruction *I = cast<Instruction>(&*Users[i]);
+
       if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
-        // Lowering all @llvm.objectsize calls first because they may
-        // use a bitcast/GEP of the alloca we are removing.
         if (II->getIntrinsicID() == Intrinsic::objectsize) {
           Value *Result =
               lowerObjectSizeCall(II, DL, &TLI, /*MustSucceed=*/true);
           replaceInstUsesWith(*I, Result);
           eraseInstFromFunction(*I);
           Users[i] = nullptr; // Skip examining in the next loop.
         }
-        // Erase llvm.invariant.start because we expect that it is used by
-        // llvm.invariant.end that we will remove below.
-        if (II->getIntrinsicID() == Intrinsic::invariant_start) {
-          replaceInstUsesWith(*I, UndefValue::get(I->getType()));
-          eraseInstFromFunction(*I);
-          Users[i] = nullptr; // Skip examining in the next loop.
-        }
       }
     }
     for (unsigned i = 0, e = Users.size(); i != e; ++i) {

llvm/test/Transforms/InstCombine/builtin-object-size-ptr.ll

@@ -28,41 +28,6 @@ define i32 @foo() #0 {
   ret i32 %conv
 }
 
-; This used to crash while erasing instructions:
-; https://bugs.llvm.org/show_bug.cgi?id=43723
-
-define void @PR43723() {
-; CHECK-LABEL: @PR43723(
-; CHECK-NEXT:    ret void
-;
-  %tab = alloca [10 x i8], align 16
-  %t0 = bitcast [10 x i8]* %tab to i8*
-  call void @llvm.memset.p0i8.i64(i8* align 16 %t0, i8 9, i64 10, i1 false)
-  %t1 = call {}* @llvm.invariant.start.p0i8(i64 10, i8* align 16 %t0)
-  call void @llvm.invariant.end.p0i8({}* %t1, i64 10, i8* align 16 %t0)
-  ret void
-
-  uselistorder i8* %t0, { 1, 0, 2 }
-}
-
-define void @unknown_use_of_invariant_start({}** %p) {
-; CHECK-LABEL: @unknown_use_of_invariant_start(
-; CHECK-NEXT:    [[T1:%.*]] = call {}* @llvm.invariant.start.p0i8(i64 10, i8* align 16 undef)
-; CHECK-NEXT:    store {}* [[T1]], {}** [[P:%.*]], align 8
-; CHECK-NEXT:    ret void
-;
-  %tab = alloca [10 x i8], align 16
-  %t0 = bitcast [10 x i8]* %tab to i8*
-  call void @llvm.memset.p0i8.i64(i8* align 16 %t0, i8 9, i64 10, i1 false)
-  %t1 = call {}* @llvm.invariant.start.p0i8(i64 10, i8* align 16 %t0)
-  call void @llvm.invariant.end.p0i8({}* %t1, i64 10, i8* align 16 %t0)
-  store {}* %t1, {}** %p
-  ret void
-}
-
 declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture) #1
 declare i64 @llvm.objectsize.i64.p0i8(i8*, i1) #2
 declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture) #1
-declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg) #0
-declare {}* @llvm.invariant.start.p0i8(i64 immarg, i8* nocapture) #0
-declare void @llvm.invariant.end.p0i8({}*, i64 immarg, i8* nocapture) #0