[LV] Strip unreachable SCEV-check blocks (#130079)

emitSCEVChecks checks if SCEVCheckCond matches zero, and returns
nullptr. However, it sets SCEVCheckCond as used before it does this,
which prevents it from being removed during cleanup, resulting in
unreachable blocks being emitted. Fix this.

Author: artagnon

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2094,18 +2094,11 @@ class GeneratedRTChecks {
   /// depending on the generated condition.
   BasicBlock *emitSCEVChecks(BasicBlock *Bypass,
                              BasicBlock *LoopVectorPreHeader) {
-    if (!SCEVCheckCond)
+    using namespace llvm::PatternMatch;
+    if (!SCEVCheckCond || match(SCEVCheckCond, m_ZeroInt()))
       return nullptr;
 
-    Value *Cond = SCEVCheckCond;
-    // Mark the check as used, to prevent it from being removed during cleanup.
-    SCEVCheckCond = nullptr;
-    if (auto *C = dyn_cast<ConstantInt>(Cond))
-      if (C->isZero())
-        return nullptr;
-
     auto *Pred = LoopVectorPreHeader->getSinglePredecessor();
-
     BranchInst::Create(LoopVectorPreHeader, SCEVCheckBlock);
     // Create new preheader for vector loop.
     if (OuterLoop)
@@ -2119,10 +2112,14 @@ class GeneratedRTChecks {
     DT->addNewBlock(SCEVCheckBlock, Pred);
     DT->changeImmediateDominator(LoopVectorPreHeader, SCEVCheckBlock);
 
-    BranchInst &BI = *BranchInst::Create(Bypass, LoopVectorPreHeader, Cond);
+    BranchInst &BI =
+        *BranchInst::Create(Bypass, LoopVectorPreHeader, SCEVCheckCond);
     if (AddBranchWeights)
       setBranchWeights(BI, SCEVCheckBypassWeights, /*IsExpected=*/false);
     ReplaceInstWithInst(SCEVCheckBlock->getTerminator(), &BI);
+
+    // Mark the check as used, to prevent it from being removed during cleanup.
+    SCEVCheckCond = nullptr;
     return SCEVCheckBlock;
   }
 

llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll

@@ -793,8 +793,6 @@ define void @multiple_exit_conditions(ptr %src, ptr noalias %dst) #1 {
 ; DEFAULT-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[DST]], [[ENTRY:%.*]] ]
 ; DEFAULT-NEXT:    [[BC_RESUME_VAL1:%.*]] = phi i64 [ 512, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY]] ]
 ; DEFAULT-NEXT:    br label [[LOOP:%.*]]
-; DEFAULT:       vector.scevcheck:
-; DEFAULT-NEXT:    unreachable
 ; DEFAULT:       loop:
 ; DEFAULT-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[PTR_IV_NEXT:%.*]], [[LOOP]] ]
 ; DEFAULT-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL1]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
@@ -857,8 +855,6 @@ define void @multiple_exit_conditions(ptr %src, ptr noalias %dst) #1 {
 ; PRED-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[DST]], [[ENTRY:%.*]] ]
 ; PRED-NEXT:    [[BC_RESUME_VAL2:%.*]] = phi i64 [ [[IND_END1]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY]] ]
 ; PRED-NEXT:    br label [[LOOP:%.*]]
-; PRED:       vector.scevcheck:
-; PRED-NEXT:    unreachable
 ; PRED:       loop:
 ; PRED-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[PTR_IV_NEXT:%.*]], [[LOOP]] ]
 ; PRED-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL2]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]

llvm/test/Transforms/LoopVectorize/X86/pr36524.ll

@@ -40,8 +40,6 @@ define void @foo(ptr %ptr, ptr %ptr.2) {
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 80, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ], [ 0, [[VECTOR_MEMCHECK]] ]
 ; CHECK-NEXT:    [[BC_RESUME_VAL2:%.*]] = phi i64 [ 82, [[MIDDLE_BLOCK]] ], [ 2, [[ENTRY]] ], [ 2, [[VECTOR_MEMCHECK]] ]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       vector.scevcheck:
-; CHECK-NEXT:    unreachable
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[CAN_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[CAN_IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[TMP9:%.*]] = phi i64 [ [[BC_RESUME_VAL2]], [[SCALAR_PH]] ], [ [[TMP12:%.*]], [[LOOP]] ]