[LV] Handle known-false mem runtime checks in GeneratedRTChecks.

Handle mem checks known to be false in getMemRuntimeChecks the same way
as SCEV checks known to be false in getSCEVChecks. This ensures such
redundant check blocks are not added in the first place.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2021,6 +2021,9 @@ class GeneratedRTChecks {
   /// Retrieves the MemCheckCond and MemCheckBlock that were generated as IR
   /// outside VPlan.
   std::pair<Value *, BasicBlock *> getMemRuntimeChecks() {
+    using namespace llvm::PatternMatch;
+    if (MemRuntimeCheckCond && match(MemRuntimeCheckCond, m_ZeroInt()))
+      return {nullptr, nullptr};
     return {MemRuntimeCheckCond, MemCheckBlock};
   }
 

llvm/test/Transforms/LoopVectorize/runtime-check-known-true.ll

@@ -24,9 +24,7 @@ define void @test_runtime_check_known_false_after_construction(ptr %start.1, ptr
 ; CHECK-NEXT:    [[TMP7:%.*]] = sub i3 [[TMP5]], [[TMP6]]
 ; CHECK-NEXT:    [[TMP8:%.*]] = zext i3 [[TMP7]] to i64
 ; CHECK-NEXT:    [[IDENT_CHECK:%.*]] = icmp ne i64 [[TMP8]], 0
-; CHECK-NEXT:    br i1 [[IDENT_CHECK]], label %[[SCALAR_PH]], label %[[VECTOR_MEMCHECK:.*]]
-; CHECK:       [[VECTOR_MEMCHECK]]:
-; CHECK-NEXT:    br i1 false, label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
+; CHECK-NEXT:    br i1 [[IDENT_CHECK]], label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
 ; CHECK:       [[VECTOR_PH]]:
 ; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i64 [[TMP4]], 4
 ; CHECK-NEXT:    [[N_VEC:%.*]] = sub i64 [[TMP4]], [[N_MOD_VF]]
@@ -56,8 +54,8 @@ define void @test_runtime_check_known_false_after_construction(ptr %start.1, ptr
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 [[TMP4]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label %[[EXIT:.*]], label %[[SCALAR_PH]]
 ; CHECK:       [[SCALAR_PH]]:
-; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[TMP10]], %[[MIDDLE_BLOCK]] ], [ [[START_1]], %[[ENTRY]] ], [ [[START_1]], %[[VECTOR_SCEVCHECK]] ], [ [[START_1]], %[[VECTOR_MEMCHECK]] ]
-; CHECK-NEXT:    [[BC_RESUME_VAL5:%.*]] = phi ptr [ [[TMP12]], %[[MIDDLE_BLOCK]] ], [ [[START_2_DIFF]], %[[ENTRY]] ], [ [[START_2_DIFF]], %[[VECTOR_SCEVCHECK]] ], [ [[START_2_DIFF]], %[[VECTOR_MEMCHECK]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[TMP10]], %[[MIDDLE_BLOCK]] ], [ [[START_1]], %[[ENTRY]] ], [ [[START_1]], %[[VECTOR_SCEVCHECK]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL5:%.*]] = phi ptr [ [[TMP12]], %[[MIDDLE_BLOCK]] ], [ [[START_2_DIFF]], %[[ENTRY]] ], [ [[START_2_DIFF]], %[[VECTOR_SCEVCHECK]] ]
 ; CHECK-NEXT:    br label %[[LOOP:.*]]
 ; CHECK:       [[LOOP]]:
 ; CHECK-NEXT:    [[PTR_IV_1:%.*]] = phi ptr [ [[PTR_IV_1_NEXT:%.*]], %[[LOOP]] ], [ [[BC_RESUME_VAL]], %[[SCALAR_PH]] ]