[VPlan] Don't build recipes for unconditional switches

In #157322 we crash because we try to infer a type for a VPReplicate switch recipe.

My understanding was that these switches should be removed by VPlanPredicator, but this switch survived through it because it was unconditional, i.e. had no cases other than the default case.

This fixes #157322 by not emitting any recipes for unconditional switches to begin with, similar to how we treat unconditional branches.

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -193,6 +193,9 @@ void PlainCFGBuilder::createVPInstructionsForVPBB(VPBasicBlock *VPBB,
     }
 
     if (auto *SI = dyn_cast<SwitchInst>(Inst)) {
+      // Don't emit recipes for unconditional switch instructions.
+      if (SI->getNumCases() == 0)
+        continue;
       SmallVector<VPValue *> Ops = {getOrCreateVPOperand(SI->getCondition())};
       for (auto Case : SI->cases())
         Ops.push_back(getOrCreateVPOperand(Case.getCaseValue()));

llvm/test/Transforms/LoopVectorize/predicate-switch.ll

@@ -490,18 +490,106 @@ loop.latch:
 exit:
   ret void
 }
+
+define void @switch_unconditional(ptr %start) {
+; IC1-LABEL: define void @switch_unconditional(
+; IC1-SAME: ptr [[START:%.*]]) {
+; IC1-NEXT:  [[ENTRY:.*:]]
+; IC1-NEXT:    br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
+; IC1:       [[VECTOR_PH]]:
+; IC1-NEXT:    br label %[[VECTOR_BODY:.*]]
+; IC1:       [[VECTOR_BODY]]:
+; IC1-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; IC1-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
+; IC1-NEXT:    [[TMP0:%.*]] = icmp eq i64 [[INDEX_NEXT]], 100
+; IC1-NEXT:    br i1 [[TMP0]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
+; IC1:       [[MIDDLE_BLOCK]]:
+; IC1-NEXT:    br label %[[EXIT:.*]]
+; IC1:       [[SCALAR_PH]]:
+; IC1-NEXT:    br label %[[LOOP:.*]]
+; IC1:       [[LOOP]]:
+; IC1-NEXT:    [[IV:%.*]] = phi i64 [ 0, %[[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], %[[LATCH:.*]] ]
+; IC1-NEXT:    [[GEP:%.*]] = getelementptr i32, ptr [[START]], i64 [[IV]]
+; IC1-NEXT:    [[X:%.*]] = load i32, ptr [[GEP]], align 4
+; IC1-NEXT:    switch i32 [[X]], label %[[FOO:.*]] [
+; IC1-NEXT:    ]
+; IC1:       [[FOO]]:
+; IC1-NEXT:    br label %[[LATCH]]
+; IC1:       [[LATCH]]:
+; IC1-NEXT:    [[IV_NEXT]] = add i64 [[IV]], 1
+; IC1-NEXT:    [[CMP:%.*]] = icmp eq i64 [[IV_NEXT]], 100
+; IC1-NEXT:    br i1 [[CMP]], label %[[EXIT]], label %[[LOOP]], !llvm.loop [[LOOP7:![0-9]+]]
+; IC1:       [[EXIT]]:
+; IC1-NEXT:    ret void
+;
+; IC2-LABEL: define void @switch_unconditional(
+; IC2-SAME: ptr [[START:%.*]]) {
+; IC2-NEXT:  [[ENTRY:.*:]]
+; IC2-NEXT:    br i1 false, label %[[SCALAR_PH:.*]], label %[[VECTOR_PH:.*]]
+; IC2:       [[VECTOR_PH]]:
+; IC2-NEXT:    br label %[[VECTOR_BODY:.*]]
+; IC2:       [[VECTOR_BODY]]:
+; IC2-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; IC2-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
+; IC2-NEXT:    [[TMP0:%.*]] = icmp eq i64 [[INDEX_NEXT]], 100
+; IC2-NEXT:    br i1 [[TMP0]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP6:![0-9]+]]
+; IC2:       [[MIDDLE_BLOCK]]:
+; IC2-NEXT:    br label %[[EXIT:.*]]
+; IC2:       [[SCALAR_PH]]:
+; IC2-NEXT:    br label %[[LOOP:.*]]
+; IC2:       [[LOOP]]:
+; IC2-NEXT:    [[IV:%.*]] = phi i64 [ 0, %[[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], %[[LATCH:.*]] ]
+; IC2-NEXT:    [[GEP:%.*]] = getelementptr i32, ptr [[START]], i64 [[IV]]
+; IC2-NEXT:    [[X:%.*]] = load i32, ptr [[GEP]], align 4
+; IC2-NEXT:    switch i32 [[X]], label %[[FOO:.*]] [
+; IC2-NEXT:    ]
+; IC2:       [[FOO]]:
+; IC2-NEXT:    br label %[[LATCH]]
+; IC2:       [[LATCH]]:
+; IC2-NEXT:    [[IV_NEXT]] = add i64 [[IV]], 1
+; IC2-NEXT:    [[CMP:%.*]] = icmp eq i64 [[IV_NEXT]], 100
+; IC2-NEXT:    br i1 [[CMP]], label %[[EXIT]], label %[[LOOP]], !llvm.loop [[LOOP7:![0-9]+]]
+; IC2:       [[EXIT]]:
+; IC2-NEXT:    ret void
+;
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %latch ]
+  %gep = getelementptr i32, ptr %start, i64 %iv
+  %x = load i32, ptr %gep
+  switch i32 %x, label %foo []
+
+foo:
+  br label %latch
+
+latch:
+  %iv.next = add i64 %iv, 1
+  %cmp = icmp eq i64 %iv.next, 100
+  br i1 %cmp, label %exit, label %loop
+
+exit:
+  ret void
+}
+
+
 ;.
 ; IC1: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
 ; IC1: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
 ; IC1: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
 ; IC1: [[LOOP3]] = distinct !{[[LOOP3]], [[META2]], [[META1]]}
 ; IC1: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]], [[META2]]}
 ; IC1: [[LOOP5]] = distinct !{[[LOOP5]], [[META2]], [[META1]]}
+; IC1: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]], [[META2]]}
+; IC1: [[LOOP7]] = distinct !{[[LOOP7]], [[META2]], [[META1]]}
 ;.
 ; IC2: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
 ; IC2: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
 ; IC2: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
 ; IC2: [[LOOP3]] = distinct !{[[LOOP3]], [[META2]], [[META1]]}
 ; IC2: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]], [[META2]]}
 ; IC2: [[LOOP5]] = distinct !{[[LOOP5]], [[META2]], [[META1]]}
+; IC2: [[LOOP6]] = distinct !{[[LOOP6]], [[META1]], [[META2]]}
+; IC2: [[LOOP7]] = distinct !{[[LOOP7]], [[META2]], [[META1]]}
 ;.