[LV] Do not check widening decision for instrs outside of loop.

[fhahn]

Summary

No widening decisions will be computed for instructions outside the
loop. Do not try to get a widening decision. The load/store will be just
a scalar load, so treating at as normal should be fine I think.

Fixes PR46950.

[fhahn]

Reference correct PR in test name.

[dmgreen]

Oh right. Sorry I saw that bug but didn't put two and two together somehow.

Thanks for the fix. LGTM

[fhahn]

In D85087#2189402, @dmgreen wrote:

Oh right. Sorry I saw that bug but didn't put two and two together somehow.

Thanks for the fix. LGTM

Thanks!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

unsigned LoopVectorizationCostModel::getInstructionCost(Instruction *I,
   case Instruction::Trunc:
   case Instruction::FPTrunc:
   case Instruction::BitCast: {
     // Computes the CastContextHint from a Load/Store instruction.
     auto ComputeCCH = [&](Instruction *I) -> TTI::CastContextHint {
       assert((isa<LoadInst>(I) || isa<StoreInst>(I)) &&
              "Expected a load or a store!");
 
-      if (VF == 1)
+      if (VF == 1 || !TheLoop->contains(I))
         return TTI::CastContextHint::Normal;
 
       switch (getWideningDecision(I, VF)) {
       case LoopVectorizationCostModel::CM_GatherScatter:
         return TTI::CastContextHint::GatherScatter;
       case LoopVectorizationCostModel::CM_Interleave:
         return TTI::CastContextHint::Interleave;
       case LoopVectorizationCostModel::CM_Scalarize:

llvm/test/Transforms/LoopVectorize/AArch64/pr46950-load-cast-context-crash.ll

+; RUN: opt -loop-vectorize %s -mtriple=arm64-apple-iphoneos -S | FileCheck %s
+
+; CHECK-LABEL: define void @test(
+; CHECK: vector.body
+
+define void @test(i64* %dst, i32* %src) {
+entry:
+  %l = load i32, i32* %src
+  br label %loop.ph
+
+loop.ph:
+  br label %loop
+
+loop:
+  %iv = phi i64 [ 0, %loop.ph ], [ %iv.next, %loop ]
+  %l.cast = sext i32 %l to i64
+  %dst.idx = getelementptr i64, i64* %dst, i64 %iv
+  store i64 %l.cast, i64* %dst.idx
+  %iv.next = add nuw nsw i64 %iv, 1
+  %cmp9.us = icmp ult i64 %iv.next, 20
+  br i1 %cmp9.us, label %loop, label %exit
+
+exit:
+  ret void
+}