[LV] Fix to pr37515, FP primary induction crashes LV

[hsaito]

Summary

LV never had an ability to convert FP primary induction to INT primary induction, but it has been lacking an appropriate safety check.
Whether it's legal to convert FP primary induction to INT primary induction and if so under what conditions are debatable, but bailing
out when it's not proven safe (and currently never proven to be safe as far as LV's existing code is concerned) is a valid thing to do.

The bug may be introduced (or exposed depending on how you see it) by FP induction support ( approx. 2yrs ago?).

[hsaito]

lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

729

In pr37515, Ayal suggested going just "if (!WidestIndTy)". I agree.

He also suggested adding an assert for non-NULL IdxTy in the following place. Also makes sense.

This line in InnerLoopVectorizer::getOrCreateTripCount():

Type *IdxTy = Legal->getWidestInductionType();

[hsaito]

Will add a LIT test.

[hsaito]

LIT test added. ORE messaging doesn't seem to be working here (I see just "loop not vectorized" without the rest of the string), but that should be fixed separately.

lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

for (Instruction &I : *BB) {
                   << "value cannot be used outside the loop");
         return false;
       }
     } // next instr.
   }
 
   if (!PrimaryInduction) {
     LLVM_DEBUG(dbgs() << "LV: Did not find one integer induction var.\n");
-    if (Inductions.empty()) {
+    if (Inductions.empty() || !WidestIndTy) {
       ORE->emit(createMissedAnalysis("NoInductionVariable")
                 << "loop induction variable could not be identified");
       return false;
     }
   }
 
   // Now we know the widest induction type, check if our found induction
   // is the same size. If it's not, unset it here and InnerLoopVectorizer

test/Transforms/LoopVectorize/pr37515.ll

+; RUN: opt -passes='loop-vectorize' -S -pass-remarks-missed=loop-vectorize < %s 2>&1 | FileCheck %s
+;
+; FP primary induction is not supported in LV. Make sure Legal bails out.
+;
+; CHECK: loop not vectorized
+
+define void @PR37515() {
+entry:
+  br label %loop
+
+loop:
+  %p = phi float [ 19.0, %entry ], [ %a, %loop ]
+  %a = fadd fast float %p, -1.0
+  %m = fmul fast float %a, %a
+  %c = fcmp fast ugt float %a, 2.0
+  br i1 %c, label %loop, label %exit
+
+exit:
+  unreachable
+}