[llvm] [LoopPredication] Fix division by zero in case of zero branch weights (PR #66506)

[Danila Malyutin via llvm-commits]

================
@@ -1,12 +1,31 @@
-; XFAIL: *
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
 ; RUN: opt -S -loop-predication-skip-profitability-checks=false -passes='require<scalar-evolution>,loop-mssa(loop-predication)' %s | FileCheck %s
 
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: nocallback nofree nosync willreturn
 declare void @llvm.experimental.guard(i1, ...) #0
 
+; Check that LoopPredication doesn't crash on all-zero branch weights
 define void @foo() {
+; CHECK-LABEL: define void @foo() {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       Header:
+; CHECK-NEXT:    [[J2:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[J_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    call void (i1, ...) @llvm.experimental.guard(i1 false, i32 0) [ "deopt"() ]
+; CHECK-NEXT:    [[J_NEXT]] = add i64 [[J2]], 1
+; CHECK-NEXT:    br i1 false, label [[LATCH]], label [[EXIT:%.*]]
+; CHECK:       Latch:
+; CHECK-NEXT:    [[SPECULATE_TRIP_COUNT:%.*]] = icmp ult i64 [[J2]], 0
+; CHECK-NEXT:    br i1 [[SPECULATE_TRIP_COUNT]], label [[HEADER]], label [[COMMON_RET_LOOPEXIT:%.*]], !prof [[PROF0:![0-9]+]]
+; CHECK:       common.ret.loopexit:
+; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       exit:
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
----------------
danilaml wrote:

Right now, it just checks that there are simply no crashes. `experimental_guard` is required otherwise the LoopPredication pass would not run
```cpp
// There is nothing to do if the module doesn't use guards
  auto *GuardDecl =
      M->getFunction(Intrinsic::getName(Intrinsic::experimental_guard));
  bool HasIntrinsicGuards = GuardDecl && !GuardDecl->use_empty();
  auto *WCDecl = M->getFunction(
      Intrinsic::getName(Intrinsic::experimental_widenable_condition));
  bool HasWidenableConditions =
      PredicateWidenableBranchGuards && WCDecl && !WCDecl->use_empty();
  if (!HasIntrinsicGuards && !HasWidenableConditions)
    return false;
```

The test was reduced with bugpoint and llvm-reduce, so I don't know if it can be meaningfully reduced further (also why it doesn't really do much if there is no crash).

Trying to make it output something more meaningful would likely make the test bigger (and would require assertions).

https://github.com/llvm/llvm-project/pull/66506