[FuzzMutate] Fix getWeight of InstDeleterIRStrategy

[liblor]

Summary

The comment states the following, for calculating the Line variable:

Draw a line starting from when we only have 1k left and increasing linearly to double the current weight.

However, the value was not calculated as described. Instead, it would result in a negative value, which resulted in the function always returning 0 afterwards.

// Invariant: CurrentSize <= MaxSize - 200
// Invariant: CurrentWeight >= 0
int Line = (-2 * CurrentWeight) * (MaxSize - CurrentSize + 1000);
// {Line <= 0}

This commit fixes the issue and linearly interpolates as described.

[liblor]

@bogner I don't have commit access. Can you commit it?

[liblor]

In D96207#2589537, @liblor wrote:

@bogner I don't have commit access. Can you commit it?

Ping

[liblor]

@bogner Ping, could someone commit?

[xgupta]

CC @fhahn who has last committed to this file.

[bogner]

Oh sorry, I missed the comments asking for someone to commit this. I'll commit it for you later today.

llvm/lib/FuzzMutate/IRMutator.cpp

 
 uint64_t InstDeleterIRStrategy::getWeight(size_t CurrentSize, size_t MaxSize,
                                           uint64_t CurrentWeight) {
   // If we have less than 200 bytes, panic and try to always delete.
   if (CurrentSize > MaxSize - 200)
     return CurrentWeight ? CurrentWeight * 100 : 1;
   // Draw a line starting from when we only have 1k left and increasing linearly
   // to double the current weight.
-  int Line = (-2 * CurrentWeight) * (MaxSize - CurrentSize + 1000);
+  int64_t Line = (-2 * static_cast<int64_t>(CurrentWeight)) *
+                 (static_cast<int64_t>(MaxSize) -
+                  static_cast<int64_t>(CurrentSize) - 1000) /
+                 1000;
   // Clamp negative weights to zero.
   if (Line < 0)
     return 0;
   return Line;
 }
 
 void InstDeleterIRStrategy::mutate(Function &F, RandomIRBuilder &IB) {
   auto RS = makeSampler<Instruction *>(IB.Rand);