[llvm] 7e976cd - [FuzzMutate] Fix getWeight of InstDeleterIRStrategy

[Justin Bogner via llvm-commits]

Author: Justin Bogner
Date: 2021-06-08T11:14:33-07:00
New Revision: 7e976cd4568517df4020845682d31f326907a5df

URL: https://github.com/llvm/llvm-project/commit/7e976cd4568517df4020845682d31f326907a5df
DIFF: https://github.com/llvm/llvm-project/commit/7e976cd4568517df4020845682d31f326907a5df.diff

LOG: [FuzzMutate] Fix getWeight of InstDeleterIRStrategy

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.

Patch by Loris Reiff. Thanks!

Differential Revision: https://reviews.llvm.org/D96207

Added: 
    

Modified: 
    llvm/lib/FuzzMutate/IRMutator.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/FuzzMutate/IRMutator.cpp b/llvm/lib/FuzzMutate/IRMutator.cpp
index 33b90097ab2c..0cd0f538fdbc 100644
--- a/llvm/lib/FuzzMutate/IRMutator.cpp
+++ b/llvm/lib/FuzzMutate/IRMutator.cpp
@@ -143,7 +143,10 @@ uint64_t InstDeleterIRStrategy::getWeight(size_t CurrentSize, size_t MaxSize,
     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;