[MemoryLocation] Support strncpy in getForArgument.

The size argument of strncpy can be used as bound for the size of
its pointer arguments.

strncpy is guaranteed to write N bytes and reads up to N bytes.

Reviewed By: xbolva00

Differential Revision: https://reviews.llvm.org/D114871

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/MemoryLocation.cpp b/llvm/lib/Analysis/MemoryLocation.cpp
index b4eb06d..854ba83 100644
--- a/llvm/lib/Analysis/MemoryLocation.cpp
+++ b/llvm/lib/Analysis/MemoryLocation.cpp
@@ -223,6 +223,18 @@ MemoryLocation MemoryLocation::getForArgument(const CallBase *Call,
       }
       return MemoryLocation(Arg, Size, AATags);
     }
+    case LibFunc_strncpy: {
+      assert((ArgIdx == 0 || ArgIdx == 1) &&
+             "Invalid argument index for strncpy");
+      LocationSize Size = LocationSize::afterPointer();
+      if (const auto *Len = dyn_cast<ConstantInt>(Call->getArgOperand(2))) {
+        // strncpy is guaranteed to write Len bytes, but only reads up to Len
+        // bytes.
+        Size = ArgIdx == 0 ? LocationSize::precise(Len->getZExtValue())
+                           : LocationSize::upperBound(Len->getZExtValue());
+      }
+      return MemoryLocation(Arg, Size, AATags);
+    }
     case LibFunc_memset_pattern16:
       assert((ArgIdx == 0 || ArgIdx == 1) &&
              "Invalid argument index for memset_pattern16");