[MemoryLocation] Support strncpy in getForArgument.

[fhahn]

Summary

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.

llvm/lib/Analysis/MemoryLocation.cpp

case LibFunc_memset_chk: {
       LocationSize Size = LocationSize::afterPointer();
       if (const auto *Len = dyn_cast<ConstantInt>(Call->getArgOperand(2))) {
         // memset_chk writes at most Len bytes. It may write less, if Len
         // exceeds the specified max size and aborts.
         Size = LocationSize::upperBound(Len->getZExtValue());
       }
       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");
       if (ArgIdx == 1)
         return MemoryLocation(Arg, LocationSize::precise(16), AATags);
       if (const ConstantInt *LenCI =
               dyn_cast<ConstantInt>(Call->getArgOperand(2)))
         return MemoryLocation(Arg, LocationSize::precise(LenCI->getZExtValue()),

llvm/test/Analysis/BasicAA/libfuncs.ll

 declare i8* @strncpy(i8*, i8*, i64)
 
 define i8* @test_strncpy_const_size(i8* noalias %a, i8* noalias %b) {
 ; CHECK-LABEL: Function: test_strncpy_const_size
 ; CHECK:       Just Mod:  Ptr: i8* %a <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Ref:  Ptr: i8* %b <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %res <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.1 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
-; CHECK-NEXT:  Just Mod:  Ptr: i8* %a.gep.5 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %a.gep.5 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.1 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
-; CHECK-NEXT:  Just Ref:  Ptr: i8* %b.gep.5 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
+; CHECK-NEXT:  NoModRef:  Ptr: i8* %b.gep.5 <->  %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
 ;
 entry:
   %res = tail call i8* @strncpy(i8* %a, i8* %b, i64 4)
   %a.gep.1 = getelementptr i8, i8* %a, i32 1
   store i8 0, i8* %a.gep.1
   %a.gep.5 = getelementptr i8, i8* %a, i32 5
   store i8 1, i8* %a.gep.5
   %b.gep.1 = getelementptr i8, i8* %b, i32 1