[clang-tidy] Fix a false positive in google-runtime-memset

google-runtime-memset no longer issues a warning if the fill char value
is known to be an invalid fill char count.

Namely, it no longer warns for these:

  memset(p, 0, 0);
  memset(p, -1, 0);

In both cases, swapping the last two args would either be useless (there is
no actual bug) or wrong (it would introduce a bug).

Patch by Matt Armstrong!

llvm-svn: 257320

2 files changed, 17 insertions, 11 deletions

diff --git a/clang-tools-extra/clang-tidy/google/MemsetZeroLengthCheck.cpp b/clang-tools-extra/clang-tidy/google/MemsetZeroLengthCheck.cpp
index 4c0b8e01f22..bcf4b5b3b61 100644
--- a/clang-tools-extra/clang-tidy/google/MemsetZeroLengthCheck.cpp
+++ b/clang-tools-extra/clang-tidy/google/MemsetZeroLengthCheck.cpp
@@ -51,25 +51,29 @@ static StringRef getAsString(const MatchFinder::MatchResult &Result,
 void MemsetZeroLengthCheck::check(const MatchFinder::MatchResult &Result) {
   const auto *Call = Result.Nodes.getNodeAs<CallExpr>("decl");
 
+  // Note, this is:
+  // void *memset(void *buffer, int fill_char, size_t byte_count);
+  // Arg1 is fill_char, Arg2 is byte_count.
   const Expr *Arg1 = Call->getArg(1);
   const Expr *Arg2 = Call->getArg(2);
 
-  // Try to evaluate the second argument so we can also find values that are not
-  // just literals.
+  // Return if `byte_count` is not zero at compile time.
   llvm::APSInt Value1, Value2;
   if (Arg2->isValueDependent() ||
       !Arg2->EvaluateAsInt(Value2, *Result.Context) || Value2 != 0)
     return;
 
-  // If both arguments evaluate to zero emit a warning without fix suggestions.
+  // Return if `fill_char` is known to be zero or negative at compile
+  // time. In these cases, swapping the args would be a nop, or
+  // introduce a definite bug. The code is likely correct.
   if (!Arg1->isValueDependent() &&
       Arg1->EvaluateAsInt(Value1, *Result.Context) &&
-      (Value1 == 0 || Value1.isNegative())) {
-    diag(Call->getLocStart(), "memset of size zero");
+      (Value1 == 0 || Value1.isNegative()))
     return;
-  }
 
-  // Emit a warning and fix-its to swap the arguments.
+  // `byte_count` is known to be zero at compile time, and `fill_char` is
+  // either not known or known to be a positive integer. Emit a warning
+  // and fix-its to swap the arguments.
   auto D = diag(Call->getLocStart(),
                 "memset of size zero, potentially swapped arguments");
   SourceRange LHSRange = Arg1->getSourceRange();
diff --git a/clang-tools-extra/test/clang-tidy/google-runtime-memset-zero-length.cpp b/clang-tools-extra/test/clang-tidy/google-runtime-memset-zero-length.cpp
index db75cb0fedd..7599c755b48 100644
--- a/clang-tools-extra/test/clang-tidy/google-runtime-memset-zero-length.cpp
+++ b/clang-tools-extra/test/clang-tidy/google-runtime-memset-zero-length.cpp
@@ -48,13 +48,15 @@ void foo(void *a, int xsize, int ysize) {
 
   memset(a, -1, sizeof(int));
   memset(a, 0xcd, 1);
+
+  // Don't warn when the fill char and the length are both known to be
+  // zero.  No bug is possible.
+  memset(a, 0, v);
   memset(a, v, 0);
-// CHECK-MESSAGES: :[[@LINE-1]]:3: warning: memset of size zero
-// CHECK-FIXES: memset(a, v, 0);
 
+  // -1 is clearly not a length by virtue of being negative, so no warning
+  // despite v == 0.
   memset(a, -1, v);
-// CHECK-MESSAGES: :[[@LINE-1]]:3: warning: memset of size zero
-// CHECK-FIXES: memset(a, -1, v);
 
   memtmpl<0, int>();
 }