[clang] [analyzer] Use AllocaRegion in MallocChecker (PR #72402)

[Endre Fülöp via cfe-commits]

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@@ -266,13 +266,18 @@ void CheckUseZeroAllocated1(void) {
 }
 
 char CheckUseZeroAllocated2(void) {
+  // FIXME: The return value of `alloca()` is modeled with `AllocaRegion`
+  // instead of `SymbolicRegion`, so the current implementation of
+  // `MallocChecker::checkUseZeroAllocated()` cannot handle it; and we get an
+  // unrelated, but suitable warning from core.uninitialized.UndefReturn.
   char *p = alloca(0);
-  return *p; // expected-warning {{Use of memory allocated with size zero}}
+  return *p; // expected-warning {{Undefined or garbage value returned to caller}}
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gamesh411 wrote:

Even if it is not the real question, what we are to do with the 0-size `alloca` calls, but just to highlight some practical concerns, I found these sources:
https://discourse.llvm.org/t/malloc-free-and-alloca-with-zero-size/9284/3
https://stackoverflow.com/questions/8036654/what-does-alloca0-do-and-return-on-various-platforms

So `alloca(0)` sometimes has a special meaning. If we can give more specific error messages in these cases, I would prefer to handle those error messages in the more specific checker.
Even if ArrayBoundV2 has more user-friendly and mature error reporting (and would cover this case strictly speaking), making this more specific checker emit better diagnostics as well is something worth considering IMO.

https://github.com/llvm/llvm-project/pull/72402