[llvm-branch-commits] [clang] [clang] "modular_format" attribute for functions using format strings (PR #147431)

[Aaron Ballman via llvm-branch-commits]

================
@@ -9427,3 +9427,37 @@ diagnostics with code like:
   __attribute__((nonstring)) char NotAStr[3] = "foo"; // Not diagnosed
   }];
 }
+
+def ModularFormatDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``modular_format`` attribute can be applied to a function that bears the
+``format`` attribute (or standard library functions) to indicate that the
+implementation is modular on the format string argument. When the format string
+for a given call is constant, the compiler may redirect the call to the symbol
+given as the first argument to the attribute (the modular implementation
+function).
+
+The second argument is a implementation name, and the remaining arguments are
+aspects of the format string for the compiler to report. If the compiler does
+not understand a aspect, it must summarily report that the format string has
+that aspect.
+
+The compiler reports an aspect by issing a relocation for the symbol
+``<impl_name>_<aspect>``. This arranges for code and data needed to support the
+aspect of the implementation to be brought into the link to satisfy weak
+references in the modular implemenation function.
+
+For example, say ``printf`` is annotated with
+``modular_format(__modular_printf, __printf, float)``. Then, a call to
+``printf(var, 42)`` would be untouched. A call to ``printf("%d", 42)`` would
+become a call to ``__modular_printf`` with the same arguments, as would
----------------
AaronBallman wrote:

So will any call to `printf` with a constant format specifier string be rewritten to call `__modular_printf`?

Also, who is responsible for writing these attributes? Are they only in the libc implementation, or can a user write one of these themselves on their own declarations? I'm asking because I wonder about compatibility; e.g., the call dispatches to `__modular_printf` but that doesn't know about some particular extension being used in the format specifier and so the code appears to misbehave.

https://github.com/llvm/llvm-project/pull/147431