[libc] add compile option for printf arg type array

[michaelrj]

Summary

This patch allows for adjusting the size of the array that printf uses
to track the types of arguments in index mode. This is useful for
optimizing the tradeoff between memory usage and performance.

[sivachandra]

libc/src/stdio/printf_core/parser.h

51

Normally, config params like this are all put in one single .h file, say in this printf_parser_config.h:

#ifndef LLVM_LIBC_PRINTF_INDEX_ARR_LEN
// Explain what this param is about.
#define LLVM_LIBC_PRINTF_INDEX_ARR_LEN 128
#endif

Include printf_parser_config.h in parser.h. Not sure if we really want a separate config file for the parser, may be just a printf_config.h file is sufficient.

[michaelrj]

move the config into a separate file.

[michaelrj]

rebase

[sivachandra]

libc/src/stdio/printf_core/printf_config.h

17 ↗

(On Diff #455281)

Nit: Give an example of what that "severity" will look like.

[michaelrj]

add description of how the index array being too small effects runtime.

libc/src/stdio/printf_core/parser.h

#ifndef LLVM_LIBC_PRINTF_DISABLE_INDEX_MODE
   // a relatively compact manner.
   struct TypeDesc {
     uint8_t size;
     PrimaryType primary_type;
     constexpr bool operator==(const TypeDesc &other) const {
       return (size == other.size) && (primary_type == other.primary_type);
     }
   };
-  // TODO: Make this size configurable via a compile option.
-  static constexpr size_t DESC_ARR_LEN = 32;
+// This buffer is always initialized when printf is called. In cases where index
+// mode is necessary but memory is limited, or when index mode performance is
+// important and memory is available, this compile option provides a knob to
+// adjust memory usage to an appropriate level. 128 is picked as the default
+// size since that's big enough to handle even extreme cases and the runtime
+// penalty for not having enough space is severe.
+#ifndef LLVM_LIBC_PRINTF_INDEX_ARR_LEN
+  static constexpr size_t DESC_ARR_LEN = 128;
+#else
+  static constexpr size_t DESC_ARR_LEN = LLVM_LIBC_PRINTF_INDEX_ARR_LEN;
+#endif
   // desc_arr stores the sizes of the variables in the ArgList. This is used in
   // index mode to reduce repeated string parsing. The sizes are stored as
   // TypeDesc objects, which store the size as well as minimal type information.
   // This is necessary because some systems separate the floating point and
   // integer values in va_args.
-  TypeDesc desc_arr[DESC_ARR_LEN];
+  TypeDesc desc_arr[DESC_ARR_LEN] = {{0, Integer}};
 
   // TODO: Look into object stores for optimization.
 
 #endif // LLVM_LIBC_PRINTF_DISABLE_INDEX_MODE
 
 public:
 #ifndef LLVM_LIBC_PRINTF_DISABLE_INDEX_MODE
   Parser(const char *__restrict new_str, internal::ArgList &args)
-      : str(new_str), args_cur(args), args_start(args) {
-    inline_memset(reinterpret_cast<char *>(desc_arr), 0,
-                  DESC_ARR_LEN * sizeof(TypeDesc));
-  }
+      : str(new_str), args_cur(args), args_start(args) {}
 #else
   Parser(const char *__restrict new_str, internal::ArgList &args)
       : str(new_str), args_cur(args) {}
 #endif // LLVM_LIBC_PRINTF_DISABLE_INDEX_MODE
 
   // get_next_section will parse the format string until it has a fully
   // specified format section. This can either be a raw format section with no
   // conversion, or a format section with a conversion that has all of its