[libc-commits] [PATCH] D131993: [libc] add compile option for printf arg type array

[Michael Jones via Phabricator via libc-commits]

michaelrj updated this revision to Diff 455971.
michaelrj marked an inline comment as done.
michaelrj added a comment.

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


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D131993/new/

https://reviews.llvm.org/D131993

Files:
  libc/src/stdio/printf_core/parser.h
  libc/src/stdio/printf_core/printf_config.h


Index: libc/src/stdio/printf_core/printf_config.h
===================================================================
--- /dev/null
+++ libc/src/stdio/printf_core/printf_config.h
@@ -0,0 +1,34 @@
+//===-- Printf Configuration Handler ----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PRINTF_CONFIG_H
+#define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PRINTF_CONFIG_H
+
+// The index array 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.
+// When an index mode argument is requested, if its index is before the most
+// recently read index, then the arg list must be restarted from the beginning,
+// and all of the arguments before the new index must be requested with the
+// correct types. The index array caches the types of the values in the arg
+// list. For every number between the last index cached in the array and the
+// requested index, the format string must be parsed again to find the
+// type of that index. As an example, if the format string has 20 indexes, and
+// the index array is 10, then when the 20th index is requested the first 10
+// types can be found immediately, and then the format string must be parsed 10
+// times to find the types of the next 10 arguments.
+#ifndef LLVM_LIBC_PRINTF_INDEX_ARR_LEN
+#define LLVM_LIBC_PRINTF_INDEX_ARR_LEN 128
+#endif
+
+// TODO(michaelrj): Move the other printf configuration options into this file.
+
+#endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PRINTF_CONFIG_H
Index: libc/src/stdio/printf_core/parser.h
===================================================================
--- libc/src/stdio/printf_core/parser.h
+++ libc/src/stdio/printf_core/parser.h
@@ -11,6 +11,7 @@
 
 #include "src/__support/arg_list.h"
 #include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/printf_config.h"
 
 #include <stddef.h>
 
@@ -41,8 +42,10 @@
       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;
+
+  // Defined in printf_config.h
+  static constexpr size_t DESC_ARR_LEN = LLVM_LIBC_PRINTF_INDEX_ARR_LEN;
+
   // 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.


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