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Re: [PATCH] convert braced initializers to strings (PR 71625)


On 07/31/2018 07:38 AM, Jason Merrill wrote:
On Tue, Jul 31, 2018 at 9:51 AM, Martin Sebor <msebor@gmail.com> wrote:
The middle-end contains code to determine the lengths of constant
character arrays initialized by string literals.  The code is used
in a number of optimizations and warnings.

However, the code is unable to deal with constant arrays initialized
using the braced initializer syntax, as in

  const char a[] = { '1', '2', '\0' };

The attached patch extends the C and C++ front-ends to convert such
initializers into a STRING_CST form.

The goal of this work is to both enable existing optimizations for
such arrays, and to help detect bugs due to using non-nul terminated
arrays where nul-terminated strings are expected.  The latter is
an extension of the GCC 8 _Wstringop-overflow and
-Wstringop-truncation warnings that help detect or prevent reading
past the end of dynamically created character arrays.  Future work
includes detecting potential past-the-end reads from uninitialized
local character arrays.

  && TYPE_MAIN_VARIANT (TREE_TYPE (valtype)) == char_type_node)

Why? Don't we want this for other character types as well?

It suppresses narrowing warnings for things like

  signed char a[] = { 0xff };

(there are a couple of tests that exercise this).

At the same time, STRING_CST is supposed to be able to represent
strings of any integer type so there should be a way to make it
work.  On the flip side, recent discussions of changes in this
area suggest there may be bugs in the wide character handling of
STRING_CST so those would need to be fixed before relying on it
for robust support.

In any case, if you have a suggestion for how to make it work for
at least the narrow character types I'll adjust the patch.

Martin


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