[cfe-dev] Treating constant strings as dynamically initialized globals

[Eli Friedman]

On Mon, Jun 17, 2013 at 1:50 AM, Alexey Samsonov <samsonov at google.com>wrote:

> Hi cfe-dev!
>
> Consider the following code:
> $ cat str.cc
> extern const char* const AAA;
> extern const char* const BBB;
> const char* const AAA = "aaa";
> const char* const BBB = AAA;
>
> "g++ -O0" and "clang++ -O0" behave differently: gcc emits both AAA and BBB
> as
> linker-initialized constants, while Clang initializes BBB during static
> initialization:
> $ g++ -O0 -c str.cc -o a.o ; objdump -d a.o
> a.o:     file format elf64-x86-64
> $ ./bin/clang++ -O0 -c str.cc -o a.o ; objdump -d a.o
> a.o:     file format elf64-x86-64
> Disassembly of section .text.startup:
> 0000000000000000 <__cxx_global_var_init>:
>    0: 55                   push   %rbp
>    1: 48 89 e5             mov    %rsp,%rbp
>    4: 48 8b 04 25 00 00 00 mov    0x0,%rax
>    b: 00
>    c: 48 89 04 25 00 00 00 mov    %rax,0x0
>   13: 00
>   14: 5d                   pop    %rbp
>   15: c3                   retq
>   16: 66 2e 0f 1f 84 00 00 nopw   %cs:0x0(%rax,%rax,1)
>   1d: 00 00 00
> 0000000000000020 <_GLOBAL__I_a>:
>   20: 55                   push   %rbp
>   21: 48 89 e5             mov    %rsp,%rbp
>   24: e8 d7 ff ff ff       callq  0 <__cxx_global_var_init>
>   29: 5d                   pop    %rbp
>   2a: c3                   retq
>
>
> Is Clang correct here? I imagine that user who wrote the code might expect
> the
> "constant strings" to go in .rodata and be safely accessible during static
> initialization.
>
>
clang is correct in the sense that the C++ standard doesn't require the
compiler to use static initialization for BBB.  [basic.start.init] is the
relevant section in the standard; "AAA" doesn't count as a constant
expression.  You can "fix" this using constexpr.

-Eli
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20130617/9c3345b7/attachment.html>