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

[Alexey Samsonov]

On Mon, Jun 17, 2013 at 7:28 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

>
> 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.
>

I see.  Another question is whether Clang should always do constant
initialization, although it's not required.


-- 
Alexey Samsonov, MSK
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