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

[Alexey Samsonov]

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.

-- 
Alexey Samsonov, MSK
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