https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125262
Bug ID: 125262
Summary: constexpr asm doesn't work with std::string
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ivan.lazaric.gcc at gmail dot com
Target Milestone: ---
In an asm statement AssemblerInstructions/AssemblerTemplate does not
have to be a string literal, it can also be a (parenthesized) constant
expression.
https://gcc.gnu.org/onlinedocs/gcc-16.1.0/gcc/Asm-constexprs.html
```
The constant expression can return a container with data () and size ()
member functions, following similar rules as the C++26 static_assert message.
```
Consider the following code snippet:
```cpp
#include <string>
#include <string_view>
#ifndef __GXX_CONSTEXPR_ASM__
# error "missing support for constexpr asm"
#endif
asm((std::string_view("")));
asm((std::string("")));
```
Compiler flags: "-std=c++26"
First asm statement with `std::string_view` works,
while the second one with `std::string` errors, with diagnostic:
```
<source>:9:21: error:
'std::string{std::__cxx11::basic_string<char>::_Alloc_hider{((char*)(&<anonymous>.std::__cxx11::basic_string<char>::<anonymous>.std::__cxx11::basic_string<char>::<unnamed
union>::_M_local_buf))}, 0, std::__cxx11::basic_string<char>::<unnamed
union>{char [16]{0, '\000', '\000', '\000', '\000', '\000', '\000', '\000',
'\000', '\000', '\000', '\000', '\000', '\000', '\000', '\000'}}}' is not a
constant expression
9 | asm((std::string("")));
| ^
<source>:9:21: error: 'std::__cxx11::basic_string<char>(((const char*)""),
std::allocator<char>())' is not a constant expression because it refers to an
incompletely initialized variable
```
Godbolt: https://godbolt.org/z/MaPv8bz4h
Noting that this is easy to avoid via `std::define_static_string()`
```cpp
consteval std::string_view simplify(const std::string& str) {
return std::string_view(std::define_static_string(str), str.size());
}
asm((simplify(std::string(""))));
```
