cxxrtl: reduce stack space used by the debug_info() function
#4380
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In C and C++, a `\x` escape sequence consumes as many hexadecimal digits as there are available, so it is not composable with arbitrary alnum characters afterwards. An octal escape sequence like `\000` always has fixed width, avoiding an issue where `\x01c` and `\x1c` produce the same string.
Before this commit, the creation of (constant) attribute maps caused `debug_info()` (which is built with `__attribute__((optnone))`) to consume large amounts of stack space; up to tens of megabytes. This caused problems particularly on macOS, where the default stack size is 512 KiB. After this commit, `std::map` objects are no longer created inline in the `debug_info()` function, but are compiled to and then expanded from a string literal in a subroutine call. This reduces stack space usage by about 50%.
Before this commit, this function would create a temporary `std::string` per debug item (and scope). After this commit, an additional overload is used to push that down the call stack. This reduces stack usage by about 50% more on top of the previous commit.
This commit uses parameter packs to sink `debug_item()` construction into the `debug_info()`-specific `add()` overload. This makes the stack space use sub-linear in typical case rather than linear (which is still the worst case). Oddly, the stack slots that get allocated now are all for the `0` literal for `lsb_offset`. This could be fixed by allocating numbers statically but the existing reduction in stack use of ~98% for a representative example (Minerva SoC) should be enough.
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The large amount of stack space used by this function is problematic especially on macOS, where the default stack size is 512 KiB.