Ensure that symbols explicitly* assigned a section name are placed into a section with a compatible entry size.
This is done by creating multiple sections with the same name** if incompatible symbols are explicitly given the name of an incompatible section, whilst:
- Avoiding using uniqued sections where possible (for readability and to maximize compatibly with assemblers).
- Creating as few SHF_MERGE sections as possible (for efficiency).
Given that each symbol is assigned to a section in a single pass, we must decide which section each symbol is assigned to without seeing the properties of all symbols. A stable and easy to understand assignment is desirable. The following rules facilitate this: The "generic" section for a given section name will be mergeable if the name is a mergeable "default" section name (such as .debug_str), a mergeable "implicit" section name (such as .rodata.str2.2), or MC has already created a mergeable "generic" section for the given section name (e.g. in response to a section directive in inline assembly). Otherwise, the "generic" section for a given name is non-mergeable; and, non-mergeable symbols are assigned to the "generic" section, while mergeable symbols are assigned to uniqued sections.
"default" sections are those always created by MC initially, e.g. .text or .debug_str.
"implicit" sections are those created normally by MC in response to the symbols that it encounters, i.e. in the absence of an explicit section name assignment on the symbol, e.g. a function foo might be placed into a .text.foo section.
"generic" sections are those that are referred to when a unique section ID is not supplied, e.g. if there are multiple unique .bob sections then ".quad .bob" will reference the generic .bob section. Typically, the generic section is just the first section of a given name to be created. Default sections are always generic.
* Typically, section names might be explicitly assigned in source code using a language extension e.g. a section attribute: _attribute_ ((section ("section-name"))) - https://clang.llvm.org/docs/AttributeReference.html
** I refer to such sections as unique/uniqued sections. In assembly the ", unique," assembly syntax is used to express such sections.
See https://reviews.llvm.org/D68101 for previous discussions leading to this patch.
Some minor fixes were required to LLVM's tests, for tests had been using the old behavior - which allowed for explicitly assigning globals with incompatible entry sizes to a section.