# Clarification of Address Range Overlap Rules for Functions and Basic Blocks
## Background Modern compiler optimizations and linker features can result in functions or basic blocks sharing the same address ranges in the final binary. Two common cases are: 1. Identical Code Folding (ICF) at link time, where identical functions are merged to reduce code size 2. Basic block section optimizations (e.g., `-fbasic-block-sections=all`), where basic blocks from different functions may be reordered and identical blocks merged The current DWARF 5 specification's statement in Section 2.17.3 (p. 53, line 15) that "Bounded range entries in a range list may not overlap" is overly restrictive and doesn't reflect valid cases where identical code sequences are merged. The specification needs to differentiate between valid identical overlaps and invalid partial overlaps, both within range lists and between different `DW_TAG_subprogram` entries. ## Overview This proposal seeks to: 1. Correct and clarify the rules regarding overlapping ranges within a single range list 2. Clarify rules for overlapping function ranges specified via DW_AT_low_pc/DW_AT_high_pc 3. Address the validity of identical overlapping ranges resulting from code merging optimizations ## Proposed Changes In Section 2.17 (Location Descriptions), replace the text on p. 53, line 15: From: ``` Bounded range entries in a range list may not overlap. There is no requirement that the entries be ordered in any particular way. ``` To: ``` Bounded range entries in a range list must not partially overlap. However, range entries may be exactly identical (having the same low and high PC values), which commonly occurs when compiler or linker optimizations merge identical code sequences. A partial overlap occurs when two ranges share some but not all addresses. There is no requirement that the entries be ordered in any particular way. Similarly, when ranges are specified using DW_AT_low_pc and DW_AT_high_pc attributes, multiple debugging information entries (such as DW_TAG_subprogram entries) may specify identical ranges when they represent code that has been merged by optimizations. However, partial overlaps between ranges are not permitted, as this would create ambiguity in determining which code sequence is being executed at a given address. ``` ## Dependencies None ## References 1. LLVM Pull Request demonstrating the need for this clarification (+ additional discussion): https://github.com/llvm/llvm-project/pull/117952
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