forked from OSchip/llvm-project
7cfa74fc69
Summary: The code in DWARFCompileUnit::BuildAddressRangeTable tries hard to avoid relying on DW_AT_low/high_pc for compile unit range information, and this logic is a big cause of llvm/lldb divergence in the lowest layers of dwarf parsing code. The implicit assumption in that code is that this information (as opposed to DW_AT_ranges) is unreliable. However, I have not been able to verify that assumption. It is definitely not true for all present-day compilers (gcc, clang, icc), and it was also not the case for the historic compilers that I have been able to get a hold of (thanks Matt Godbolt). All compiler included in my research either produced correct DW_AT_ranges or .debug_aranges entries, or they produced no DW_AT_hi/lo pc at all. The detailed findings are: - gcc >= 4.4: produces DW_AT_ranges and .debug_aranges - 4.1 <= gcc < 4.4: no DW_AT_ranges, no DW_AT_high_pc, .debug_aranges present. The upper version range here is uncertain as godbolt.org does not have intermediate versions. - gcc < 4.1: no versions on godbolt.org - clang >= 3.5: produces DW_AT_ranges, and (optionally) .debug_aranges - 3.4 <= clang < 3.5: no DW_AT_ranges, no DW_AT_high_pc, .debug_aranges present. - clang <= 3.3: no DW_AT_ranges, no DW_AT_high_pc, no .debug_aranges - icc >= 16.0.1: produces DW_AT_ranges - icc < 16.0.1: no functional versions on godbolt.org (some are present but fail to compile) Based on this analysis, I believe it is safe to start trusting DW_AT_low/high_pc information in dwarf as well as remove the code for manually reconstructing range information by traversing the DIE structure, and just keep the line table fallback. The only compilers where this will change behavior are pre-3.4 clangs, which are almost 7 years old now. However, the functionality should remain unchanged because we will be able to reconstruct this information from the line table, which seems to be needed for some line-tables-only scenarios anyway (haven't researched this too much, but at least some compilers seem to emit DW_AT_ranges even in these situations). In addition, benchmarks showed that for these compilers computing the ranges via line tables is noticably faster than doing so via the DIE tree. Other advantages include simplifying the code base, removing some untested code (the only test changes are recent tests with overly reduced synthetic dwarf), and increasing llvm convergence. Tags: #lldb Differential Revision: https://reviews.llvm.org/D78489 |
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| LICENSE.TXT | ||
| use_lldb_suite_root.py | ||