[PATCH] D60470: [DWARF] Prefer larger DW_AT_low_pc when constructing aranges

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

> Any idea why this problem hasn't been much of an issue until now/recently (both your change here, and the LLD change being reviewed)?

These `.debug_info` sections associated with garbage collected sections have low_pc/high_pc pairs in this form: `[0, high_pc)`  (their `DW_AT_low_pc` relocations resolve to zero).

ld.bfd sets default image base to 0x400000 for x86_64, thus the first good CU will have `DW_AT_low_pc` slightly larger than 0x400000. That bad CU needs to have `DW_AT_high_pc` larger than 0x400000 (4MiB) to overlap with a good CU. I think it is not common to have a >4MiB text section with `-ffunction-sections -fdata-sections -Wl,--gc-sections`.

lld has a different section layout. Its default image base is 0x200000 (2MiB) but it places potentially large sections `.dynsym` `.rodata` `.eh_frame` before `.text`. If the application is complex enough to have >2MiB `.text.*`, I'm fairly confident it should have sufficient large `.rodata` to move `.text` far away from the bad CU.

  [12] .rodata           PROGBITS        000000000020b950 00b950 006118 00 AMS  0   0 16                       
  ...
  [15] .eh_frame         PROGBITS        0000000000222330 022330 01a494 00   A  0   0  8                       
  [16] .text             PROGBITS        000000000023d000 03d000 0f6339 00  AX  0   0 16 


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  https://reviews.llvm.org/D60470/new/

https://reviews.llvm.org/D60470