[PATCH] Loosen the Gold ODR checker to only compare filenames

[Jeffrey Yasskin]

Here's a tweaked version in response to Ian's review.

On Thu, Feb 3, 2011 at 3:09 PM, Jeffrey Yasskin <jyasskin@google.com> wrote:
> This patch should do a better job. I went back to the
> substring-comparison the original code used, and chopped off the line
> number in addition to the directory. The code to remove the directory
> is different because the original code wasn't a transitive relation:
> it reported that "c:1" < "d/a:1" < "a/b:1" < "c:1".
>
> 2011-02-03  Jeffrey Yasskin  <jyasskin@google.com>
>
>      * symtab.cc: Sort by just the filename.
>
> On Thu, Feb 3, 2011 at 2:10 PM, Jeffrey Yasskin <jyasskin@google.com> wrote:
>> Whoops, please don't submit this. The strings that Source_location
>> points at don't live long enough to use them in the way I'm using
>> them. I'll send a working patch shortly.
>>
>> On Wed, Feb 2, 2011 at 10:44 PM, Jeffrey Yasskin <jyasskin@google.com> wrote:
>>> Code compiled with different flags, especially -O, may have a
>>> different line number for the first instruction in a function. This
>>> produces false positives in the ODR checker when linking object files
>>> that should be ABI-compatible.
>>>
>>> The best fixes would be to A) look at the DW_AT_decl_line of the
>>> DW_TAG_subprogram for the function, but this would require gold to
>>> parse a whole new debug section, or B) hash the ODR-relevant aspects
>>> of each function into a new dwarf attribute, but this would require
>>> gcc to produce the hash and gold to parse a whole new debug section.
>>>
>>> Instead, loosening the ODR check to allow a function's definitions to
>>> be from anywhere within the same file removes the false positives with
>>> much less work, and would have caused very few extra false negatives
>>> in Google's codebase.
>>>
>>> 2011-02-02  Jeffrey Yasskin  <jyasskin@google.com>
>>>
>>>       * dwarf_reader.h: Add a Source_location type, and change the
>>> addr2line functions to return it.
>>>       * dwarf_reader.cc: Implement Source_location, and change the
>>> addr2line functions to return it.
>>>       * symtab.cc: Sort by just the filename.
>>>       * object.cc: Convert a Source_location return to a std::string.
>>>
>>
>
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