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

[Jeffrey Yasskin <jyasskin@google.com>]

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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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Index: gold/symtab.cc
===================================================================
RCS file: /cvs/src/src/gold/symtab.cc,v
retrieving revision 1.146
diff -u -r1.146 symtab.cc
--- gold/symtab.cc	24 Jan 2011 21:48:40 -0000	1.146
+++ gold/symtab.cc	4 Feb 2011 19:19:49 -0000
@@ -3012,11 +3012,12 @@
 // We check for ODR violations by looking for symbols with the same
 // name for which the debugging information reports that they were
 // defined in different source locations.  When comparing the source
-// location, we consider instances with the same base filename and
-// line number to be the same.  This is because different object
-// files/shared libraries can include the same header file using
-// different paths, and we don't want to report an ODR violation in
-// that case.
+// location, we consider instances with the same base filename to be
+// the same.  This is because different object files/shared libraries
+// can include the same header file using different paths, and
+// different optimization settings can make the line number appear to
+// be a couple lines off, and we don't want to report an ODR violation
+// in those cases.
 
 // This struct is used to compare line information, as returned by
 // Dwarf_line_info::one_addr2line.  It implements a < comparison
@@ -3027,13 +3028,28 @@
   bool
   operator()(const std::string& s1, const std::string& s2) const
   {
-    std::string::size_type pos1 = s1.rfind('/');
-    std::string::size_type pos2 = s2.rfind('/');
-    if (pos1 == std::string::npos
-	|| pos2 == std::string::npos)
-      return s1 < s2;
-    return s1.compare(pos1, std::string::npos,
-		      s2, pos2, std::string::npos) < 0;
+    // Inputs should be of the form "dirname/filename:linenum" where
+    // "dirname/" is optional.  We want to compare just the filename.
+
+    // Find the last '/' and ':' in each string.
+    std::string::size_type s1begin = s1.rfind('/');
+    std::string::size_type s2begin = s2.rfind('/');
+    std::string::size_type s1end = s1.rfind(':');
+    std::string::size_type s2end = s2.rfind(':');
+    // If there was no '/' in a string, start at the beginning.
+    if (s1begin == std::string::npos)
+      s1begin = 0;
+    if (s2begin == std::string::npos)
+      s2begin = 0;
+    // If the ':' appeared in the directory name, compare to the end
+    // of the string.
+    if (s1end < s1begin)
+      s1end = s1.size();
+    if (s2end < s2begin)
+      s2end = s2.size();
+    // Compare takes lengths, not end indices.
+    return s1.compare(s1begin, s1end - s1begin,
+		      s2, s2begin, s2end - s2begin) < 0;
   }
 };