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Slow algorithm used for string merging

From: Stephan Tobies
Subject: Slow algorithm used for string merging
Date: Tue, 30 Mar 2004 07:54:31 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.5) Gecko/20031007


I have a project where switching from gcc-2.95.x to gcc 3.2.2 has brought linking time from 10s to ~1h. (Yes, it is a really large project.) After some profiling, I have found that the merge_strings routine seems to be responsible to most of this time. Further analysis shows that the number of collisions in the hash table is causing the trouble.

I have seen other references to this problem, where the suggestion was to use the --fno-merge-strings option to gcc to solve the problem. Unfortunately, this does not really help this much because most of the strings that seem to be merged are debug information, which is always stored as mergable strings in the object files. Ok, there should be an option in gcc that also allows to generate debug info as non-mergable, but that does not solve the problem that the algorithm for string merging used by ld is rather poor and could use some improvement.

The --traditional-format option does not seem to solve the problem (at least not in my experiments) - I assume that this option is either ignored when generating stabs debug info, or it is only considered when writing thigs to a file, but not during the merging phase, which causes the hash tables to be built and then thrown away.

My question is: are there plans to work on this issue, or are there even some patches floating around that would solve this? Before I sit down and start writing patches for ld, I would like to check if there is not somebody else who has already done so, and, of course, if such patches would be welcome by the ld maintainers.

Best regards

Stephan Tobies

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