[PATCH] D41734: [DebugInfo][PDB] Fix too many FPM blocks being written in some cases

Zachary Turner via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Jan 4 13:25:12 PST 2018

zturner added a comment.

No worries on the repro, I actually got one myself, and I now actually understand what's causing the problem.

Honestly the repro is trivial.  All you need to do is edit `MappedBlockStreamTest.cpp` line 512 to look like this:

  constexpr uint32_t NumFileBlocks = 4096 + 1;

And it will crash.  No other number works except +1, so -1 doesn't work.  But because of this I now think I know *why* it is crashing.  At every stride there are **two** FPMs.  So basically FPM0 starts at block 1 and has a new piece every 4096 blocks.  FPM1 starts at block 2 and has a new piece every 4096 blocks.  So this means FPM0 has a piece at 4097 and FPM1 has a piece at 4098.  I think this is what's causing the problem.  We create an MSF file that only has 4097 blocks *total*, and then we try to "initialize" the FPM page at block 4098.

I think the fix is probably as simple as

  if (TotalFileBlocks - 1 % BlockSize == 0)

I'm playing around with this locally in the meantime.



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