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

Colden Cullen via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Thu Jan 4 10:37:56 PST 2018

colden created this revision.
colden added a reviewer: zturner.
Herald added a subscriber: JDevlieghere.

The current interval counting math includes the two following issues:

- The Super Block (and Fpm0 when using Fpm1) are included in the block count, even though they don't have associated free page maps
- When calculating the number of existing free page maps, we should divide by block size + 1, to include the FPMs associated with the blocks

This error only occurs when the total number of blocks present is within 1 (up or down) of a multiple of 4096, so it's pretty rare. Mine was hitting right at 12289.

I'm 90% sure this is the correct fix for this, but if anyone can point out a better place I'm happy to poke around some more.




Index: include/llvm/DebugInfo/MSF/MSFCommon.h
--- include/llvm/DebugInfo/MSF/MSFCommon.h
+++ include/llvm/DebugInfo/MSF/MSFCommon.h
@@ -110,12 +110,15 @@
 inline uint32_t getNumFpmIntervals(const MSFLayout &L,
                                    bool IncludeUnusedFpmData = false) {
+  // Need to subtract the free block map index from the number of blocks
+  // so the blocks before Fpm (SB & Fpm0 if using Fpm1) aren't included
+  // Need to add 1 to block size so the interval includes the already present FPM blocks
   if (IncludeUnusedFpmData)
-    return divideCeil(L.SB->NumBlocks, L.SB->BlockSize);
+    return divideCeil(L.SB->NumBlocks - L.SB->FreeBlockMapBlock, L.SB->BlockSize + 1);
   // We want the minimum number of intervals required, where each interval can
   // represent BlockSize * 8 blocks.
-  return divideCeil(L.SB->NumBlocks, 8 * L.SB->BlockSize);
+  return divideCeil(L.SB->NumBlocks - L.SB->FreeBlockMapBlock, 8 * L.SB->BlockSize);
 Error validateSuperBlock(const SuperBlock &SB);

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