[PATCH] D22693: More strongly separate the PDB reading interfaces and PDB writing interfaces

[Zachary Turner via llvm-commits]

zturner added inline comments.

================
Comment at: lib/DebugInfo/Msf/MappedBlockStream.cpp:191
@@ -162,4 +190,3 @@
   uint32_t NumAdditionalBlocks =
-      llvm::alignTo(Size - BytesFromFirstBlock, Msf.getBlockSize()) /
-      Msf.getBlockSize();
+      llvm::alignTo(Size - BytesFromFirstBlock, BlockSize) / BlockSize;
 
----------------
majnemer wrote:
> amccarth wrote:
> > Is this correct?  If `Size` is two blocks and `BytesFromFirstBlock` is less than `BlockSize`, then we'll get two additional blocks instead of one.
> Perhaps:
>   uint32_t NumAdditionalBlocks = (llvm::alignTo(Size, BlockSize) / BlockSize) - 1;
> ?
Are you sure?  Here's a concrete example:

```
// Starting from the beginning of block 2, read 2 full blocks of data.
BlockSize = 4096
Offset = 8192
Size = 8192

  // BlockNum = 8192 / 4096 = 2;
  uint32_t BlockNum = Offset / BlockSize;

  // OffsetInBlock = 8192 % 4096 = 0;
  uint32_t OffsetInBlock = Offset % BlockSize;

  // BytesFromFirstBlock = min(8192, 4096 - 0) = 4096
  uint32_t BytesFromFirstBlock = std::min(Size, BlockSize - OffsetInBlock);

  // NumAdditionalBlocks = alignTo(8192 - 4096, 4096) / 4096 = 4096 / 4096 = 1
  uint32_t NumAdditionalBlocks =
      llvm::alignTo(Size - BytesFromFirstBlock, BlockSize) / BlockSize;
```




https://reviews.llvm.org/D22693