[PATCH] D104745: [llvm-cov] Enforce alignment of function records

[serge via Phabricator via llvm-commits]

serge-sans-paille added a comment.

In D104745#2834796 <https://reviews.llvm.org/D104745#2834796>, @vsk wrote:

> The requirement is that the advancing by sizeof(CovMapFunctionRecordV3) bytes in the record buffer advances to the start of the next record. The address of the buffer doesn't need the same alignment.

The function `advanceByOne(...)` starts with the following assert: ` assert(isAddrAligned(Align(8), this) && "Function record not aligned");` so I tend to think each record needs to be aligned, including the first.

Take the case of the first record created, and assume it has a size multiple of 8. In that case, upon creation no padding is added (independently of it's start address). That's what the snippet below does

  while (FuncRecords.size() % 8 != 0)
    FuncRecords += '\0';

Now let's assume the start address is not a multiple of 8. In that case next record won't have an address aligned to 8, etc.

A potential patch would be yo compute padding based on the start address instead of the size, but then when the std::string grows, the alignment of the data may change upon reallocation. Thus my patch.
I also tend to think that using a MemoryBuffer conveys more meaning than an `std::string` for what is actually a view on raw data.


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