[PATCH] D112465: [llvm-profgen] Avoid writing any data to CSNameTableSection for the empty CSNameTable

[Lei Wang via Phabricator via llvm-commits]

wlei added inline comments.


================
Comment at: llvm/lib/ProfileData/SampleProfWriter.cpp:255-256
 std::error_code SampleProfileWriterExtBinaryBase::writeCSNameTableSection() {
+  if (CSNameTable.empty())
+    return sampleprof_error::success;
+
----------------
wenlei wrote:
> `SampleProfileReaderExtBinaryBase::readCSNameTableSec` would still expect to read a zero here, and if we skip writing the zero, the read will get some garbage data? Or did I miss anything?
> 
> Also this is inconsistent with how we write other sections - we don't omit zero size.
You see the header reader logic below, at the beginning of the loop, it will skip the zero size entry (` if (!Entry.Size)`). so it won't call the `readCSNameTableSec`. Also it has the sanity check after read(`if (Data != SecStart + SecSize)`)

```
  for (auto &Entry : SecHdrTable) {
    // Skip empty section.
    if (!Entry.Size)
      continue;
   ...
    if (std::error_code EC = readOneSection(SecStart, SecSize, Entry))
      return EC;
    if (Data != SecStart + SecSize)
      return sampleprof_error::malformed;
   ....
   
```

I feel like each section doesn't follow strong encoding convention. `NameTable` is encoded the size probably because it won't have a zero size(if the profile is not empty).
Like the ProfileSymbolList, it can be a zero section size:
```
  if (ProfSymList && ProfSymList->size() > 0)
    if (std::error_code EC = ProfSymList->write(*OutputStream))
      return EC;

  return sampleprof_error::success;
```







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https://reviews.llvm.org/D112465