Diagnostic message (error) related to ThinLTO caching needs to be downgraded to a remark

[kromanova]

Summary

This is a fix for PR #46392 (Diagnostic message (error) related to ThinLTO caching needs to be downgraded to a remark).

There are diagnostic messages related to ThinLTO caching that contain the word "error", but they are really just notices/remarks for users, and they don't cause a build failure. The word "error" appearing can be confusing to users, and may even cause deeper problems.

User's build system might be designed to interpret any error messages (even a benign error message as the one above) reported by the compiler as a build failure, thus causing the build to fail "needlessly". In short, the term "error" in this diagnostic is misleading at best, and may be causing build systems to fail at worst.

We would like to downgrade this "error" message to a diagnostic remark.

There is another error message that needs to be downgraded to a remark, since it’s benign too...

errs() << "error: can't reload cached file '" << CacheEntryPath
      << "': " << EC.message() << "\n";

[steven_wu]

LGTM

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

if (!Err)
       return std::string(OutputPath.str());
     // Hard linking failed, try to copy.
     Err = sys::fs::copy_file(CacheEntryPath, OutputPath);
     if (!Err)
       return std::string(OutputPath.str());
     // Copy failed (could be because the CacheEntry was removed from the cache
     // in the meantime by another process), fall back and try to write down the
     // buffer to the output.
-    errs() << "error: can't link or copy from cached entry '" << CacheEntryPath
+    errs() << "remark: can't link or copy from cached entry '" << CacheEntryPath
            << "' to '" << OutputPath << "'\n";
   }
   // No cache entry, just write out the buffer.
   std::error_code Err;
   raw_fd_ostream OS(OutputPath, Err, sys::fs::OF_None);
   if (Err)
     report_fatal_error("Can't open output '" + OutputPath + "'\n");
   OS << OutputBuffer.getBuffer();
for (auto IndexCount : ModulesOrdering) {
             // Releasing the buffer from the heap and reloading it from the
             // cache file with mmap helps us to lower memory pressure.
             // The freed memory can be used for the next input file.
             // The final binary link will read from the VFS cache (hopefully!)
             // or from disk (if the memory pressure was too high).
             auto ReloadedBufferOrErr = CacheEntry.tryLoadingBuffer();
             if (auto EC = ReloadedBufferOrErr.getError()) {
               // On error, keep the preexisting buffer and print a diagnostic.
-              errs() << "error: can't reload cached file '" << CacheEntryPath
+              errs() << "remark: can't reload cached file '" << CacheEntryPath
                      << "': " << EC.message() << "\n";
             } else {
               OutputBuffer = std::move(*ReloadedBufferOrErr);
             }
           }
           ProducedBinaries[count] = std::move(OutputBuffer);
           return;
         }