[lld] [llvm] [DTLTO][LLD][ELF] Support bitcode members of thin archives (PR #149425)

[via llvm-commits]

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@@ -1753,6 +1754,36 @@ static uint8_t getOsAbi(const Triple &t) {
   }
 }
 
+// For DTLTO, bitcode member names must be valid paths to files on disk.
+// For thin archives, resolve `memberPath` relative to the archive's location.
+// Returns true if adjusted; false otherwise. Non-thin archives are unsupported.
+static bool dtltoAdjustMemberPathIfThinArchive(Ctx &ctx, StringRef archivePath,
+                                               std::string &memberPath) {
+  assert(!archivePath.empty() && !ctx.arg.dtltoDistributor.empty());
+
+  // Read the archive header to determine if it's a thin archive.
+  auto bufferOrErr =
+      MemoryBuffer::getFileSlice(archivePath, sizeof(ThinArchiveMagic) - 1, 0);
+  if (std::error_code ec = bufferOrErr.getError()) {
+    ErrAlways(ctx) << "cannot open " << archivePath << ": " << ec.message();
+    return false;
+  }
+
+  if (!bufferOrErr->get()->getBuffer().starts_with(ThinArchiveMagic))
+    return false;
+
+  SmallString<64> resolvedPath;
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bd1976bris wrote:

Done. I used `SmallString<128>`.

https://github.com/llvm/llvm-project/pull/149425