[clang] [Driver] Improve error when a compiler-rt library is not found (PR #81037)

[Nico Weber via cfe-commits]

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@@ -656,19 +656,29 @@ std::string ToolChain::getCompilerRT(const ArgList &Args, StringRef Component,
   // Check for runtime files in the new layout without the architecture first.
   std::string CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/false);
+  SmallString<128> Path;
   for (const auto &LibPath : getLibraryPaths()) {
     SmallString<128> P(LibPath);
     llvm::sys::path::append(P, CRTBasename);
     if (getVFS().exists(P))
       return std::string(P);
+    if (Path.empty())
+      Path = P;
   }
+  if (getTriple().isOSAIX())
+    Path.clear();
 
-  // Fall back to the old expected compiler-rt name if the new one does not
-  // exist.
+  // Check the filename for the old layout if the new one does not exist.
   CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/true);
-  SmallString<128> Path(getCompilerRTPath());
-  llvm::sys::path::append(Path, CRTBasename);
+  SmallString<128> OldPath(getCompilerRTPath());
+  llvm::sys::path::append(OldPath, CRTBasename);
+  if (Path.empty() || getVFS().exists(OldPath))
+    return std::string(OldPath);
+
+  // If none is found, use a file name from the new layout, which may get
+  // printed in an error message, aiding users in knowing what Clang is
+  // looking for.
----------------
nico wrote:

This is a behavior change: In distributed build environments, neither lib file exists at compile time. Previously, this would result in the "old" style, now it results in the "new" style (which we disable everywhere since it causes all kinds of issues – from what I can tell, we're not alone in this).

Is there some way we can tell clang that we always want the old style here, independent of what's on disk?

https://github.com/llvm/llvm-project/pull/81037