[ThinLTO] Don't try to replay cache if no hash was computed

There are certain cases where an bitcode arrives in ThinLTOCodeGenerator
and it lacks information to compute a cache key for incremental builds.
In those cases, don't try to replay the output object file from an
non-existing location.

rdar://106432595

Author: cachemeifyoucan

llvm/lib/LTO/ThinLTOCodeGenerator.cpp

@@ -452,18 +452,20 @@ class FileModuleCacheEntry : public ModuleCacheEntry {
     sys::fs::remove(OutputPath);
     // Cache is enabled, hard-link the entry (or copy if hard-link fails).
     std::string CacheEntryPath = getEntryPath();
-    auto Err = sys::fs::create_hard_link(CacheEntryPath, OutputPath);
-    if (!Err)
-      return Error::success();
-    // Hard linking failed, try to copy.
-    Err = sys::fs::copy_file(CacheEntryPath, OutputPath);
-    if (!Err)
-      return Error::success();
-    // 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() << "remark: can't link or copy from cached entry '" << CacheEntryPath
-           << "' to '" << OutputPath << "'\n";
+    if (!CacheEntryPath.empty()) {
+      auto Err = sys::fs::create_hard_link(CacheEntryPath, OutputPath);
+      if (!Err)
+        return Error::success();
+      // Hard linking failed, try to copy.
+      Err = sys::fs::copy_file(CacheEntryPath, OutputPath);
+      if (!Err)
+        return Error::success();
+      // 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() << "remark: can't link or copy from cached entry '"
+             << CacheEntryPath << "' to '" << OutputPath << "'\n";
+    }
     // Fallback to default.
     return ModuleCacheEntry::writeObject(OutputBuffer, OutputPath);
   }

llvm/test/ThinLTO/X86/cache-no-replay.ll

@@ -0,0 +1,16 @@
+; No hash produced, empty file module entry.
+; RUN: opt -module-summary %s -o %t.bc
+
+; RUN: rm -rf %t.cache.noindex && mkdir %t.cache.noindex
+; RUN: llvm-lto -thinlto-action=run -exported-symbol=globalfunc %t.bc \
+; RUN:   -thinlto-cache-dir %t.cache.noindex -thinlto-save-objects %t.save.noindex | FileCheck %s --allow-empty
+
+; CHECK-NOT: remarks
+
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.11.0"
+
+define void @globalfunc() #0 {
+entry:
+  ret void
+}