Swift MCCAS object files are 0 bytes on cache miss

When clang caching and MCCAS are enabled, on a cache miss, the object
file is not written to in MachOCASWriter, instead the replay code is
rerun and the object file is serialized and written out. However, in
swift, the replay code isn't run on a cache miss at all, and it expects
the object writer to write to the object file.

This patch makes it so that on a cache miss, the MachOCASWriter always
writes to the raw_ostream buffer for the object file and doesn't
serialize the object file in the replay code.

(cherry picked from commit ce65bfa)

Author: rastogishubham

clang/lib/Frontend/CompileJobCache.cpp

@@ -724,7 +724,7 @@ Error ObjectStoreCachingOutputs::finishComputedResult(
 std::optional<int> ObjectStoreCachingOutputs::replayCachedResult(
     const llvm::cas::CASID &ResultCacheKey, llvm::cas::ObjectRef ResultID,
     bool JustComputedResult) {
-  if (JustComputedResult && !ComputedJobNeedsReplay)
+  if (JustComputedResult && !WriteOutputAsCASID)
     return std::nullopt;
 
   // FIXME: Stop calling report_fatal_error().
@@ -745,10 +745,7 @@ std::optional<int> ObjectStoreCachingOutputs::replayCachedResult(
 Expected<std::optional<int>> ObjectStoreCachingOutputs::replayCachedResult(
     const llvm::cas::CASID &ResultCacheKey,
     clang::cas::CompileJobCacheResult &Result, bool JustComputedResult) {
-  // FIXME: The correct fix for MCCAS replay is that you have an official CASID
-  // file output going all the way down into ObjectWriter, we can remove this
-  // callback and special case.
-  if (JustComputedResult && !ComputedJobNeedsReplay)
+  if (JustComputedResult && !WriteOutputAsCASID)
     return std::nullopt;
 
   llvm::cas::ObjectStore &CAS = Result.getCAS();

clang/test/CAS/cas-backend.c

@@ -1,7 +1,7 @@
 // RUN: rm -rf %t && mkdir -p %t
 // RUN: llvm-cas --cas %t/cas --ingest %s > %t/casid
 //
-// RUN: %clang -cc1 -triple x86_64-apple-macos11 -fcas-backend \
+// RUN: %clang -cc1 -fcas-emit-casid-file -triple x86_64-apple-macos11 -fcas-backend \
 // RUN:   -fcas-path %t/cas -fcas-fs @%t/casid -fcache-compile-job \
 // RUN:   -Rcompile-job-cache %s -emit-obj -o %t/output.o \
 // RUN:   -debug-info-kind=standalone -dwarf-version=4 -debugger-tuning=lldb \
@@ -11,7 +11,7 @@
 // RUN: ls %t/output.o && rm %t/output.o
 // RUN: ls %t/deps.d && mv %t/deps.d %t/deps.d.orig
 //
-// RUN: CLANG_CAS_BACKEND_SAVE_CASID_FILE=1 %clang -cc1 \
+// RUN: %clang -cc1 -fcas-emit-casid-file\
 // RUN:   -triple x86_64-apple-macos11 -fcas-backend \
 // RUN:   -fcas-path %t/cas -fcas-fs @%t/casid -fcache-compile-job \
 // RUN:   -Rcompile-job-cache %s -emit-obj -o %t/output.o \

clang/test/CAS/casid-output-test.cpp

@@ -0,0 +1,25 @@
+// RUN: rm -rf %t && mkdir -p %t
+
+// Check if -fcasid-output works on a cache miss with file based caching
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas CLANG_CACHE_DISABLE_MCCAS=1 %clang-cache %clang -target x86_64-apple-macos11 -Xclang -fcasid-output -g -c %s -o %t/test.o
+// RUN: cat %t/test.o | FileCheck %s
+// RUN: rm -rf %t/test.o
+// Check if -fcasid-output works on a cache hit with file based caching
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas CLANG_CACHE_DISABLE_MCCAS=1 %clang-cache %clang -target x86_64-apple-macos11 -Xclang -fcasid-output -g -c %s -o %t/test.o
+// RUN: cat %t/test.o | FileCheck %s
+// RUN: rm -rf %t/test.o
+// RUN: rm -rf %t/cas
+
+// Check if -fcasid-output works on a cache miss with MCCAS
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas %clang-cache %clang -target x86_64-apple-macos11 -Xclang -fcasid-output -g -c %s -o %t/test.o
+// RUN: cat %t/test.o | FileCheck %s
+// RUN: rm -rf %t/test.o
+
+// Check if -fcasid-output works on a cache hit with MCCAS
+// RUN: env LLVM_CACHE_CAS_PATH=%t/cas %clang-cache %clang -target x86_64-apple-macos11 -Xclang -fcasid-output -g -c %s -o %t/test.o
+// RUN: cat %t/test.o | FileCheck %s
+
+// CHECK: llvmcas://{{[a-f0-9]+}}
+
+
+void foo() {}

llvm/lib/MC/MachOCASWriter.cpp

@@ -76,6 +76,7 @@ uint64_t MachOCASWriter::writeObject(MCAssembler &Asm) {
           inconvertibleErrorCode(),
           "CASBackend output round-trip verification error");
 
+    OS << ObjectBuffer;
     return Error::success();
   };
 
@@ -85,11 +86,6 @@ uint64_t MachOCASWriter::writeObject(MCAssembler &Asm) {
   // If there is a callback, then just hand off the result through callback.
   if (ResultCallBack) {
     cantFail((*ResultCallBack)(CASObj.getID()));
-    if (Mode == CASBackendMode::Verify) {
-      if (auto E = VerifyObject())
-        report_fatal_error(std::move(E));
-    }
-    return 0;
   }
 
   switch (Mode) {
@@ -103,17 +99,8 @@ uint64_t MachOCASWriter::writeObject(MCAssembler &Asm) {
     break;
   }
   case CASBackendMode::Verify: {
-    SmallString<512> ObjectBuffer;
-    raw_svector_ostream ObjectOS(ObjectBuffer);
-    auto E = SerializeObjectFile(CASObj, CAS, ObjectOS);
-    if (E)
+    if (auto E = VerifyObject())
       report_fatal_error(std::move(E));
-
-    if (!ObjectBuffer.equals(InternalBuffer))
-      report_fatal_error("CASBackend output round-trip verification error");
-
-    OS << ObjectBuffer;
-    break;
   }
   }