Add support for LTO plugin option obj-path

[rdhindsa]

Summary

Add support for LTO plugin option obj-path

[pcc]

lld/ELF/LTO.cpp

282

You don't need this part any more. With thinlto-index-only, the obj-path file is not a "temporary" file but rather one of the final outputs.

302–307

I'd drop this part. We only need to support obj-path with thinlto-index-only.

[pcc]

lld/ELF/LTO.cpp

283

I wouldn't save the buffer at all if obj-path is not passed.

[rdhindsa]

lld/ELF/LTO.cpp

283

Wouldn't we want to store the generated file, if obj-path is not passed? Since we know that Buff[0] is not empty, aren't we supposed to store the content? In that case, is obj-path a requirement for thinlto-index-only?

[pcc]

lld/ELF/LTO.cpp

283

If you happen to know that all input files do not use full LTO then there is no need to pass obj-path. The linker shouldn't write a file to an arbitrary location in that case.

[rdhindsa]

Removed the test which ensured that regular LTO file was written with index only option. It needs to have obj-path option to write that file.

[pcc]

LGTM

[ruiu]

Please consider splitting "thinlto.ll" test file as it contains all tests for thinlto, and that's not desirabl, as small test files are more manageable than one large test file.

lld/ELF/Config.h

   llvm::StringRef Init;
   llvm::StringRef LTOAAPipeline;
   llvm::StringRef LTONewPmPasses;
+  llvm::StringRef LTOObjPath;
   llvm::StringRef LTOSampleProfile;
   llvm::StringRef MapFile;
   llvm::StringRef OutputFile;

lld/ELF/Driver.cpp

       Config->LTODebugPassManager = true;
     } else if (S.startswith("sample-profile=")) {
       Config->LTOSampleProfile = S.substr(15);
+    } else if (S.startswith("obj-path=")) {
+      Config->LTOObjPath = S.substr(9);
     } else if (S == "thinlto-index-only") {
       Config->ThinLTOIndexOnly = true;
     } else if (S.startswith("thinlto-index-only=")) {

lld/ELF/LTO.cpp

         openFile(Path + ".imports");
     }
 
-    if (Config->SaveTemps)
+    if (Config->LTOObjPath.empty())
       saveBuffer(Buff[0], Config->OutputFile + ".lto.o");
+    else
+      saveBuffer(Buff[0], Config->LTOObjPath);
 
     // ThinLTO with index only option is required to generate only the index
     // files. After that, we exit from linker and ThinLTO backend runs in a

lld/test/ELF/lto/thinlto.ll

 ; stable on the final output file itself.
 ; RUN: ld.lld -shared %t.o %t2.o -o %t2
 
+; Test to ensure that thinlto-index-only with obj-path creates the file.
+; RUN: rm -f %t5.o
+; RUN: ld.lld --plugin-opt=thinlto-index-only --plugin-opt=obj-path=%t5.o -shared %t.o %t2.o -o %t4
+; RUN: llvm-readobj -h %t5.o | FileCheck %s --check-prefix=FORMAT
+; RUN: llvm-nm %t5.o | count 0
+
 ; NM: T f
 ; NM1: T f
 ; NM1-NOT: U g