[yaml2obj] Add a `-E` flag to preprocess only.

[simon_tatham]

Summary

If you're having trouble getting a yaml2obj macro expansion to do what
you want, it's useful to be able to print the output of the
preprocessing to see what your macros expanded to before going
into the YAML processing phase.

yaml2obj has its own preprocessing system which isn't the same as any
other well-known thing like cpp. So there's no way to do this macro
expansion via another tool: yaml2obj will have to do it itself.

In this commit I add an -E flag to yaml2obj to do that.

[simon_tatham]

I wrote this patch in order to use it myself. Naturally the problem I was trying to debug turned out to be embarrassingly trivial – I hadn't made a subtle mistake in the macro syntax, I'd put my -D options on completely the wrong command line. But now I've written the patch anyway it seems a pity to waste it :-)

[probinson]

One nit on the help text and LGTM.

llvm/tools/yaml2obj/yaml2obj.cpp

43

Option help text generally omits the final period.

[simon_tatham]

llvm/tools/yaml2obj/yaml2obj.cpp

43

Oh yes. I was fooled by the period in the help text for -D immediately above, but of course, that's not a final period, and the followup sentence doesn't have one.

[MaskRay]

LGTM. Note: if I proposed the patch, I would wait a bit longer before landing.

[simon_tatham]

Oh yes, sorry about that. Switching a little too fast between projects with completely different code review practices :-(

[jhenderson]

llvm/test/tools/yaml2obj/preprocess-only.test

4–8

Probably not worth changing the test now that it's written (although I won't object if you choose to change things), but I think you could have avoided this by having the "YAML" in a separate file to the test in this case right?

[simon_tatham]

llvm/test/tools/yaml2obj/preprocess-only.test

4–8

I suppose so, although if anyone wants to update this test later (say if they were to add extra features to the preprocessor), that trades off one inconvenience for another – this way they get to put each test preprocessor input and its expected expansion together and see them both at once, even if they have to write the expansion in the form of a slightly awkward regex.

llvm/test/tools/yaml2obj/preprocess-only.test

+# RUN: yaml2obj -E -Dfoo=wibble %s | FileCheck %s
+
+This is a test of yaml2obj's pure preprocessing mode, so it doesn't
+have to contain valid YAML, or any YAML at all. But we do have to be
+careful with the FileCheck CHECK directives, because they'll be
+emitted into the preprocessed output, and risk matching themselves!
+For that reason, each one matches only at the start of a line.
+
+Expand a macro:
+[[foo]]            # CHECK: {{^wibble}}
+
+Expand an undefined macro:
+[[bar]]            # CHECK: {{^\[\[bar\]\]}}
+
+Expand an undefined macro where we provided a default value:
+[[baz=123]]        # CHECK: {{^123}}

llvm/tools/yaml2obj/yaml2obj.cpp

cl::opt<std::string> Input(cl::Positional, cl::desc("<input file>"),
                            cl::init("-"), cl::cat(Cat));
 
 cl::list<std::string>
     D("D", cl::Prefix,
       cl::desc("Defined the specified macros to their specified "
                "definition. The syntax is <macro>=<definition>"),
       cl::cat(Cat));
 
+cl::opt<bool> PreprocessOnly("E", cl::desc("Just print the preprocessed file."),
+                             cl::cat(Cat));
+
 cl::opt<unsigned>
     DocNum("docnum", cl::init(1),
            cl::desc("Read specified document from input (default = 1)"),
            cl::cat(Cat));
 
 static cl::opt<uint64_t> MaxSize(
     "max-size", cl::init(10 * 1024 * 1024),
     cl::desc(
int main(int argc, char **argv) {
   ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
       MemoryBuffer::getFileOrSTDIN(Input);
   if (!Buf)
     return 1;
 
   Optional<std::string> Buffer = preprocess(Buf.get()->getBuffer(), ErrHandler);
   if (!Buffer)
     return 1;
+
+  if (PreprocessOnly) {
+    Out->os() << Buffer;
+  } else {
     yaml::Input YIn(*Buffer);
 
     if (!convertYAML(YIn, Out->os(), ErrHandler, DocNum,
                      MaxSize == 0 ? UINT64_MAX : MaxSize))
       return 1;
+  }
 
   Out->keep();
   Out->os().flush();
   return 0;
 }