[yaml2obj] - Add a way to set default values for macros used in a YAML.

[grimar]

Summary

Currently we have to override all macros that are declared. But in many
cases it is convenient to use default values and to override only
a particular one or two.

This provides a way to set a default value for any macro:

Symbols:
  - Name: [[FOO=foo]]

[MaskRay]

llvm/test/tools/yaml2obj/macro.yaml

68

The two YAML !ELF documents can be merged to make the file simpler.

llvm/tools/yaml2obj/yaml2obj.cpp

84

a default macro value if present.

"YAML description" is implied from the context.

90

!Default.empty() || MacroExpr.endswith("=")

[jhenderson]

Thanks for looking at this. I've had a few times where this might have been useful already. Do we need a way of escaping the = in the event somebody wants to explicitly include it? Not sure I can think of a use-case for that, so probably not, but something to think about.

LGTM, with @MaskRay's comments and one nit addressed.

llvm/tools/yaml2obj/yaml2obj.cpp

83

a value provided -> the provided value

[grimar]

In D82455#2113349, @jhenderson wrote:

Do we need a way of escaping the = in the event somebody wants to explicitly include it? Not sure I can think of a use-case for that, so probably not, but something to think about.

I thought that we might add a --nomacro option. But I doubt that we really need it too.

[grimar]

• Addressed review comments.

[jhenderson]

Latest version LGTM.

llvm/test/tools/yaml2obj/macro.yaml

FileHeader:
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Symbols:
   - Name: "a[["
   - Name: "[[a]"
   - Name: "[[a[[a]]"
   - Name: "[[a][[a]][[b]]"
+
+## Check that it is possible to set a default value for a macro in the YAML description.
+## Check that we are able to provide an empty default value for a macro.
+# RUN: yaml2obj --docnum=4 %s -o %t4.1
+# RUN: llvm-nm --no-sort %t4.1 | FileCheck %s --check-prefix=DEFAULT
+
+# DEFAULT:      U _foo_
+# DEFAULT-NEXT: U __
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Symbols:
+  - Name: _[[FOO=foo]]_
+  - Name: _[[BAR=]]_
+
+## Check that it is possible to override a default macro value.
+# RUN: yaml2obj --docnum=4 %s -o %t4.2 -DFOO=bar -DBAR=foo
+# RUN: llvm-nm --no-sort %t4.2 | FileCheck %s --check-prefix=OVERRIDE-DEFAULT1
+
+# OVERRIDE-DEFAULT1:      U _bar_
+# OVERRIDE-DEFAULT1-NEXT: U _foo_
+
+# RUN: yaml2obj --docnum=4 %s -o %t4.3 -DFOO=bar=foo -DBAR=foo=bar
+# RUN: llvm-nm --no-sort %t4.3 | FileCheck %s --check-prefix=OVERRIDE-DEFAULT2
+
+# OVERRIDE-DEFAULT2:      U _bar=foo_
+# OVERRIDE-DEFAULT2-NEXT: U _foo=bar_

llvm/tools/yaml2obj/yaml2obj.cpp

for (StringRef Define : D) {
     }
   }
 
   std::string Preprocessed;
   while (!Buf.empty()) {
     if (Buf.startswith("[[")) {
       size_t I = Buf.find_first_of("[]", 2);
       if (Buf.substr(I).startswith("]]")) {
-        StringRef Macro = Buf.substr(2, I - 2);
+        StringRef MacroExpr = Buf.substr(2, I - 2);
+        StringRef Macro;
+        StringRef Default;
+        std::tie(Macro, Default) = MacroExpr.split('=');
+
+        // When the -D option is requested, we use the provided value.
+        // Otherwise we use a default macro value if present.
         auto It = Defines.find(Macro);
-        if (It != Defines.end()) {
-          Preprocessed += It->second;
+        Optional<StringRef> Value;
+        if (It != Defines.end())
+          Value = It->second;
+        else if (!Default.empty() || MacroExpr.endswith("="))
+          Value = Default;
+
+        if (Value) {
+          Preprocessed += *Value;
           Buf = Buf.substr(I + 2);
           continue;
         }
       }
     }
 
     Preprocessed += Buf[0];
     Buf = Buf.substr(1);