[llvm-dev] RFC: Add a preprocessor to yaml2obj (and other YAML tools)

[James Henderson via llvm-dev]

As someone who suggested this kind of functionality in a review earlier,
this will certainly be useful, I think. I think the syntax makes sense to
me, if we allow for unrecognised macros to just be treated as part of the
input string. This means we don't have to worry about complexities like
escaping "[[" etc. In the (unlikely) event that somebody's YAML needs to
include the literal "[[FOO]]", they simply should not also use -DFOO - use
a different name instead, e.g. -DBAR.

On Mon, 3 Feb 2020 at 22:04, Fangrui Song <maskray at google.com> wrote:

> I am adding -D k=v to yaml2obj, similar to clang -D. This makes it easy
> to generate {32-bit,64-bit} x {big-endian,little-endian} tests.
>
>    --- !ELF
>    FileHeader:
>      Class:   ELFCLASS[[BITS]]
>      Data:    ELFDATA2[[ENCODE]]
>      Type:    ET_DYN
>      Machine: EM_X86_64
>
> # RUN: yaml2obj -D BITS=32 -D ENCODE=LSB %s -o %t.32le
> # RUN: yaml2obj -D BITS=32 -D ENCODE=MSB %s -o %t.32le
> # RUN: yaml2obj -D BITS=64 -D ENCODE=LSB %s -o %t.64le
> # RUN: yaml2obj -D BITS=64 -D ENCODE=MSB %s -o %t.64be
>
> See https://reviews.llvm.org/D73828 for examples how -D simplifies tests.
>
> Do people think it may be useful in other YAML tools? If yes, I'll move
> the yaml2obj implementation (https://reviews.llvm.org/D73821 ) to
> include/llvm/Support/YAMLTraits.h llvm::yaml::Input so that other YAML
> tools can use the feature.
>
> Do people prefer a different syntax? I think [[PATTERN]] is nice because
> it is what FileCheck -DFILE=... uses:
>
>    # CHECK: ... [[FILE]]
>
>    FileCheck only preprocesses patterns in CHECK lines.
>    D73821 preprocesses both comment lines (which include CHECK lines) and
> non-comment lines (which include YAML).
>    It is not a problem that the YAML preprocessor also processes CHECK
> lines, because tokens on a comment line will be ignored.
>
> If -D UNDEF= is not specified, should [[UNDEF]] in the source be
> considered an error?
> I think it is fine not to treat it as an error because there can be
> legitimate use cases of unterminated [[, for example, [[ in a string
> literal.
> YAML parsing is complex. I don't expect the preprocessor to be smart
> enough to recognize string literals. (llvm/lib/Support/YAMLParser.cpp does
> not seem to provide raw strings of
> spaces and comments. Hooking a preprocessor into the scanner does not seem
> to be simple.)
>
> Do people know other preprocessing features which may be useful?
>
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