Support partial preprocessing with clang

[jpeach]

This one made me scratch my head a bit because I assumed it was a Bazel problem and didn't look in the llama docs for it 😂

ERROR: /home/jpeach/.cache/bazel/_bazel_jpeach/bca846880028b2eb76664b66797b90da/external/com_google_protobuf/BUILD:129:11: Compiling src/google/protobuf/stubs/stringprintf.cc failed: (Exit 1): llamacc failed: error executing command 
  (cd /home/jpeach/.cache/bazel/_bazel_jpeach/bca846880028b2eb76664b66797b90da/sandbox/linux-sandbox/12/execroot/build && \
  exec env - \
    PATH=/bin:/usr/bin:/usr/local/bin \
    PWD=/proc/self/cwd \
  /home/jpeach/src/envoy-build/bazel/llamacc -U_FORTIFY_SOURCE -fstack-protector -Wall -fno-omit-frame-pointer -g0 -O2 '-D_FORTIFY_SOURCE=1' -DNDEBUG -ffunction-sections -fdata-sections '-std=c++0x' -MD -MF bazel-out/k8-opt-exec-2B5CBBC6/bin/external/com_google_protobuf/_objs/protobuf_lite/stringprintf.d '-frandom-seed=bazel-out/k8-opt-exec-2B5CBBC6/bin/external/com_google_protobuf/_objs/protobuf_lite/stringprintf.o' -iquote external/com_google_protobuf -iquote bazel-out/k8-opt-exec-2B5CBBC6/bin/external/com_google_protobuf -isystem external/com_google_protobuf/src -isystem bazel-out/k8-opt-exec-2B5CBBC6/bin/external/com_google_protobuf/src -g0 -g0 -DHAVE_PTHREAD -DHAVE_ZLIB -Woverloaded-virtual -Wno-sign-compare -Wno-unused-function -Wno-write-strings -Wno-deprecated-declarations -Wno-builtin-macro-redefined '-D__DATE__="redacted"' '-D__TIMESTAMP__="redacted"' '-D__TIME__="redacted"' -c external/com_google_protobuf/src/google/protobuf/stubs/stringprintf.cc -o bazel-out/k8-opt-exec-2B5CBBC6/bin/external/com_google_protobuf/_objs/protobuf_lite/stringprintf.o)
Execution platform: @local_config_platform//:host

Use --sandbox_debug to see verbose messages from the sandbox
clang: error: unknown argument: '-fdirectives-only'
Target @envoy//source/exe:envoy-static failed to build

-fdirectives-only doesn't work with clang. Perhaps there's an alternative that works with both compilers? There is a short thread discussing how clang's -frewrite-includes differs, but maybe the difference doesn't matter for llama?

[nelhage]

Yeah, I think -frewrite-includes would work for us, but I'm not sure if there's an easy way to autodetect which one to use. Could add another configuration flag, I guess -- LLAMACC_CC_IS_CLANG or something -- but that seems mildly unpleasant

[jpeach]

FWIW, I tried this quick hack with the Envoy build, and it ended up not being able to resolve libstdc++ types properly. Not sure whether there's something more needed to get partial pre-processing to work with clang, or whether this Bael making some aggressive assumptions.

--- cmd/llamacc/main.go
+++ cmd/llamacc/main.go
@@ -196,7 +196,7 @@ func buildLocalPreprocess(ctx context.Context, client *daemon.Client, cfg *Confi
                preprocessor.Args = []string{comp.LocalCompiler(cfg)}
                preprocessor.Args = append(preprocessor.Args, comp.LocalArgs...)
                if !cfg.FullPreprocess {
-                       preprocessor.Args = append(preprocessor.Args, "-fdirectives-only")
+                       preprocessor.Args = append(preprocessor.Args, "-frewrite-includes")
                }
                preprocessor.Args = append(preprocessor.Args, "-E", "-o", "-", comp.Input)
                preprocessor.Stdout = &preprocessed
@@ -224,7 +224,7 @@ func buildLocalPreprocess(ctx context.Context, client *daemon.Client, cfg *Confi
        args.Args = []string{comp.RemoteCompiler(cfg)}
        args.Args = append(args.Args, comp.RemoteArgs...)
        if !cfg.FullPreprocess {
-               args.Args = append(args.Args, "-fdirectives-only", "-fpreprocessed")
+               args.Args = append(args.Args, "-frewrite-includes")
        }
        args.Args = append(args.Args, "-x", comp.PreprocessedLanguage, "-o", comp.Output, "-")

[jpeach]

OK, I figured this one out. We need to pass all the Defs along to the remote build as well as the -frewrite-includes flag. Clang also needs different constants in preprocessedLang (ie. "c" and "c++").