[flang][driver] Introduce FCC_OVERRIDE_OPTIONS.

clang/include/clang/Driver/Driver.h

@@ -879,7 +879,7 @@ llvm::Error expandResponseFiles(SmallVectorImpl<const char *> &Args,
 /// See applyOneOverrideOption.
 void applyOverrideOptions(SmallVectorImpl<const char *> &Args,
                           const char *OverrideOpts,
-                          llvm::StringSet<> &SavedStrings,
+                          llvm::StringSet<> &SavedStrings, StringRef EnvVar,
                           raw_ostream *OS = nullptr);
 
 } // end namespace driver

clang/lib/Driver/Driver.cpp

@@ -7289,7 +7289,7 @@ static void applyOneOverrideOption(raw_ostream &OS,
 void driver::applyOverrideOptions(SmallVectorImpl<const char *> &Args,
                                   const char *OverrideStr,
                                   llvm::StringSet<> &SavedStrings,
-                                  raw_ostream *OS) {
+                                  StringRef EnvVar, raw_ostream *OS) {
   if (!OS)
     OS = &llvm::nulls();
 
@@ -7298,7 +7298,7 @@ void driver::applyOverrideOptions(SmallVectorImpl<const char *> &Args,
     OS = &llvm::nulls();
   }
 
-  *OS << "### CCC_OVERRIDE_OPTIONS: " << OverrideStr << "\n";
+  *OS << "### " << EnvVar << ": " << OverrideStr << "\n";
 
   // This does not need to be efficient.
 

clang/tools/driver/driver.cpp

@@ -305,7 +305,7 @@ int clang_main(int Argc, char **Argv, const llvm::ToolContext &ToolContext) {
   if (const char *OverrideStr = ::getenv("CCC_OVERRIDE_OPTIONS")) {
     // FIXME: Driver shouldn't take extra initial argument.
     driver::applyOverrideOptions(Args, OverrideStr, SavedStrings,
-                                 &llvm::errs());
+                                 "CCC_OVERRIDE_OPTIONS", &llvm::errs());
   }
 
   std::string Path = GetExecutablePath(ToolContext.Path, CanonicalPrefixes);

flang/docs/FlangDriver.md

@@ -614,3 +614,28 @@ nvfortran defines `-fast` as
  - `-Mcache_align`: there is no equivalent flag in Flang or Clang.
  - `-Mflushz`: flush-to-zero mode - when `-ffast-math` is specified, Flang will
    link to `crtfastmath.o` to ensure denormal numbers are flushed to zero.
+
+
+## FCC_OVERRIDE_OPTIONS
+
+The environment variable `FCC_OVERRIDE_OPTIONS` can be used to apply a list of
+edits to the input argument lists. The value of this environment variable is
+a space separated list of edits to perform. These edits are applied in order to
+the input argument lists. Edits should be one of the following forms:
+
+- `#`: Silence information about the changes to the command line arguments.
+
+- `^FOO`: Add `FOO` as a new argument at the beginning of the command line.
+
+- `+FOO`: Add `FOO` as a new argument at the end of the command line.
+
+- `s/XXX/YYY/`: Substitute the regular expression `XXX` with `YYY` in the
+  command line.
+
+- `xOPTION`: Removes all instances of the literal argument `OPTION`.
+
+- `XOPTION`: Removes all instances of the literal argument `OPTION`, and the
+  following argument.
+
+- `Ox`: Removes all flags matching `O` or `O[sz0-9]` and adds `Ox` at the end
+  of the command line.

flang/test/Driver/fcc_override.f90

@@ -0,0 +1,12 @@
+! RUN: env FCC_OVERRIDE_OPTIONS="#+-Os +-Oz +-O +-O3 +-Oignore +a +b +c xb Xa Omagic ^-###  " %flang -target x86_64-unknown-linux-gnu %s -O2 b -O3 2>&1 | FileCheck %s
+! RUN: env FCC_OVERRIDE_OPTIONS="x-Werror +-g" %flang -target x86_64-unknown-linux-gnu -Werror %s -c -### 2>&1 | FileCheck %s -check-prefix=RM-WERROR
+
+! CHECK: "-fc1"
+! CHECK-NOT: "-Oignore"
+! CHECK: "-Omagic"
+! CHECK-NOT: "-Oignore"
+
+! RM-WERROR: ### FCC_OVERRIDE_OPTIONS: x-Werror +-g
+! RM-WERROR-NEXT: ### Deleting argument -Werror
+! RM-WERROR-NEXT: ### Adding argument -g at end
+! RM-WERROR-NOT: "-Werror"

flang/tools/flang-driver/driver.cpp

@@ -111,6 +111,13 @@ int main(int argc, const char **argv) {
     }
   }
 
+  llvm::StringSet<> SavedStrings;
+  // Handle FCC_OVERRIDE_OPTIONS, used for editing a command line behind the
+  // scenes.
+  if (const char *OverrideStr = ::getenv("FCC_OVERRIDE_OPTIONS"))
+    clang::driver::applyOverrideOptions(args, OverrideStr, SavedStrings,
+                                        "FCC_OVERRIDE_OPTIONS", &llvm::errs());
+
   // Not in the frontend mode - continue in the compiler driver mode.
 
   // Create DiagnosticsEngine for the compiler driver