driver: Don't warn about assembler flags being unused when not assembling

clang currently warns when passing flags for the assembler (e.g.
-Wa,-mbig-obj) to an invocation that doesn't run the assembler (e.g.
-E).

At first sight, that makes sense -- the flag really is unused. But many
other flags don't have an effect if no assembler runs (e.g.
-fno-integrated-as, -ffunction-sections, and many others), and those
currently don't warn. So this seems more like a side effect of how
CollectArgsForIntegratedAssembler() is implemented than like an
intentional feature.

Since it's a bit inconvenient when debugging builds and adding -E,
always call CollectArgsForIntegratedAssembler() to make sure assembler
args always get claimed. Currently, this affects only these flags:
-mincremental-linker-compatible, -mimplicit-it= (on ARM), -Wa, -Xassembler

It does have the side effect that assembler options now need to be valid
even if -E is passed. Previously, `-Wa,-mbig-obj` would error for
non-coff output only if the assembler ran, now it always errors. This
too makes assembler flags more consistent with all the other flags and
seems like a progression.

Fixes PR42066.

Differential Revision: https://reviews.llvm.org/D64527

llvm-svn: 365703

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3545,6 +3545,18 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // Select the appropriate action.
   RewriteKind rewriteKind = RK_None;
 
+  // If CollectArgsForIntegratedAssembler() isn't called below, call it here
+  // with a dummy args list to mark assembler flags as used even when not
+  // running an assembler. Otherwise, clang would emit "argument unused"
+  // warnings for assembler flags when e.g. adding "-E" to flags while debugging
+  // something. That'd be somewhat inconvenient, and it's also inconsistent with
+  // most other flags -- we don't warn on -ffunction-sections not being used
+  // in -E mode either for example, even though it's not really used either.
+  if (!isa<AssembleJobAction>(JA)) {
+    ArgStringList DummyArgs;
+    CollectArgsForIntegratedAssembler(C, Args, DummyArgs, D);
+  }
+
   if (isa<AnalyzeJobAction>(JA)) {
     assert(JA.getType() == types::TY_Plist && "Invalid output type.");
     CmdArgs.push_back("-analyze");

clang/test/Driver/as-options.s

@@ -35,3 +35,14 @@
 // RUN:   | FileCheck %s
 
 // CHECK: "-I" "foo_dir"
+
+// Test that assembler options don't cause warnings when there's no assembler
+// stage.
+
+// RUN: %clang -mincremental-linker-compatible -E -o /dev/null %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
+// RUN: %clang -mimplicit-it=always -target armv7-linux-gnueabihf -E %s \
+// RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
+// RUN: %clang -Wa,-mbig-obj -target i386-pc-windows -E %s \
+// RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
+// WARN-NOT: unused