Revert "[Driver] Delete -mimplicit-it="

This reverts commit 2919222d80.

That commit broke backwards compatibility. Additionally, the
replacement, -Wa,-mimplicit-it, isn't yet supported by any stable
release of Clang.

See D102812 for a fix for the error cases when callers specify both
-mimplicit-it and -Wa,-mimplicit-it.

clang/include/clang/Driver/Options.td

@@ -3017,6 +3017,7 @@ def mtls_size_EQ : Joined<["-"], "mtls-size=">, Group<m_Group>, Flags<[NoXarchOp
   HelpText<"Specify bit size of immediate TLS offsets (AArch64 ELF only): "
            "12 (for 4KB) | 24 (for 16MB, default) | 32 (for 4GB) | 48 (for 256TB, needs -mcmodel=large)">,
   MarshallingInfoInt<CodeGenOpts<"TLSSize">>;
+def mimplicit_it_EQ : Joined<["-"], "mimplicit-it=">, Group<m_Group>;
 def mdefault_build_attributes : Joined<["-"], "mdefault-build-attributes">, Group<m_Group>;
 def mno_default_build_attributes : Joined<["-"], "mno-default-build-attributes">, Group<m_Group>;
 def mconstant_cfstrings : Flag<["-"], "mconstant-cfstrings">, Group<clang_ignored_m_Group>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2393,6 +2393,22 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
                    DefaultIncrementalLinkerCompatible))
     CmdArgs.push_back("-mincremental-linker-compatible");
 
+  switch (C.getDefaultToolChain().getArch()) {
+  case llvm::Triple::arm:
+  case llvm::Triple::armeb:
+  case llvm::Triple::thumb:
+  case llvm::Triple::thumbeb:
+    if (Arg *A = Args.getLastArg(options::OPT_mimplicit_it_EQ)) {
+      StringRef Value = A->getValue();
+      if (!AddARMImplicitITArgs(Args, CmdArgs, Value))
+        D.Diag(diag::err_drv_unsupported_option_argument)
+            << A->getOption().getName() << Value;
+    }
+    break;
+  default:
+    break;
+  }
+
   // If you add more args here, also add them to the block below that
   // starts with "// If CollectArgsForIntegratedAssembler() isn't called below".
 
@@ -4337,6 +4353,16 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       Args.ClaimAllArgs(options::OPT_mno_relax_all);
       Args.ClaimAllArgs(options::OPT_mincremental_linker_compatible);
       Args.ClaimAllArgs(options::OPT_mno_incremental_linker_compatible);
+      switch (C.getDefaultToolChain().getArch()) {
+      case llvm::Triple::arm:
+      case llvm::Triple::armeb:
+      case llvm::Triple::thumb:
+      case llvm::Triple::thumbeb:
+        Args.ClaimAllArgs(options::OPT_mimplicit_it_EQ);
+        break;
+      default:
+        break;
+      }
     }
     Args.ClaimAllArgs(options::OPT_Wa_COMMA);
     Args.ClaimAllArgs(options::OPT_Xassembler);

clang/test/Driver/arm-implicit-it.s

@@ -0,0 +1,24 @@
+// RUN: %clang -target armv7--none-eabi -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-DEFAULT
+
+// RUN: %clang -target armv7--none-eabi -mimplicit-it=arm -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-ARM
+
+// RUN: %clang -target armv7--none-eabi -mimplicit-it=thumb -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-THUMB
+
+// RUN: %clang -target armv7--none-eabi -mimplicit-it=never -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NEVER
+
+// RUN: %clang -target armv7--none-eabi -mimplicit-it=always -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-ALWAYS
+
+// RUN: %clang -target armv7--none-eabi -mimplicit-it=thisisnotavalidoption -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-INVALID
+
+// CHECK-DEFAULT-NOT: "-arm-implicit-it
+// CHECK-ARM: "-arm-implicit-it=arm"
+// CHECK-THUMB: "-arm-implicit-it=thumb"
+// CHECK-NEVER: "-arm-implicit-it=never"
+// CHECK-ALWAYS: "-arm-implicit-it=always"
+// CHECK-INVALID: error: unsupported argument 'thisisnotavalidoption' to option 'mimplicit-it='

clang/test/Driver/arm-target-as-mimplicit-it.s

@@ -20,6 +20,13 @@
 // RUN: %clang -target arm-linux-gnueabi -### -Wa,-mimplicit-it=always,-mimplicit-it=arm %s 2>&1 | FileCheck %s --check-prefix=ARM
 // RUN: %clang -target arm-linux-gnueabi -### -Wa,-mimplicit-it=always,-mimplicit-it=thumb %s 2>&1 | FileCheck %s --check-prefix=THUMB
 
+/// Mix -implicit-it= (compiler) with -Wa,-mimplicit-it= (assembler), assembler
+/// takes priority. -mllvm -arm-implicit-it= will be repeated, with the
+/// assembler flag appearing last (latter wins).
+// RUN: %clang -target arm-linux-gnueabi -### -mimplicit-it=never -Wa,-mimplicit-it=always %S/Inputs/wildcard1.c 2>&1 | FileCheck %s --check-prefix=NEVER_ALWAYS
+// RUN: %clang -target arm-linux-gnueabi -### -mimplicit-it=always -Wa,-mimplicit-it=never %S/Inputs/wildcard1.c 2>&1 | FileCheck %s --check-prefix=ALWAYS_NEVER
+// RUN: %clang -target arm-linux-gnueabi -### -Wa,-mimplicit-it=never -mimplicit-it=always %S/Inputs/wildcard1.c 2>&1 | FileCheck %s --check-prefix=ALWAYS_NEVER
+
 /// Test invalid input.
 // RUN: %clang -target arm-linux-gnueabi -### -Wa,-mimplicit-it=foo %s 2>&1 | FileCheck %s --check-prefix=INVALID
 // RUN: %clang -target arm-linux-gnueabi -### -Xassembler -mimplicit-it=foo %s 2>&1 | FileCheck %s --check-prefix=XINVALID
@@ -31,5 +38,7 @@
 // NEVER: "-mllvm" "-arm-implicit-it=never"
 // ARM: "-mllvm" "-arm-implicit-it=arm"
 // THUMB: "-mllvm" "-arm-implicit-it=thumb"
+// NEVER_ALWAYS: "-mllvm" "-arm-implicit-it=never" "-mllvm" "-arm-implicit-it=always"
+// ALWAYS_NEVER: "-mllvm" "-arm-implicit-it=always" "-mllvm" "-arm-implicit-it=never"
 // INVALID: error: unsupported argument '-mimplicit-it=foo' to option 'Wa,'
 // XINVALID: error: unsupported argument '-mimplicit-it=foo' to option 'Xassembler'

clang/test/Driver/as-options.s

@@ -53,6 +53,20 @@
 // RUN:   -o /dev/null -x assembler-with-cpp %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
 
+// RUN: %clang -mimplicit-it=always -target armv7-linux-gnueabi -E \
+// RUN:   -fintegrated-as -o /dev/null -x c++ %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=NOWARN --allow-empty %s
+// RUN: %clang -mimplicit-it=always -target armv7-linux-gnueabi -E \
+// RUN:   -fno-integrated-as -o /dev/null -x c++ %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
+
+// RUN: %clang -mimplicit-it=always -target armv7-linux-gnueabi -E \
+// RUN:   -fintegrated-as -o /dev/null -x assembler-with-cpp %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=NOWARN --allow-empty %s
+// RUN: %clang -mimplicit-it=always -target armv7-linux-gnueabi -E \
+// RUN:   -fno-integrated-as -o /dev/null -x assembler-with-cpp %s 2>&1 \
+// RUN:   | FileCheck --check-prefix=WARN --allow-empty %s
+
 // RUN: %clang -Wa,-mbig-obj -target i386-pc-windows -E -fintegrated-as \
 // RUN:   -o /dev/null -x c++ %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=NOWARN --allow-empty %s