Add -mno-movt frontend option, to disable movt/movw on ARM

[dim]

Summary

In rL256641, @davide turned off movt generation by default for FreeBSD.
This was because our ld is very old, and did not support the relocations
for it. However, Ian Lepore added the support very recently, so we
would like to revert rL256641, and replace it with a new -mno-movt
frontend option. This way, it can be turned off when needed.

[davide]

I'm not opposed to this but ... this will likely it only 11 (and maybe 10.3 if backported) -- what about people running <= 10.2 ? Isn't this a problem for them?

[davide]

Hmm, probably they can use the new frontend options so it's fine. LGTM.

[emaste]

This LGTM

[echristo]

Sure.

-eric

[dim]

In D15899#319902, @davide wrote:

I'm not opposed to this but ... this will likely it only 11 (and maybe 10.3 if backported) -- what about people running <= 10.2 ? Isn't this a problem for them?

People running ports-provided or hand-built clang 3.8.0 or higher on older FreeBSD's can use -fno-movt. For clang versions 3.7.x and earlier, they can still use the old style -mllvm -arm-use-movt=0.

We'll fix the FreeBSD build system so it uses the right options for older and newer clang versions.

[davide]

In D15899#319918, @dim wrote:

In D15899#319902, @davide wrote:

I'm not opposed to this but ... this will likely it only 11 (and maybe 10.3 if backported) -- what about people running <= 10.2 ? Isn't this a problem for them?

People running ports-provided or hand-built clang 3.8.0 or higher on older FreeBSD's can use -fno-movt. For clang versions 3.7.x and earlier, they can still use the old style -mllvm -arm-use-movt=0.

We'll fix the FreeBSD build system so it uses the right options for older and newer clang versions.

Sounds like a plan.

[dim]

Rename -fno-movt to -mno-movt.

include/clang/Driver/Options.td

 def mno_thumb : Flag<["-"], "mno-thumb">, Group<m_arm_Features_Group>;
 def mrestrict_it: Flag<["-"], "mrestrict-it">, Group<m_arm_Features_Group>,
   HelpText<"Disallow generation of deprecated IT blocks for ARMv8. It is on by default for ARMv8 Thumb mode.">;
 def mno_restrict_it: Flag<["-"], "mno-restrict-it">, Group<m_arm_Features_Group>,
   HelpText<"Allow generation of deprecated IT blocks for ARMv8. It is off by default for ARMv8 Thumb mode">;
 def marm : Flag<["-"], "marm">, Alias<mno_thumb>;
 def ffixed_r9 : Flag<["-"], "ffixed-r9">, Group<m_arm_Features_Group>,
   HelpText<"Reserve the r9 register (ARM only)">;
+def mno_movt : Flag<["-"], "mno-movt">, Group<m_arm_Features_Group>,
+  HelpText<"Disallow use of movt/movw pairs (ARM only)">;
 def mcrc : Flag<["-"], "mcrc">, Group<m_arm_Features_Group>,
   HelpText<"Allow use of CRC instructions (ARM only)">;
 def mnocrc : Flag<["-"], "mnocrc">, Group<m_arm_Features_Group>,
   HelpText<"Disallow use of CRC instructions (ARM only)">;
 def mlong_calls : Flag<["-"], "mlong-calls">, Group<m_arm_Features_Group>,
   HelpText<"Generate an indirect jump to enable jumps further than 64M">;
 def mno_long_calls : Flag<["-"], "mno-long-calls">, Group<m_arm_Features_Group>,
   HelpText<"Restore the default behaviour of not generating long calls">;

lib/Driver/Tools.cpp

static void getARMTargetFeatures(const ToolChain &TC,
   }
 
   // llvm does not support reserving registers in general. There is support
   // for reserving r9 on ARM though (defined as a platform-specific register
   // in ARM EABI).
   if (Args.hasArg(options::OPT_ffixed_r9))
     Features.push_back("+reserve-r9");
 
-  // The kext and FreeBSD linkers don't know how to deal with movw/movt.
-  if (KernelOrKext || Triple.isOSFreeBSD())
+  // The kext linker doesn't know how to deal with movw/movt.
+  if (KernelOrKext || Args.hasArg(options::OPT_mno_movt))
     Features.push_back("+no-movt");
 }
 
 void Clang::AddARMTargetArgs(const llvm::Triple &Triple, const ArgList &Args,
                              ArgStringList &CmdArgs, bool KernelOrKext) const {
   // Select the ABI to use.
   // FIXME: Support -meabi.
   // FIXME: Parts of this are duplicated in the backend, unify this somehow.

test/Driver/arm-no-movt.c

 // RUN: %clang -target armv7-apple-darwin -### %s 2>&1 \
 // RUN:    | FileCheck %s -check-prefix CHECK-DEFAULT
 
 // RUN: %clang -target armv7-apple-darwin -mkernel -### %s 2>&1 \
 // RUN:    | FileCheck %s -check-prefix CHECK-KERNEL
 
-// RUN: %clang -target armv7-gnueabi-freebsd11 -### %s 2>&1 \
-// RUN:    | FileCheck %s -check-prefix CHECK-FREEBSD
+// RUN: %clang -target armv7-none-gnueabi -mno-movt -### %s 2>&1 \
+// RUN:    | FileCheck %s -check-prefix CHECK-NO-MOVT
 
 // CHECK-DEFAULT-NOT: "-target-feature" "+no-movt"
 
 // CHECK-KERNEL: "-target-feature" "+no-movt"
 
-// CHECK-FREEBSD: "-target-feature" "+no-movt"
+// CHECK-NO-MOVT: "-target-feature" "+no-movt"