[ARM] Add option to force fast-isel

[olista01]

Summary

The ARM backend has some logic that only allows the fast-isel to be enabled for subtargets where it is known to be stable. This adds a backend option to override this and force the fast-isel to be used for any target, to allow it to be tested.

This is an ARM-specific option, because no other backend disables the fast-isel on a per-subtarget basis.

[echristo]

By "for testing only" I assume you mean "build some stuff I've got" rather than "for use in llvm/tests" right?

On the assumption your answer is "yes", LGTM and thanks!

-eric

[olista01]

By "for testing only" I assume you mean "build some stuff I've got" rather than "for use in llvm/tests" right?

Yep, this is to allow me to run csmith on some bare-metal targets.

Committed revision 248369.

lib/Target/ARM/ARMSubtarget.cpp

IT(cl::desc("IT block support"), cl::Hidden, cl::init(DefaultIT),
    cl::values(clEnumValN(DefaultIT, "arm-default-it",
                          "Generate IT block based on arch"),
               clEnumValN(RestrictedIT, "arm-restrict-it",
                          "Disallow deprecated IT based on ARMv8"),
               clEnumValN(NoRestrictedIT, "arm-no-restrict-it",
                          "Allow IT blocks based on ARMv7"),
               clEnumValEnd));
 
+/// ForceFastISel - Use the fast-isel, even for subtargets where it is not
+/// currently supported (for testing only).
+static cl::opt<bool>
+ForceFastISel("arm-force-fast-isel",
+               cl::init(false), cl::Hidden);
+
 /// initializeSubtargetDependencies - Initializes using a CPU and feature string
 /// so that we can use initializer lists for subtarget initialization.
 ARMSubtarget &ARMSubtarget::initializeSubtargetDependencies(StringRef CPU,
                                                             StringRef FS) {
   initializeEnvironment();
   initSubtargetFeatures(CPU, FS);
   return *this;
 }
bool ARMSubtarget::useMovt(const MachineFunction &MF) const {
   // NOTE Windows on ARM needs to use mov.w/mov.t pairs to materialise 32-bit
   // immediates as it is inherently position independent, and may be out of
   // range otherwise.
   return !NoMovt && hasV6T2Ops() &&
          (isTargetWindows() || !MF.getFunction()->optForMinSize());
 }
 
 bool ARMSubtarget::useFastISel() const {
+  // Enable fast-isel for any target, for testing only.
+  if (ForceFastISel)
+    return true;
+
   // Limit fast-isel to the targets that are or have been tested.
   if (!hasV6Ops())
     return false;
 
   // Thumb2 support on iOS; ARM support on iOS, Linux and NaCl.
   return TM.Options.EnableFastISel &&
          ((isTargetMachO() && !isThumb1Only()) ||
           (isTargetLinux() && !isThumb()) || (isTargetNaCl() && !isThumb()));
 }