MIPS: add build time option to disable madd.fmt

[wzssyqa]

Summary

MIPS: add build time option to disable madd.fmt

Some of MIPS CPU has a buggy madd.fmt/msub.fmt insns, Loongson is
an example.

To support all CPUs for a binary Linux distribution - like Debian,
the only choice is to disable madd.fmt/msub.fmt.

So here we add a build time option -DLLVM_DISABLE_MIPS_MADD4.
It does the same thing as the gcc build time option: --with(out)-madd4.

If llvm is built with this option, users can still enable/disable
madd.fmt/msub.fmt by runtime option -mmadd4/-mno-madd4.

[wzssyqa]

No need to block list r6 since the maddf.fmt has 3 reg operand.

[atanasyan]

Maybe it's better to switch off generation of the madd.fmt by default if the problem is so wide spread? I'm concerned that AFAIK no one other architectures supported by LLVM has configuration options like the proposed LLVM_DISABLE_MIPS_MADD4.

[wzssyqa]

In D83778#2152926, @atanasyan wrote:

Maybe it's better to switch off generation of the madd.fmt by default if the problem is so wide spread? I'm concerned that AFAIK no one other architectures supported by LLVM has configuration options like the proposed LLVM_DISABLE_MIPS_MADD4.

currently Loongson is the only known CPU has this problem.
I don't know whether changing the default behavior is a good idea.

I think that we should have a talk with people about: whether and how llvm add some arch-specfied buildtime options.

[MaskRay]

llvm/lib/Target/Mips/MipsSubtarget.cpp

268

+nomadd4 & -nomadd4 below is fine. However, DisableMadd4 = true; is ok.
Can the disabling done in lib/Driver when the problematic CPUs are detected?

[wzssyqa]

For source distributions, like gentoo, it is OK. while for binary distributions like Debian, Fedora, we need to distribute the single binary object to user.

Except the program itself can do auto detect and dispatch different code, we have no way to assume it.
So, the only choice for binary distribution is to disable these insns fully.

[wzssyqa]

The downstream distribution can disable it by patch.

llvm/CMakeLists.txt

 if( LLVM_USE_INTEL_JITEVENTS )
   # Verify we are on a supported platform
   if( NOT CMAKE_SYSTEM_NAME MATCHES "Windows" AND NOT CMAKE_SYSTEM_NAME MATCHES "Linux" )
     message(FATAL_ERROR
       "Intel JIT API support is available on Linux and Windows only.")
   endif()
 endif( LLVM_USE_INTEL_JITEVENTS )
 
+# Loongson has bug on madd.fmt/msub.fmt instructions. gcc has a buildtime option: --without-madd4/--with-madd4
+# Users can still enable/disable madd.fmt/msub.fmt when runtime with option: -mmadd4/-mno-madd4
+option(LLVM_DISABLE_MIPS_MADD4
+  "Disable MIPS madd.fmt/msub.fmt instructions by default."
+  OFF)
+
 option(LLVM_USE_OPROFILE
   "Use opagent JIT interface to inform OProfile about JIT code" OFF)
 
 option(LLVM_EXTERNALIZE_DEBUGINFO
   "Generate dSYM files and strip executables and libraries (Darwin Only)" OFF)
 
 set(LLVM_CODESIGNING_IDENTITY "" CACHE STRING
   "Sign executables and dylibs with the given identity or skip if empty (Darwin Only)")

llvm/include/llvm/Config/llvm-config.h.cmake

 #cmakedefine LLVM_NATIVE_TARGETMC LLVMInitialize${LLVM_NATIVE_ARCH}TargetMC
 
 /* Define if this is Unixish platform */
 #cmakedefine LLVM_ON_UNIX ${LLVM_ON_UNIX}
 
 /* Define if we have the Intel JIT API runtime support library */
 #cmakedefine01 LLVM_USE_INTEL_JITEVENTS
 
+/* Define if wish disable MIPS madd.fmt/msub.fmt instructions by default. */
+#cmakedefine01 LLVM_DISABLE_MIPS_MADD4
+
 /* Define if we have the oprofile JIT-support library */
 #cmakedefine01 LLVM_USE_OPROFILE
 
 /* Define if we have the perf JIT-support library */
 #cmakedefine01 LLVM_USE_PERF
 
 /* Major version of the LLVM API */
 #define LLVM_VERSION_MAJOR ${LLVM_VERSION_MAJOR}

llvm/lib/Target/Mips/MipsSubtarget.cpp

: MipsGenSubtargetInfo(TT, CPU, FS), MipsArchVersion(MipsDefault),
       InstrInfo(
           MipsInstrInfo::create(initializeSubtargetDependencies(CPU, FS, TM))),
       FrameLowering(MipsFrameLowering::create(*this)),
       TLInfo(MipsTargetLowering::create(TM, *this)) {
 
   if (MipsArchVersion == MipsDefault)
     MipsArchVersion = Mips32;
 
+  if (hasMips32r6() || hasMips64r6())
+    DisableMadd4 = false;
+
   // Don't even attempt to generate code for MIPS-I and MIPS-V. They have not
   // been tested and currently exist for the integrated assembler only.
   if (MipsArchVersion == Mips1)
     report_fatal_error("Code generation for MIPS-I is not implemented", false);
   if (MipsArchVersion == Mips5)
     report_fatal_error("Code generation for MIPS-V is not implemented", false);
 
   // Check if Architecture and ABI are compatible.

 CodeGenOpt::Level MipsSubtarget::getOptLevelToEnablePostRAScheduler() const {
   return CodeGenOpt::Aggressive;
 }
 
 MipsSubtarget &
 MipsSubtarget::initializeSubtargetDependencies(StringRef CPU, StringRef FS,
                                                const TargetMachine &TM) {
   StringRef CPUName = MIPS_MC::selectMipsCPU(TM.getTargetTriple(), CPU);
+  SubtargetFeatures Features(FS);
 
   // Parse features string.
   ParseSubtargetFeatures(CPUName, FS);
   // Initialize scheduling itinerary for the specified CPU.
   InstrItins = getInstrItineraryForCPU(CPUName);
 
   if (InMips16Mode && !IsSoftFloat)
     InMips16HardFloat = true;
 
   if (StackAlignOverride)
     stackAlignment = *StackAlignOverride;
   else if (isABI_N32() || isABI_N64())
     stackAlignment = Align(16);
   else {
     assert(isABI_O32() && "Unknown ABI for stack alignment!");
     stackAlignment = Align(8);
   }
 
   if ((isABI_N32() || isABI_N64()) && !isGP64bit())
     report_fatal_error("64-bit code requested on a subtarget that doesn't "
                        "support it!");
 
+#if LLVM_DISABLE_MIPS_MADD4
+  DisableMadd4 = true;
+#endif
+  for (const std::string &Feature : Features.getFeatures()) {
+    if (Feature == "+nomadd4")
+      DisableMadd4 = true;
+    else if (Feature == "-nomadd4")
+      DisableMadd4 = false;
+  }
+
   return *this;
 }
 
 bool MipsSubtarget::useConstantIslands() {
   LLVM_DEBUG(dbgs() << "use constant islands " << Mips16ConstantIslands
                     << "\n");
   return Mips16ConstantIslands;
 }