[mips] Don't use FastISel when -mxgot is present

[smaksimovic]

Summary

The clang compiler by default uses FastISel when invoked with -O0, which is also the default.
In that case, passing of -mxgot doesn't get honored, i.e. the code path that is to deal with large got is not taken.
Clang produces same output regardless of -mxgot being present or not.
This change checks whether -mxgot is passed as an option, and turns off FastISel if it is.

[sdardis]

LGTM. Comment inlined.

lib/Target/Mips/MipsISelLowering.cpp

474 ↗

(On Diff #100390)

Also, update this list of reasons why we disable fastisel to include the xgot case.

[smaksimovic]

Comment updated.

llvm/trunk/lib/Target/Mips/MipsISelLowering.cpp

MipsTargetLowering::createFastISel(FunctionLoweringInfo &funcInfo,
   const MipsTargetMachine &TM =
       static_cast<const MipsTargetMachine &>(funcInfo.MF->getTarget());
 
   // We support only the standard encoding [MIPS32,MIPS32R5] ISAs.
   bool UseFastISel = TM.Options.EnableFastISel && Subtarget.hasMips32() &&
                      !Subtarget.hasMips32r6() && !Subtarget.inMips16Mode() &&
                      !Subtarget.inMicroMipsMode();
 
-  // Disable if we don't generate PIC or the ABI isn't O32.
-  if (!TM.isPositionIndependent() || !TM.getABI().IsO32())
+  // Disable if either of the following is true:
+  // We do not generate PIC, the ABI is not O32, LargeGOT is being used.
+  if (!TM.isPositionIndependent() || !TM.getABI().IsO32() || LargeGOT)
     UseFastISel = false;
 
   return UseFastISel ? Mips::createFastISel(funcInfo, libInfo) : nullptr;
 }
 
 EVT MipsTargetLowering::getSetCCResultType(const DataLayout &, LLVMContext &,
                                            EVT VT) const {
   if (!VT.isVector())

llvm/trunk/test/CodeGen/Mips/biggot.ll

 ; RUN: llc -march=mipsel -mxgot -relocation-model=pic < %s | FileCheck %s -check-prefix=O32
 ; RUN: llc -march=mips64el -mcpu=mips64r2 -mxgot -relocation-model=pic < %s | \
 ; RUN: FileCheck %s -check-prefix=N64
+; RUN: llc -march=mipsel -mxgot -relocation-model=pic -fast-isel < %s | FileCheck %s -check-prefix=O32
+; RUN: llc -march=mips64el -mcpu=mips64r2 -mxgot -relocation-model=pic -fast-isel < %s | \
+; RUN: FileCheck %s -check-prefix=N64
 
 @v0 = external global i32
 
 define void @foo1() nounwind {
 entry:
 ; O32-LABEL: foo1:
 ; O32: lui $[[R0:[0-9]+]], %got_hi(v0)
 ; O32: addu  $[[R1:[0-9]+]], $[[R0]], ${{[a-z0-9]+}}