[X86][GlobalISel] Support G_FCMP for scalar cases (#123598)

With this patch, we are enabling comparisons for Long double (80) types
on X87 stack. It lowers G_FCMP.

---------

Co-authored-by: mattarde <[email protected]>

Author: mahesh-attarde

llvm/lib/Target/X86/GISel/X86InstructionSelector.cpp

@@ -1048,17 +1048,29 @@ bool X86InstructionSelector::selectFCmp(MachineInstr &I,
     break;
   }
 
+  assert((LhsReg.isVirtual() && RhsReg.isVirtual()) &&
+         "Both arguments of FCMP need to be virtual!");
+  auto *LhsBank = RBI.getRegBank(LhsReg, MRI, TRI);
+  auto *RhsBank = RBI.getRegBank(RhsReg, MRI, TRI);
+  assert((LhsBank == RhsBank) &&
+         "Both banks assigned to FCMP arguments need to be same!");
+
   // Compute the opcode for the CMP instruction.
   unsigned OpCmp;
   LLT Ty = MRI.getType(LhsReg);
   switch (Ty.getSizeInBits()) {
   default:
     return false;
   case 32:
-    OpCmp = X86::UCOMISSrr;
+    OpCmp = LhsBank->getID() == X86::PSRRegBankID ? X86::UCOM_FpIr32
+                                                  : X86::UCOMISSrr;
     break;
   case 64:
-    OpCmp = X86::UCOMISDrr;
+    OpCmp = LhsBank->getID() == X86::PSRRegBankID ? X86::UCOM_FpIr64
+                                                  : X86::UCOMISDrr;
+    break;
+  case 80:
+    OpCmp = X86::UCOM_FpIr80;
     break;
   }
 

llvm/lib/Target/X86/GISel/X86LegalizerInfo.cpp

@@ -452,10 +452,9 @@ X86LegalizerInfo::X86LegalizerInfo(const X86Subtarget &STI,
 
   // fp comparison
   getActionDefinitionsBuilder(G_FCMP)
-      .legalIf([=](const LegalityQuery &Query) {
-        return (HasSSE1 && typePairInSet(0, 1, {{s8, s32}})(Query)) ||
-               (HasSSE2 && typePairInSet(0, 1, {{s8, s64}})(Query));
-      })
+      .legalFor(HasSSE1 || UseX87, {s8, s32})
+      .legalFor(HasSSE2 || UseX87, {s8, s64})
+      .legalFor(UseX87, {s8, s80})
       .clampScalar(0, s8, s8)
       .clampScalar(1, s32, HasSSE2 ? s64 : s32)
       .widenScalarToNextPow2(1);

llvm/lib/Target/X86/GISel/X86RegisterBankInfo.cpp

@@ -321,8 +321,8 @@ X86RegisterBankInfo::getInstrMapping(const MachineInstr &MI) const {
 
     unsigned Size = Ty1.getSizeInBits();
     (void)Size;
-    assert((Size == 32 || Size == 64) && "Unsupported size for G_FCMP");
-
+    assert((Size == 32 || Size == 64 || Size == 80) &&
+           "Unsupported size for G_FCMP");
     auto FpRegBank = getPartialMappingIdx(MI, Ty1, /* isFP= */ true);
     OpRegBankIdx = {PMI_GPR8,
                     /* Predicate */ PMI_None, FpRegBank, FpRegBank};