Fix for PR26356

[nemanjai]

Summary

Fast Isel fails when trying to load an unsigned immediate greater than 0x7FFF. The li instruction takes a signed immediate so it isn't appropriate for such values.

[echristo]

I think all we actually need to check is whether or not the sign extended value is in UInt16 rather than the zero extended value.

I've gone ahead and done this and I'll commit it in a few.

[nemanjai]

Thanks Eric. I'll abandon the revision.
I only implemented it this way because for some reason, I didn't want to fall through and get the load immediate shifted. But that really doesn't matter in this context.

lib/Target/PowerPC/PPCFastISel.cpp

unsigned PPCFastISel::PPCMaterializeInt(const ConstantInt *CI, MVT VT,
   // If the constant is in range, use a load-immediate.
   if (UseSExt && isInt<16>(CI->getSExtValue())) {
     unsigned Opc = (VT == MVT::i64) ? PPC::LI8 : PPC::LI;
     unsigned ImmReg = createResultReg(RC);
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(Opc), ImmReg)
         .addImm(CI->getSExtValue());
     return ImmReg;
   } else if (!UseSExt && isUInt<16>(CI->getZExtValue())) {
+    // The li instruction loads signed 16-bit immediates so we
+    // need an extra step for unsigned values with the high bit set.
+    if (CI->getZExtValue() > 0x7FFF) {
+      unsigned LdImm = (VT == MVT::i64) ? PPC::LI8 : PPC::LI;
+      unsigned OrImm = (VT == MVT::i64) ? PPC::ORI8 : PPC::ORI;
+      unsigned ResReg = createResultReg(RC);
+      unsigned TmpReg = createResultReg(RC);
+      BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(LdImm), TmpReg)
+          .addImm(0);
+      BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(OrImm), ResReg)
+          .addReg(TmpReg).addImm(CI->getZExtValue());
+      return ResReg;
+    }
     unsigned Opc = (VT == MVT::i64) ? PPC::LI8 : PPC::LI;
     unsigned ImmReg = createResultReg(RC);
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(Opc), ImmReg)
         .addImm(CI->getZExtValue());
     return ImmReg;
   }
 
   // Construct the constant piecewise.

test/CodeGen/PowerPC/pr26356.ll

+; RUN: llc -mcpu=pwr7 -mtriple=powerpc64le-unknown-unknown -O0 < %s | FileCheck %s
+
+; Function Attrs: nounwind
+define internal zeroext i16 @zot() #0 {
+  ret i16 32768
+}
+
+; CHECK: li 3, 0
+; CHECK: ori 3, 3, 32768
+; CHECK: blr