[llvm-dev] Bug 20871 -- is there a fix or work around?

[Riyaz Puthiyapurayil via llvm-dev]

Thanks. Looks like we can do this for only __multi3. Other TImode functions (e.g. __divti3, __modti3, etc.) are not defined on 32-bit. I am not sure if any optimizations could cause those to be generated. AFAICT there is no fall back support (like what is there for MUL) in the Legalizer for wide SDIV, etc. See below:

void DAGTypeLegalizer::ExpandIntRes_SDIV(SDNode *N,
                                         SDValue &Lo, SDValue &Hi) {
  EVT VT = N->getValueType(0);
  SDLoc dl(N);
  SDValue Ops[2] = { N->getOperand(0), N->getOperand(1) };

  if (TLI.getOperationAction(ISD::SDIVREM, VT) == TargetLowering::Custom) {
    SDValue Res = DAG.getNode(ISD::SDIVREM, dl, DAG.getVTList(VT, VT), Ops);
    SplitInteger(Res.getValue(0), Lo, Hi);
    return;
  }

  RTLIB::Libcall LC = RTLIB::UNKNOWN_LIBCALL;
  if (VT == MVT::i16)
    LC = RTLIB::SDIV_I16;
  else if (VT == MVT::i32)
    LC = RTLIB::SDIV_I32;
  else if (VT == MVT::i64)
    LC = RTLIB::SDIV_I64;
  else if (VT == MVT::i128)
    LC = RTLIB::SDIV_I128;
  assert(LC != RTLIB::UNKNOWN_LIBCALL && "Unsupported SDIV!");

  SplitInteger(TLI.makeLibCall(DAG, LC, VT, Ops, true, dl).first, Lo, Hi);
}

From: Craig Topper [mailto:craig.topper at gmail.com]
Sent: Friday, October 6, 2017 11:13 PM
To: Riyaz Puthiyapurayil <Riyaz.Puthiyapurayil at synopsys.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Bug 20871 -- is there a fix or work around?

That seems like a valid fix to me.

~Craig

On Fri, Oct 6, 2017 at 9:45 PM, Riyaz Puthiyapurayil via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Ignore the suggested fix in my earlier post. How about this?

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 20c81c3..b8ebf42 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -1632,10 +1632,11 @@ X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
   if (!Subtarget.is64Bit()) {
     // These libcalls are not available in 32-bit.
     setLibcallName(RTLIB::SHL_I128, nullptr);
     setLibcallName(RTLIB::SRL_I128, nullptr);
     setLibcallName(RTLIB::SRA_I128, nullptr);
+    setLibcallName(RTLIB::MUL_I128, nullptr);
   }
   // Combine sin / cos into one node or libcall if possible.
   if (Subtarget.hasSinCos()) {
     setLibcallName(RTLIB::SINCOS_F32, "sincosf");

This will make the Legalizer choose the brute force approach below:

void DAGTypeLegalizer::ExpandIntRes_MUL(SDNode *N,
                                        SDValue &Lo, SDValue &Hi) {
...

  if (LC == RTLIB::UNKNOWN_LIBCALL || !TLI.getLibcallName(LC)) {
    // We'll expand the multiplication by brute force because we have no other
    // options. This is a trivially-generalized version of the code from
    // Hacker's Delight (itself derived from Knuth's Algorithm M from section
    // 4.3.1).

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