[SystemZ] Return a higher cost for fdiv from getUserCost()

[jonpa]

Summary

It seems that fdiv on SystemZ is so expensive that it shouldn't be moved up in the CFG by SpeculativeExecution.cpp.

Not sure if this adjustment makes sense on the target, but it seems at least on a high level to be unreasonable to speculate fdiv.

This also affects other decisions, such as the amount of unrolling, it seems.

[jonpa]

This didn't seem to do much good on any benchmarks, so abandoning.

lib/Target/SystemZ/SystemZTargetTransformInfo.h

explicit SystemZTTIImpl(const SystemZTargetMachine *TM, const Function &F)
       : BaseT(TM, F.getParent()->getDataLayout()), ST(TM->getSubtargetImpl(F)),
         TLI(ST->getTargetLowering()) {}
 
   /// \name Scalar TTI Implementations
   /// @{
 
   unsigned getInliningThresholdMultiplier() { return 3; }
 
+  unsigned getUserCost(const User *U, ArrayRef<const Value *> Operands);
+
   int getIntImmCost(const APInt &Imm, Type *Ty);
 
   int getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm, Type *Ty);
   int getIntImmCost(Intrinsic::ID IID, unsigned Idx, const APInt &Imm,
                     Type *Ty);
 
   TTI::PopcntSupportKind getPopcntSupport(unsigned TyWidth);
 

lib/Target/SystemZ/SystemZTargetTransformInfo.cpp

 #define DEBUG_TYPE "systemztti"
 
 //===----------------------------------------------------------------------===//
 //
 // SystemZ cost model.
 //
 //===----------------------------------------------------------------------===//
 
+unsigned SystemZTTIImpl::getUserCost(const User *U,
+                                     ArrayRef<const Value *> Operands) {
+  if (const Instruction *I = dyn_cast<Instruction>(U))
+    if (I->getOpcode() == Instruction::FDiv)
+      return TTI::TCC_Expensive * 2;
+  return BaseT::getUserCost(U, Operands);
+}
+
 int SystemZTTIImpl::getIntImmCost(const APInt &Imm, Type *Ty) {
   assert(Ty->isIntegerTy());
 
   unsigned BitSize = Ty->getPrimitiveSizeInBits();
   // There is no cost model for constants with a bit size of 0. Return TCC_Free
   // here, so that constant hoisting will ignore this constant.
   if (BitSize == 0)
     return TTI::TCC_Free;

test/CodeGen/SystemZ/fdiv-speculation.ll

+; RUN: opt < %s -mtriple=systemz-unknown -mcpu=z13 -speculative-execution -S \
+; RUN:  | FileCheck %s
+;
+; Test that an fdiv is not speculatively executed.
+
+define void @f(double %arg) {
+; CHECK-LABEL: bb:
+; CHECK-LABEL: bb3:
+; CHECK: fdiv
+
+bb:
+  %tmp2 = fcmp ogt double %arg, 0x3E7AD7F29ABCAF48
+  br i1 %tmp2, label %bb3, label %bb5
+
+bb3:
+  %tmp4 = fdiv double 1.000000e+00, %arg
+  br label %bb5
+
+bb5:
+  ret void
+}