[InstCombine] Add folds for (fp_binop ({s|u}itofp x), ({s|u}itofp y))

The full fold is one of the following:
1) `(fp_binop ({s|u}itofp x), ({s|u}itofp y))`
    -> `({s|u}itofp (int_binop x, y))`
2) `(fp_binop ({s|u}itofp x), FpC)`
    -> `({s|u}itofp (int_binop x, (fpto{s|u}i FpC)))`

And support the following binops:
    `fmul` -> `mul`
    `fadd` -> `add`
    `fsub` -> `sub`

Proofs: https://alive2.llvm.org/ce/z/zuacA8

The proofs timeout, so they must be reproduced locally.

Closes llvm#82555

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -2793,6 +2793,9 @@ Instruction *InstCombinerImpl::visitFSub(BinaryOperator &I) {
   if (Instruction *X = foldFNegIntoConstant(I, DL))
     return X;
 
+  if (Instruction *R = foldFBinOpOfIntCasts(I))
+    return R;
+
   Value *X, *Y;
   Constant *C;
 

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -769,6 +769,9 @@ Instruction *InstCombinerImpl::visitFMul(BinaryOperator &I) {
   if (Instruction *R = foldFPSignBitOps(I))
     return R;
 
+  if (Instruction *R = foldFBinOpOfIntCasts(I))
+    return R;
+
   // X * -1.0 --> -X
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   if (match(Op1, m_SpecificFP(-1.0)))

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1402,71 +1402,176 @@ Value *InstCombinerImpl::dyn_castNegVal(Value *V) const {
 }
 
 // Try to fold:
-//    1) (add (sitofp x), (sitofp y))
-//          -> (sitofp (add x, y))
-//    2) (add (sitofp x), FpC)
-//          -> (sitofp (add x, (fptosi FpC)))
+//    1) (fp_binop ({s|u}itofp x), ({s|u}itofp y))
+//        -> ({s|u}itofp (int_binop x, y))
+//    2) (fp_binop ({s|u}itofp x), FpC)
+//        -> ({s|u}itofp (int_binop x, (fpto{s|u}i FpC)))
 Instruction *InstCombinerImpl::foldFBinOpOfIntCasts(BinaryOperator &BO) {
-  // Check for (fadd double (sitofp x), y), see if we can merge this into an
-  // integer add followed by a promotion.
-  Value *LHS = BO.getOperand(0), *RHS = BO.getOperand(1);
-  if (SIToFPInst *LHSConv = dyn_cast<SIToFPInst>(LHS)) {
-    Value *LHSIntVal = LHSConv->getOperand(0);
-    Type *FPType = LHSConv->getType();
-
-    // TODO: This check is overly conservative. In many cases known bits
-    // analysis can tell us that the result of the addition has less significant
-    // bits than the integer type can hold.
-    auto IsValidPromotion = [](Type *FTy, Type *ITy) {
-      Type *FScalarTy = FTy->getScalarType();
-      Type *IScalarTy = ITy->getScalarType();
-
-      // Do we have enough bits in the significand to represent the result of
-      // the integer addition?
-      unsigned MaxRepresentableBits =
-          APFloat::semanticsPrecision(FScalarTy->getFltSemantics());
-      return IScalarTy->getIntegerBitWidth() <= MaxRepresentableBits;
-    };
+  Value *IntOps[2] = {nullptr, nullptr};
+  Constant *Op1FpC = nullptr;
+
+  // Check for:
+  //    1) (binop ({s|u}itofp x), ({s|u}itofp y))
+  //    2) (binop ({s|u}itofp x), FpC)
+  if (!match(BO.getOperand(0), m_SIToFP(m_Value(IntOps[0]))) &&
+      !match(BO.getOperand(0), m_UIToFP(m_Value(IntOps[0]))))
+    return nullptr;
 
-    // (fadd double (sitofp x), fpcst) --> (sitofp (add int x, intcst))
-    // ... if the constant fits in the integer value.  This is useful for things
-    // like (double)(x & 1234) + 4.0 -> (double)((X & 1234)+4) which no longer
-    // requires a constant pool load, and generally allows the add to be better
-    // instcombined.
-    if (ConstantFP *CFP = dyn_cast<ConstantFP>(RHS))
-      if (IsValidPromotion(FPType, LHSIntVal->getType())) {
-        Constant *CI = ConstantFoldCastOperand(Instruction::FPToSI, CFP,
-                                               LHSIntVal->getType(), DL);
-        if (LHSConv->hasOneUse() &&
-            ConstantFoldCastOperand(Instruction::SIToFP, CI, BO.getType(),
-                                    DL) == CFP &&
-            willNotOverflowSignedAdd(LHSIntVal, CI, BO)) {
-          // Insert the new integer add.
-          Value *NewAdd = Builder.CreateNSWAdd(LHSIntVal, CI);
-          return new SIToFPInst(NewAdd, BO.getType());
-        }
-      }
+  if (!match(BO.getOperand(1), m_Constant(Op1FpC)) &&
+      !match(BO.getOperand(1), m_SIToFP(m_Value(IntOps[1]))) &&
+      !match(BO.getOperand(1), m_UIToFP(m_Value(IntOps[1]))))
+    return nullptr;
 
-    // (fadd double (sitofp x), (sitofp y)) --> (sitofp (add int x, y))
-    if (SIToFPInst *RHSConv = dyn_cast<SIToFPInst>(RHS)) {
-      Value *RHSIntVal = RHSConv->getOperand(0);
-      // It's enough to check LHS types only because we require int types to
-      // be the same for this transform.
-      if (IsValidPromotion(FPType, LHSIntVal->getType())) {
-        // Only do this if x/y have the same type, if at least one of them has a
-        // single use (so we don't increase the number of int->fp conversions),
-        // and if the integer add will not overflow.
-        if (LHSIntVal->getType() == RHSIntVal->getType() &&
-            (LHSConv->hasOneUse() || RHSConv->hasOneUse()) &&
-            willNotOverflowSignedAdd(LHSIntVal, RHSIntVal, BO)) {
-          // Insert the new integer add.
-          Value *NewAdd = Builder.CreateNSWAdd(LHSIntVal, RHSIntVal);
-          return new SIToFPInst(NewAdd, BO.getType());
-        }
+  Type *FPTy = BO.getType();
+  Type *IntTy = IntOps[0]->getType();
+
+  // Do we have signed casts?
+  bool OpsFromSigned = isa<SIToFPInst>(BO.getOperand(0));
+
+  unsigned IntSz = IntTy->getScalarSizeInBits();
+  // This is the maximum number of inuse bits by the integer where the int -> fp
+  // casts are exact.
+  unsigned MaxRepresentableBits =
+      APFloat::semanticsPrecision(FPTy->getScalarType()->getFltSemantics());
+
+  // Cache KnownBits a bit to potentially save some analysis.
+  WithCache<const Value *> OpsKnown[2] = {IntOps[0], IntOps[1]};
+
+  // Preserve known number of leading bits. This can allow us to trivial nsw/nuw
+  // checks later on.
+  unsigned NumUsedLeadingBits[2] = {IntSz, IntSz};
+
+  auto IsNonZero = [&](unsigned OpNo) -> bool {
+    if (OpsKnown[OpNo].hasKnownBits() &&
+        OpsKnown[OpNo].getKnownBits(SQ).isNonZero())
+      return true;
+    return isKnownNonZero(IntOps[OpNo], SQ.DL);
+  };
+
+  auto IsNonNeg = [&](unsigned OpNo) -> bool {
+    if (OpsKnown[OpNo].hasKnownBits() &&
+        OpsKnown[OpNo].getKnownBits(SQ).isNonNegative())
+      return true;
+    return isKnownNonNegative(IntOps[OpNo], SQ);
+  };
+
+  // Check if we know for certain that ({s|u}itofp op) is exact.
+  auto IsValidPromotion = [&](unsigned OpNo) -> bool {
+    // If fp precision >= bitwidth(op) then its exact.
+    // NB: This is slightly conservative for `sitofp`. For signed conversion, we
+    // can handle `MaxRepresentableBits == IntSz - 1` as the sign bit will be
+    // handled specially. We can't, however, increase the bound arbitrarily for
+    // `sitofp` as for larger sizes, it won't sign extend.
+    if (MaxRepresentableBits < IntSz) {
+      // Otherwise if its signed cast check that fp precisions >= bitwidth(op) -
+      // numSignBits(op).
+      // TODO: If we add support for `WithCache` in `ComputeNumSignBits`, change
+      // `IntOps[OpNo]` arguments to `KnownOps[OpNo]`.
+      if (OpsFromSigned)
+        NumUsedLeadingBits[OpNo] = IntSz - ComputeNumSignBits(IntOps[OpNo]);
+      // Finally for unsigned check that fp precision >= bitwidth(op) -
+      // numLeadingZeros(op).
+      else {
+        NumUsedLeadingBits[OpNo] =
+            IntSz - OpsKnown[OpNo].getKnownBits(SQ).countMinLeadingZeros();
       }
     }
+    // NB: We could also check if op is known to be a power of 2 or zero (which
+    // will always be representable). Its unlikely, however, that is we are
+    // unable to bound op in any way we will be able to pass the overflow checks
+    // later on.
+
+    if (MaxRepresentableBits < NumUsedLeadingBits[OpNo])
+      return false;
+    // Signed + Mul also requires that op is non-zero to avoid -0 cases.
+    return !OpsFromSigned || BO.getOpcode() != Instruction::FMul ||
+           IsNonZero(OpNo);
+  };
+
+  // If we have a constant rhs, see if we can losslessly convert it to an int.
+  if (Op1FpC != nullptr) {
+    Constant *Op1IntC = ConstantFoldCastOperand(
+        OpsFromSigned ? Instruction::FPToSI : Instruction::FPToUI, Op1FpC,
+        IntTy, DL);
+    if (Op1IntC == nullptr)
+      return nullptr;
+    if (ConstantFoldCastOperand(OpsFromSigned ? Instruction::SIToFP
+                                              : Instruction::UIToFP,
+                                Op1IntC, FPTy, DL) != Op1FpC)
+      return nullptr;
+
+    // First try to keep sign of cast the same.
+    IntOps[1] = Op1IntC;
   }
-  return nullptr;
+
+  // Ensure lhs/rhs integer types match.
+  if (IntTy != IntOps[1]->getType())
+    return nullptr;
+
+  if (Op1FpC == nullptr) {
+    if (OpsFromSigned != isa<SIToFPInst>(BO.getOperand(1))) {
+      // If we have a signed + unsigned, see if we can treat both as signed
+      // (uitofp nneg x) == (sitofp nneg x).
+      if (OpsFromSigned ? !IsNonNeg(1) : !IsNonNeg(0))
+        return nullptr;
+      OpsFromSigned = true;
+    }
+    if (!IsValidPromotion(1))
+      return nullptr;
+  }
+  if (!IsValidPromotion(0))
+    return nullptr;
+
+  // Final we check if the integer version of the binop will not overflow.
+  BinaryOperator::BinaryOps IntOpc;
+  // Because of the precision check, we can often rule out overflows.
+  bool NeedsOverflowCheck = true;
+  // Try to conservatively rule out overflow based on the already done precision
+  // checks.
+  unsigned OverflowMaxOutputBits = OpsFromSigned ? 2 : 1;
+  unsigned OverflowMaxCurBits =
+      std::max(NumUsedLeadingBits[0], NumUsedLeadingBits[1]);
+  bool OutputSigned = OpsFromSigned;
+  switch (BO.getOpcode()) {
+  case Instruction::FAdd:
+    IntOpc = Instruction::Add;
+    OverflowMaxOutputBits += OverflowMaxCurBits;
+    break;
+  case Instruction::FSub:
+    IntOpc = Instruction::Sub;
+    OverflowMaxOutputBits += OverflowMaxCurBits;
+    break;
+  case Instruction::FMul:
+    IntOpc = Instruction::Mul;
+    OverflowMaxOutputBits += OverflowMaxCurBits * 2;
+    break;
+  default:
+    llvm_unreachable("Unsupported binop");
+  }
+  // The precision check may have already ruled out overflow.
+  if (OverflowMaxOutputBits < IntSz) {
+    NeedsOverflowCheck = false;
+    // We can bound unsigned overflow from sub to in range signed value (this is
+    // what allows us to avoid the overflow check for sub).
+    if (IntOpc == Instruction::Sub)
+      OutputSigned = true;
+  }
+
+  // Precision check did not rule out overflow, so need to check.
+  // TODO: If we add support for `WithCache` in `willNotOverflow`, change
+  // `IntOps[...]` arguments to `KnownOps[...]`.
+  if (NeedsOverflowCheck &&
+      !willNotOverflow(IntOpc, IntOps[0], IntOps[1], BO, OutputSigned))
+    return nullptr;
+
+  Value *IntBinOp = Builder.CreateBinOp(IntOpc, IntOps[0], IntOps[1]);
+  if (auto *IntBO = dyn_cast<BinaryOperator>(IntBinOp)) {
+    IntBO->setHasNoSignedWrap(OutputSigned);
+    IntBO->setHasNoUnsignedWrap(!OutputSigned);
+  }
+  if (OutputSigned)
+    return new SIToFPInst(IntBinOp, FPTy);
+  return new UIToFPInst(IntBinOp, FPTy);
 }
 
 /// A binop with a constant operand and a sign-extended boolean operand may be

llvm/test/Transforms/InstCombine/add-sitofp.ll

@@ -83,15 +83,13 @@ define float @test_2_neg(i32 %a, i32 %b) {
   ret float %res
 }
 
-; This test demonstrates overly conservative legality check. The float addition
-; can be replaced with the integer addition because the result of the operation
-; can be represented in float, but we don't do that now.
+; can be represented in float.
 define float @test_3(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test_3(
 ; CHECK-NEXT:    [[M:%.*]] = lshr i32 [[A:%.*]], 24
 ; CHECK-NEXT:    [[N:%.*]] = and i32 [[M]], [[B:%.*]]
-; CHECK-NEXT:    [[O:%.*]] = sitofp i32 [[N]] to float
-; CHECK-NEXT:    [[P:%.*]] = fadd float [[O]], 1.000000e+00
+; CHECK-NEXT:    [[TMP1:%.*]] = add nuw nsw i32 [[N]], 1
+; CHECK-NEXT:    [[P:%.*]] = sitofp i32 [[TMP1]] to float
 ; CHECK-NEXT:    ret float [[P]]
 ;
   %m = lshr i32 %a, 24