[llvm] [InstCombine] Fold fptoui (fdiv (uitofp X), C) to udiv X, C (PR #205853)

[Sungbin Jo via llvm-commits]

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@@ -2504,7 +2504,64 @@ static Instruction *foldFPtoI(Instruction &FI, InstCombiner &IC) {
   if (FPClass.isKnownNever(Mask))
     return IC.replaceInstUsesWith(FI, ConstantInt::getNullValue(FI.getType()));
 
-  return nullptr;
+  // fpto{u/s}i (fdiv ({u/s}itofp X to F), C_fp) --> {u/s}div X, C
+  //
+  // F has precision p (significand bits incl. hidden bit); C_fp is the exact FP
+  // value of the integer constant C. Given N = integer width, this is safe if:
+  //   Unsigned: C > 0 and N <= p.
+  //   Signed:   C != 0 and N - 1 <= p, excluding (X == INT_MIN, C == -1) since
+  //             sdiv INT_MIN, -1 is UB while the FP path only yields poison.
+  //             fdiv X, -1 gets transformed to fneg in InstCombine regardless.
+  //
+  // The bounds make {u/s}itofp and C_fp exact (every |int| <= 2^p is exact),
+  // and ensure the rounded quotient never crosses an integer boundary:
+  //   Rounding lemma: for 0 <= A <= 2^p, 1 <= B <= 2^p, q = floor(A/B),
+  //     trunc(R_p(A/B)) = q.
+  //   For r = A - qB > 0, m = q+1, half-gap H(m) <= q/2^p and
+  //   m - A/B = (B-r)/B >= 1/B > q/2^p >= H(m), so R_p(A/B) < m; q = 0 is
+  //   similar (H(1) = 2^(-p-1) < 2^-p <= 1/B).
+  //   Signed case: by symmetry R_p(-z) = -R_p(z), so fptosi yields s*q = sdiv.
+  bool IsSigned = FI.getOpcode() == Instruction::FPToSI;
+  Value *X;
+  const APFloat *APF;
+  if (IsSigned) {
+    if (!match(FI.getOperand(0),
+               m_OneUse(m_FDiv(m_SIToFP(m_Value(X)), m_APFloat(APF)))))
+      return nullptr;
+  } else {
+    if (!match(FI.getOperand(0),
+               m_OneUse(m_FDiv(m_UIToFP(m_Value(X)), m_APFloat(APF)))))
+      return nullptr;
+  }
+  Type *IntTy = X->getType();
+  if (!IntTy->isIntOrIntVectorTy() || FI.getType() != IntTy)
----------------
goranmoomin wrote:

Done.

https://github.com/llvm/llvm-project/pull/205853