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

[Sungbin Jo via llvm-commits]

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@@ -2507,10 +2507,74 @@ static Instruction *foldFPtoI(Instruction &FI, InstCombiner &IC) {
   return nullptr;
 }
 
+/// fpto{u/s}i (fdiv ({u/s}itofp X), C) --> {u/s}div X, C
+///
+/// Given N = integer width, p = FP mantissa width, this is safe if:
+/// Unsigned: C > 0 and N <= p.
+/// Signed: C != 0, N - 1 <= p, and not (X == INT_MIN and C == -1).
+/// Then {u/s}itofp is exact and the rounded quotient never crosses an integer,
+/// i.e. floor(rne_p(X/C)) == floor(X/C).
+///
+/// See #205305 for detailed reasoning.
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goranmoomin wrote:

Done in latest commit.

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