[libc-commits] [libc] [libc][stdfix] Implement fxdivi functions (rdivi) (PR #154914)

[via libc-commits]

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@@ -281,11 +281,18 @@ template <typename XType> LIBC_INLINE constexpr XType divi(int n, int d) {
   // to quadratic convergence. So,
   // E1 = (0.059)^2 = 0.0034
   long accum val = nrstep(d_scaled_val, initial_approx);
-  // E2 = 0.0000121
-  val = nrstep(d_scaled_val, val);
-  // E3 = 1.468e−10
-  val = nrstep(d_scaled_val, val);
-
+  auto isPowerOfTwo = [](int n) { return (n > 0) && ((n & (n - 1)) == 0); };
+  // Division with a power of 2 would generally be expected to be
+  // exact, we handle this by specially treating po2 cases and having
+  // extra iterations for them.
+  if (FXRep<XType>::FRACTION_LEN > 8 || isPowerOfTwo(cpp::abs(d))) {
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lntue wrote:

when the denominator is a power of two, it's better to just perform right shifts (be careful with signed-ness and overflow).  Then the other conditions on the fraction lengths can be `constexpr`, i.e. no runtime cost.

https://github.com/llvm/llvm-project/pull/154914