[llvm] [LoopIdiomRecognize] Enable clmul optimization for CRC loops (PR #203405)

[Piotr Fusik via llvm-commits]

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@@ -376,6 +376,74 @@ CRCTable HashRecognize::genSarwateTable(const APInt &GenPoly,
   return Table;
 }
 
+// Perform polynomial (GF(2)) floor division. This is based on the
+// floor_division(S, P) algorithm in
+// https://www.corsix.org/content/barrett-reduction-polynomials. Note that the
+// maximum degree of the returned polynomial is
+// max(0, deg(Dividend) - deg(Divisor)), but the bit width will be the same as
+// that of Dividend.
+static APInt floorDivideGF2(APInt Dividend, APInt Divisor) {
+  assert(!Divisor.isZero() && "Cannot divide by zero");
+
+  // Extend the divisor bit width to match the dividend.
+  Divisor = Divisor.zext(Dividend.getBitWidth());
+
+  // Note that getActiveBits(_) returns deg(_)+1, but the computation below
+  // still holds.
+  unsigned DivisorActiveBits = Divisor.getActiveBits();
+
+  // Q = 0
+  APInt Quotient = APInt::getZero(Dividend.getBitWidth());
+  // S != 0 and deg(S) >= deg(P)
+  while (!Dividend.isZero() && Dividend.getActiveBits() >= DivisorActiveBits) {
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pfusik wrote:

Remove the `!Dividend.isZero()` condition. For `Dividend.isZero()`, `Dividend.getActiveBits()` is zero.

https://github.com/llvm/llvm-project/pull/203405