[llvm] [AggressiveInstCombine] Recognize table based log2 and replace with ctlz+sub. (PR #185160)

[David Green via llvm-commits]

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@@ -626,6 +626,205 @@ static bool tryToRecognizeTableBasedCttz(Instruction &I, const DataLayout &DL) {
   return true;
 }
 
+// Check if this array of constants represents a log2 table.
+// Iterate over the elements from \p Table by trying to find/match all
+// the numbers from 0 to \p InputBits that should represent log2 results.
+static bool isLog2Table(Constant *Table, const APInt &Mul, const APInt &Shift,
+                        const APInt &AndMask, Type *AccessTy,
+                        unsigned InputBits, const APInt &GEPIdxFactor,
+                        const DataLayout &DL) {
+  for (unsigned Idx = 0; Idx < InputBits; Idx++) {
+    APInt Index =
+        (APInt::getLowBitsSet(InputBits, Idx + 1) * Mul).lshr(Shift) & AndMask;
+    ConstantInt *C = dyn_cast_or_null<ConstantInt>(
+        ConstantFoldLoadFromConst(Table, AccessTy, Index * GEPIdxFactor, DL));
+    if (!C || C->getValue() != Idx)
+      return false;
+  }
+
+  // Verify that an input of zero will select table index 0.
+  APInt ZeroIndex = Mul.lshr(Shift) & AndMask;
+  if (!ZeroIndex.isZero())
+    return false;
+
+  return true;
+}
+
+// Try to recognize table-based log2 implementation.
+// E.g., an exmapel in C (for more cases please the llvm/tests):
+// int f(unsigned x) {
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davemgreen wrote:

x => v

https://github.com/llvm/llvm-project/pull/185160