[llvm] [PowerPC] Fix inefficient code for __builtin_ppc_test_data_class (PR #181420)

[Lei Huang via llvm-commits]

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@@ -17198,6 +17214,173 @@ static SDValue DAGCombineAddc(SDNode *N,
   return SDValue();
 }
 
+/// Optimize zero-extension of setcc when the compared value is known to be 0
+/// or 1.
+///
+/// Pattern: zext(setcc(Value, 0, seteq/setne)) where Value is 0 or 1
+///   -> zext(xor(Value, 1))  for seteq
+///   -> zext(Value)          for setne
+///
+/// This optimization avoids the i32 -> i1 -> i32/i64 conversion sequence
+/// by keeping the value in its original i32 type throughout.
+///
+/// Example:
+///   Before: zext(setcc(test_data_class(...), 0, seteq))
+///           // test_data_class returns 0 or 1 in i32
+///           // setcc converts i32 -> i1
+///           // zext converts i1 -> i64
+///   After:  zext(xor(test_data_class(...), 1))
+///           // Stays in i32, then extends to i64
+///
+/// This is beneficial because:
+/// 1. Eliminates the setcc instruction
+/// 2. Avoids i32 -> i1 truncation
+/// 3. Keeps computation in native integer width
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lei137 wrote:

Detailed doc/comment is always good.  I was just asking whether we should be using `//` vs doxygen doc via `///`.

https://github.com/llvm/llvm-project/pull/181420