[clang] First draft: [Clang] Add constant evaluation support for x86 psadbw/p… (PR #169253)

[Simon Pilgrim via cfe-commits]

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@@ -12078,6 +12080,54 @@ static bool evalPackBuiltin(const CallExpr *E, EvalInfo &Info, APValue &Result,
   return true;
 }
 
+static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info, APValue &Result) {
+  // 1) Evaluate the arguments into APValues
+  APValue A, B;
+  if (!Evaluate(A, Info, E->getArg(0)) ||
+      !Evaluate(B, Info, E->getArg(1)))
+    return false;
+
+  if (!A.isVector() || !B.isVector())
+    return false;
+
+  unsigned Len = A.getVectorLength();
+  if (Len != 16) // psadbw128 uses 16 bytes (2 × 8)
+    return false;
+
+  // 2) Compute SAD over two 8-byte blocks
+  uint64_t Sum0 = 0;
+  uint64_t Sum1 = 0;
+
+  // bytes 0..7
+  for (unsigned i = 0; i < 8; ++i) {
+    uint64_t a = A.getVectorElt(i).getInt().getZExtValue();
+    uint64_t b = B.getVectorElt(i).getInt().getZExtValue();
+    Sum0 += (a > b ? a - b : b - a);
+  }
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RKSimon wrote:

Use APIntOps::abdu ?

https://github.com/llvm/llvm-project/pull/169253