[libc-commits] [libc] [libc] Add bigint casting between word types (PR #111914)

[via libc-commits]

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@@ -361,17 +361,90 @@ struct BigInt {
 
   LIBC_INLINE constexpr BigInt(const BigInt &other) = default;
 
-  template <size_t OtherBits, bool OtherSigned>
+  template <size_t OtherBits, bool OtherSigned, typename OtherWordType>
   LIBC_INLINE constexpr BigInt(
-      const BigInt<OtherBits, OtherSigned, WordType> &other) {
-    if (OtherBits >= Bits) { // truncate
-      for (size_t i = 0; i < WORD_COUNT; ++i)
-        val[i] = other[i];
-    } else { // zero or sign extend
-      size_t i = 0;
-      for (; i < OtherBits / WORD_SIZE; ++i)
-        val[i] = other[i];
-      extend(i, Signed && other.is_neg());
+      const BigInt<OtherBits, OtherSigned, OtherWordType> &other) {
+    using BigIntOther = BigInt<OtherBits, OtherSigned, OtherWordType>;
+    const bool should_sign_extend = Signed && other.is_neg();
+
+    if constexpr (BigIntOther::WORD_SIZE < WORD_SIZE) {
+      // OtherWordType is smaller
+      constexpr size_t WORD_SIZE_RATIO = WORD_SIZE / BigIntOther::WORD_SIZE;
+      static_assert(
+          (WORD_SIZE % BigIntOther::WORD_SIZE) == 0 &&
+          "Word types must be multiples of each other for correct conversion.");
+      if (OtherBits >= Bits) { // truncate
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lntue wrote:

I guess this can be `if constexpr` also?

https://github.com/llvm/llvm-project/pull/111914