[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
+        // for each big word
+        for (size_t i = 0; i < WORD_COUNT; ++i) {
+          WordType cur_word = 0;
+          // combine WORD_SIZE_RATIO small words into a big word
+          for (size_t j = 0; j < WORD_SIZE_RATIO; ++j)
+            cur_word |= static_cast<WordType>(other[(i * WORD_SIZE_RATIO) + j])
+                        << (BigIntOther::WORD_SIZE * j);
+
+          val[i] = cur_word;
+        }
+      } else { // zero or sign extend
+        size_t i = 0;
+        WordType cur_word = 0;
+        // for each small word
+        for (; i < BigIntOther::WORD_COUNT; ++i) {
+          // combine WORD_SIZE_RATIO small words into a big word
+          cur_word |= static_cast<WordType>(other[i])
+                      << (BigIntOther::WORD_SIZE * (i % WORD_SIZE_RATIO));
+          // if we've completed a big word, copy it into place and reset
+          if ((i % WORD_SIZE_RATIO) == WORD_SIZE_RATIO - 1) {
+            val[i / WORD_SIZE_RATIO] = cur_word;
+            cur_word = 0;
+          }
+        }
+        // Pretend there are extra words of the correct sign extension as needed
+
+        const WordType extension_bits =
+            should_sign_extend ? cpp::numeric_limits<WordType>::max()
+                               : cpp::numeric_limits<WordType>::min();
+        if ((i % WORD_SIZE_RATIO) != 0) {
+          cur_word |= static_cast<WordType>(extension_bits)
+                      << (BigIntOther::WORD_SIZE * (i % WORD_SIZE_RATIO));
+        }
+        // Copy the last word into place.
+        val[(i / WORD_SIZE_RATIO)] = cur_word;
+        extend((i / WORD_SIZE_RATIO) + 1, should_sign_extend);
+      }
+    } else if constexpr (BigIntOther::WORD_SIZE == WORD_SIZE) {
+      if (OtherBits >= Bits) { // truncate
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lntue wrote:

also `if constexpr`

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