Extract storeBitsToByte to be reused (facebookincubator#8416)

Summary:

We will need this to fix `simd::gatherBits` for non-AVX cases.

See facebookincubator#8415

Differential Revision: D52837098

velox/common/base/BitUtil.h

@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include "velox/common/base/Exceptions.h"
+
 #include <algorithm>
 #include <cstddef>
 #include <cstdint>
@@ -977,6 +979,26 @@ inline __int128_t builtin_bswap128(__int128_t value) {
 #endif
 }
 
+/// Store `bits' into the memory region pointed by `byte', at `index' (bit
+/// index).  If `kSize' is 8, we store the whole byte directly; otherwise it
+/// must be 4 and we store either the whole byte or the upper 4 bits only,
+/// depending on the `index'.
+template <int kSize>
+void storeBitsToByte(uint8_t bits, uint8_t* bytes, unsigned index) {
+  VELOX_DCHECK_EQ(index % kSize, 0);
+  VELOX_DCHECK_EQ(bits >> kSize, 0);
+  if constexpr (kSize == 8) {
+    bytes[index / 8] = bits;
+  } else {
+    VELOX_DCHECK_EQ(kSize, 4);
+    if (index % 8 == 0) {
+      bytes[index / 8] = bits;
+    } else {
+      bytes[index / 8] |= bits << 4;
+    }
+  }
+}
+
 } // namespace bits
 } // namespace velox
 } // namespace facebook

velox/common/base/tests/BitUtilTest.cpp

@@ -853,6 +853,23 @@ TEST_F(BitUtilTest, countLeadingZeros) {
   EXPECT_EQ(
       countLeadingZeros<__uint128_t>(HugeInt::build(0x08FFFFFFFFFFFFFF, 0)), 4);
 }
+
+TEST_F(BitUtilTest, storeBitsToByte) {
+  uint8_t bytes[3]{};
+  storeBitsToByte<8>(0xAA, bytes, 0);
+  ASSERT_EQ(bytes[0], 0xAA);
+  ASSERT_EQ(bytes[1], 0);
+  ASSERT_EQ(bytes[2], 0);
+  storeBitsToByte<4>(0x5, bytes, 8);
+  ASSERT_EQ(bytes[0], 0xAA);
+  ASSERT_EQ(bytes[1], 0x5);
+  ASSERT_EQ(bytes[2], 0);
+  storeBitsToByte<4>(0xA, bytes, 12);
+  ASSERT_EQ(bytes[0], 0xAA);
+  ASSERT_EQ(bytes[1], 0xA5);
+  ASSERT_EQ(bytes[2], 0);
+}
+
 } // namespace bits
 } // namespace velox
 } // namespace facebook

velox/dwio/common/DecoderUtil.cpp

@@ -92,16 +92,7 @@ bool nonNullRowsFromSparse(
     if (isDense(rows.data() + i, width)) {
       uint16_t flags = load8Bits(nulls, rows[i]) & widthMask;
       if (outputNulls) {
-        if constexpr (kStep == 8) {
-          resultNullBytes[i / 8] = flags;
-        } else {
-          VELOX_DCHECK_EQ(kStep, 4);
-          if (i % 8 == 0) {
-            resultNullBytes[i / 8] = flags;
-          } else {
-            resultNullBytes[i / 8] |= flags << 4;
-          }
-        }
+        bits::storeBitsToByte<kStep>(flags, resultNullBytes, i);
         anyNull |= flags != widthMask;
       }
       if (!flags) {
@@ -131,16 +122,7 @@ bool nonNullRowsFromSparse(
       auto next8Rows = xsimd::load_unaligned(rows.data() + i);
       uint16_t flags = simd::gather8Bits(nulls, next8Rows, width);
       if (outputNulls) {
-        if constexpr (kStep == 8) {
-          resultNullBytes[i / 8] = flags;
-        } else {
-          VELOX_DCHECK_EQ(kStep, 4);
-          if (i % 8 == 0) {
-            resultNullBytes[i / 8] = flags;
-          } else {
-            resultNullBytes[i / 8] |= flags << 4;
-          }
-        }
+        bits::storeBitsToByte<kStep>(flags, resultNullBytes, i);
         anyNull |= flags != widthMask;
       }
       if (!flags) {