[libc] [llvm] [libc] Adding a version of memset with software prefetching (PR #70857)

[via llvm-commits]

================
@@ -37,29 +40,55 @@ inline_memset_x86(Ptr dst, uint8_t value, size_t count) {
   using uint512_t = cpp::array<uint64_t, 8>;
 #endif
 
-  if (count == 0)
-    return;
-  if (count == 1)
-    return generic::Memset<uint8_t>::block(dst, value);
-  if (count == 2)
-    return generic::Memset<uint16_t>::block(dst, value);
-  if (count == 3)
-    return generic::MemsetSequence<uint16_t, uint8_t>::block(dst, value);
-  if (count <= 8)
-    return generic::Memset<uint32_t>::head_tail(dst, value, count);
-  if (count <= 16)
-    return generic::Memset<uint64_t>::head_tail(dst, value, count);
-  if (count <= 32)
-    return generic::Memset<uint128_t>::head_tail(dst, value, count);
-  if (count <= 64)
-    return generic::Memset<uint256_t>::head_tail(dst, value, count);
-  if (count <= 128)
-    return generic::Memset<uint512_t>::head_tail(dst, value, count);
-  // Aligned loop
-  generic::Memset<uint256_t>::block(dst, value);
-  align_to_next_boundary<32>(dst, count);
-  return generic::Memset<uint256_t>::loop_and_tail(dst, value, count);
-}
+  [[maybe_unused]] LIBC_INLINE static void
+  inline_memset_x86_sw_prefetching(Ptr dst, uint8_t value, size_t count) {
+    // Prefetch one cacheline
+    sw_prefetch::PrefetchW(dst + sw_prefetch::kCachelineSize);
+    if (count <= 128)
+      return generic::Memset<uint512_t>::head_tail(dst, value, count);
+    // Prefetch the next cacheline
+    sw_prefetch::PrefetchW(dst + sw_prefetch::kCachelineSize * 2);
+    // Aligned loop
+    generic::Memset<uint256_t>::block(dst, value);
+    align_to_next_boundary<32>(dst, count);
+    if (count <= 192) {
+      return generic::Memset<uint256_t>::loop_and_tail(dst, value, count);
+    } else {
+      generic::Memset<uint512_t>::block(dst, value);
+      generic::Memset<uint256_t>::block(dst + sizeof(uint512_t), value);
+      return generic::Memset<uint256_t>::loop_and_tail_prefetch<320, 128>(
+          dst, value, count);
+    }
+  }
+
+  [[maybe_unused]] LIBC_INLINE static void
+  inline_memset_x86(Ptr dst, uint8_t value, size_t count) {
+    if (count == 0)
+      return;
+    if (count == 1)
+      return generic::Memset<uint8_t>::block(dst, value);
+    if (count == 2)
+      return generic::Memset<uint16_t>::block(dst, value);
+    if (count == 3)
+      return generic::MemsetSequence<uint16_t, uint8_t>::block(dst, value);
+    if (count <= 8)
+      return generic::Memset<uint32_t>::head_tail(dst, value, count);
+    if (count <= 16)
+      return generic::Memset<uint64_t>::head_tail(dst, value, count);
+    if (count <= 32)
+      return generic::Memset<uint128_t>::head_tail(dst, value, count);
+    if (count <= 64)
+      return generic::Memset<uint256_t>::head_tail(dst, value, count);
+    if constexpr (x86::kUseSoftwarePrefetchingMemset) {
+      return inline_memset_x86_sw_prefetching(dst, value, count);
+    }
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doshimili wrote:

Removed braces

https://github.com/llvm/llvm-project/pull/70857