migrate simdForEach facebook#2: remove unroll step

Summary: detail::UnrollStep should go away, because otherwise it would be exposed to the user of `simdForEach`.

Reviewed By: yfeldblum

Differential Revision: D60587842

folly/algorithm/simd/BUCK

@@ -27,6 +27,7 @@ cpp_library(
     headers = ["SimdForEach.h"],
     exported_deps = [
         "//folly:c_portability",
+        "//folly:traits",
         "//folly/algorithm/simd/detail:unroll_utils",
     ],
 )

folly/algorithm/simd/SimdForEach.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <folly/CPortability.h>
+#include <folly/Traits.h>
 #include <folly/algorithm/simd/detail/UnrollUtils.h>
 
 #include <array>
@@ -110,10 +111,9 @@ FOLLY_ALWAYS_INLINE T* previousAlignedAddress(T* ptr, int to) {
 struct SimdForEachMainLoop {
   template <typename T, typename Delegate>
   FOLLY_ALWAYS_INLINE bool operator()(
-      int cardinal, T*& f, T* l, Delegate& delegate, detail::UnrollStep<1>)
-      const {
+      int cardinal, T*& f, T* l, Delegate& delegate, index_constant<1>) const {
     while (f != l) {
-      if (delegate.step(f, ignore_none{}, detail::UnrollStep<0>{}))
+      if (delegate.step(f, ignore_none{}, index_constant<0>{}))
         return true;
       f += cardinal;
     }
@@ -129,8 +129,8 @@ struct SimdForEachMainLoop {
     T* l;
     Delegate& delegate;
 
-    template <int i>
-    FOLLY_ALWAYS_INLINE bool operator()(detail::UnrollStep<i> unrollI) {
+    template <std::size_t i>
+    FOLLY_ALWAYS_INLINE bool operator()(index_constant<i> unrollI) {
       if (f == l)
         return true;
 
@@ -140,13 +140,10 @@ struct SimdForEachMainLoop {
     }
   };
 
-  template <typename T, typename Delegate, int unrolling>
+  template <typename T, typename Delegate, std::size_t unrolling>
   FOLLY_ALWAYS_INLINE bool operator()(
-      int cardinal,
-      T*& f,
-      T* l,
-      Delegate& delegate,
-      detail::UnrollStep<unrolling>) const {
+      int cardinal, T*& f, T* l, Delegate& delegate, index_constant<unrolling>)
+      const {
     // Not enough to fully unroll explanation.
     //
     // There are a few approaches to handle this:
@@ -206,14 +203,14 @@ FOLLY_ALWAYS_INLINE void simdForEachAligning(
   ignore_extrema ignore{static_cast<int>(f - af), 0};
   if (af != al) {
     // first chunk
-    if (delegate.step(af, ignore, detail::UnrollStep<0>{})) {
+    if (delegate.step(af, ignore, index_constant<0>{})) {
       return;
     }
     ignore.first = 0;
     af += cardinal;
 
     if (SimdForEachMainLoop{}(
-            cardinal, af, al, delegate, detail::UnrollStep<unrolling>{})) {
+            cardinal, af, al, delegate, index_constant<unrolling>{})) {
       return;
     }
 
@@ -224,7 +221,7 @@ FOLLY_ALWAYS_INLINE void simdForEachAligning(
   }
 
   ignore.last = static_cast<int>(af + cardinal - l);
-  delegate.step(af, ignore, detail::UnrollStep<0>{});
+  delegate.step(af, ignore, index_constant<0>{});
 }
 
 } // namespace simd_detail

folly/algorithm/simd/detail/BUCK

@@ -10,5 +10,6 @@ cpp_library(
     headers = ["UnrollUtils.h"],
     exported_deps = [
         "//folly:portability",
+        "//folly:traits",
     ],
 )

folly/algorithm/simd/detail/UnrollUtils.h

@@ -17,14 +17,12 @@
 #pragma once
 
 #include <folly/Portability.h>
+#include <folly/Traits.h>
 
 #include <array>
+#include <type_traits>
 
-namespace folly {
-namespace detail {
-
-template <int i>
-struct UnrollStep : std::integral_constant<int, i> {};
+namespace folly::detail {
 
 /**
  * UnrollUtils
@@ -67,13 +65,14 @@ struct UnrollUtils {
    * unrollUntil<N>(op)
    *
    *  Do operation N times or until it returns true to break.
-   *  Op accepts UnrollStep<i> so it can keep track of a step begin executed.
+   *  Op accepts integral_constant<i> so it can keep track of a step begin
+   * executed.
    *
    *  Returns wether true if it was interrupted (you can know if the op breaked)
    */
-  template <int N, typename Op>
+  template <std::size_t N, typename Op>
   FOLLY_ALWAYS_INLINE static constexpr bool unrollUntil(Op op) {
-    return unrollUntilImpl<N, 0>(op);
+    return unrollUntilImpl(op, std::make_index_sequence<N>{});
   }
 
  private:
@@ -117,18 +116,11 @@ struct UnrollUtils {
     return op(leftSum, rightSum);
   }
 
-  template <int N, int i, typename Op>
-  FOLLY_ALWAYS_INLINE static constexpr std::enable_if_t<i == N, bool>
-  unrollUntilImpl(Op) {
-    return false;
-  }
-
-  template <int N, int i, typename Op>
-  FOLLY_ALWAYS_INLINE static constexpr std::enable_if_t<i != N, bool>
-  unrollUntilImpl(Op op) {
-    return op(UnrollStep<i>{}) || unrollUntilImpl<N, i + 1>(op);
+  template <typename Op, std::size_t... i>
+  FOLLY_ALWAYS_INLINE static constexpr bool unrollUntilImpl(
+      Op op, std::index_sequence<i...>) {
+    return (... || op(index_constant<i>{}));
   }
 };
 
-} // namespace detail
-} // namespace folly
+} // namespace folly::detail

folly/algorithm/simd/detail/test/UnrollUtilsTest.cpp

@@ -51,20 +51,26 @@ TEST(UnrollUtilsTest, ArrayReduce) {
   static_assert(10 == reduceValues<1, 2, 3, 4>(), "");
 }
 
-template <int stopAt>
+template <std::size_t stopAt>
 struct UnrollUntilTestOp {
-  int* lastStep;
+  std::size_t* lastStep;
 
-  template <int i>
-  constexpr bool operator()(UnrollStep<i>) const {
+  template <std::size_t i>
+  constexpr bool operator()(folly::index_constant<i>) const {
     *lastStep = i;
     return i == stopAt;
   }
 };
 
-template <int N, int stopAt, bool expectedRes, int expectedLastStep>
+constexpr std::size_t kNoStop = std::numeric_limits<std::size_t>::max();
+
+template <
+    std::size_t N,
+    std::size_t stopAt,
+    bool expectedRes,
+    std::size_t expectedLastStep>
 constexpr bool unrollUntilTest() {
-  int lastStep = -1;
+  std::size_t lastStep = kNoStop;
   UnrollUntilTestOp<stopAt> op{&lastStep};
   bool res = UnrollUtils::unrollUntil<N>(op);
 
@@ -77,14 +83,14 @@ TEST(UnrollUtilsTest, UnrollUntil) {
           /*N*/ 0,
           /*stopAt*/ 0,
           /*expectedRes*/ false,
-          /*ExpectedLastStep*/ -1>(),
+          /*ExpectedLastStep*/ kNoStop>(),
       "");
   static_assert(
       unrollUntilTest<
           /*N*/ 0,
           /*stopAt*/ 1,
           /*expectedRes*/ false,
-          /*ExpectedLastStep*/ -1>(),
+          /*ExpectedLastStep*/ kNoStop>(),
       "");
   static_assert(
       unrollUntilTest<

folly/algorithm/simd/test/SimdForEachTest.cpp

@@ -71,7 +71,7 @@ struct TestDelegate {
           return step(
               unrolled[unrollI()],
               ignore_none{},
-              detail::UnrollStep<decltype(unrollI)::value + ('A' - 'a')>{});
+              folly::index_constant<decltype(unrollI)::value + ('A' - 'a')>{});
         });
   }
 };