[vecops] Increase size of long test RVec instances in RVec test

For the tests to make sense, some vectors need to be longer than the
maximum small vector size. This maximum size is compiler and
architecture dependent. For `RVec<int>` on ARM64 with gcc 14, the
small vector capacity turns out to be 60, which is larger than the
current test vector size of 18.

Therefore, the test is refactored to easily adapt the vector size, which
is increased to 72. A `static_assert` is also added, to make sure there
is an early compilation error if the vectors are not long enough.

Author: guitargeek

math/vecops/inc/ROOT/RVec.hxx

@@ -143,8 +143,8 @@ public:
    // and we prefer the smaller Size_T to reduce the size of each RVec object.
    using Size_T = int32_t;
 
-protected:
    void *fBeginX;
+protected:
    /// Always >= 0.
    // Type is signed only for consistency with fCapacity.
    Size_T fSize = 0;

math/vecops/test/vecops_rvec.cxx

@@ -1672,15 +1672,35 @@ TEST_P(VecOpsSwap, BothSmallVectors)
 
 TEST_P(VecOpsSwap, BothRegularVectors)
 {
-   RVec<int> fixed_vreg1{1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3};
-   RVec<int> fixed_vreg2{4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6};
-   RVec<int> fixed_vreg3{7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7};
+   constexpr std::size_t nElems = 72;
+   constexpr std::size_t smallVecSize = ROOT::Internal::VecOps::RVecInlineStorageSize<int>::value;
+   static_assert(nElems > smallVecSize,
+                 "The RVec instances in this test should be larger than the maximum small vector size!");
+   constexpr int nCycle = 3;
+
+   RVec<int> fixed_vreg1(nElems);
+   RVec<int> fixed_vreg2(nElems);
+   RVec<int> fixed_vreg3(nElems + 1);
    RVec<int> fixed_vmocksmall{0, 7};
 
-   RVec<int> vreg1{1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3};
-   RVec<int> vreg2{4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6};
-   RVec<int> vreg3{7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7};
-   RVec<int> vmocksmall{0, 7, 8, 9, 7, 8, 9, 7, 8, 9, 7, 8, 7, 8, 9, 7, 8, 9};
+   RVec<int> vreg1(nElems);
+   RVec<int> vreg2(nElems);
+   RVec<int> vreg3(nElems + 1);
+   RVec<int> vmocksmall(nElems);
+   std::cout << vmocksmall[0] << std::endl;
+
+   for (std::size_t i = 0; i < nElems; ++i) {
+      vreg1[i] = (i % nCycle) + 1;
+      vreg2[i] = vreg1[i] + nCycle;
+      vreg3[i] = vreg2[i] + nCycle;
+      fixed_vreg1[i] = vreg1[i];
+      fixed_vreg2[i] = vreg2[i];
+      fixed_vreg3[i] = vreg3[i];
+      vmocksmall[i + 1] = vreg3[i];
+   }
+   fixed_vreg3[nElems] = fixed_vreg3[0];
+   vreg3[nElems] = vreg3[0];
+
    vmocksmall.erase(vmocksmall.begin() + 2, vmocksmall.end());
    // vmocksmall is a regular vector of size 2