fix: guard reassociation_barrier entirely by XSIMD_REASSOCIATIVE_MATH

- Wrap the entire reassociation_barrier implementation (including RISC-V
  scalar overloads) in #if XSIMD_REASSOCIATIVE_MATH so the function is
  a true no-op when the compiler cannot reassociate floating-point ops.
- Remove redundant #elif XSIMD_REASSOCIATIVE_MATH inner guard; the "+m"
  fallback now applies to all unknown targets within the outer guard.
- Guard the RVV batch overload with XSIMD_REASSOCIATIVE_MATH as well.
- Update test_exp condition to also match XSIMD_REASSOCIATIVE_MATH so
  approximate comparison is used whenever reassociation is active.

Author: DiamonDinoia

include/xsimd/arch/common/xsimd_common_details.hpp

@@ -123,22 +123,19 @@ namespace xsimd
             //   RISC-V RVV "+vr" (V register; GCC 15+ / Clang 20+)
             //
             // On unknown targets the "+m" fallback spills; it is
-            // guarded by XSIMD_REASSOCIATIVE_MATH so it is only
-            // emitted when the compiler can actually reassociate.
+            // only emitted when the compiler can actually reassociate.
             template <class T>
             XSIMD_INLINE void reassociation_barrier(T& x, const char*) noexcept
             {
-#if XSIMD_WITH_INLINE_ASM && !defined(__EMSCRIPTEN__)
+#if XSIMD_REASSOCIATIVE_MATH && XSIMD_WITH_INLINE_ASM && !defined(__EMSCRIPTEN__)
 #if XSIMD_WITH_SSE2
                 __asm__ volatile("" : "+x"(x));
 #elif XSIMD_WITH_NEON || XSIMD_WITH_SVE
                 __asm__ volatile("" : "+w"(x));
 #elif XSIMD_WITH_VSX
                 __asm__ volatile("" : "+wa"(x));
-#elif XSIMD_REASSOCIATIVE_MATH
-                __asm__ volatile("" : "+m"(x));
 #else
-                (void)x;
+                __asm__ volatile("" : "+m"(x));
 #endif
 #else
                 (void)x;
@@ -148,7 +145,7 @@ namespace xsimd
             // RISC-V scalar float/double: use F/D registers instead of
             // spilling through "+m".  These overloads also serve
             // emulated batches on RISC-V via the std::array overload.
-#if XSIMD_WITH_INLINE_ASM && defined(__riscv)
+#if XSIMD_REASSOCIATIVE_MATH && XSIMD_WITH_INLINE_ASM && defined(__riscv)
             XSIMD_INLINE void reassociation_barrier(float& x, const char*) noexcept
             {
                 __asm__ volatile("" : "+f"(x));
@@ -169,7 +166,7 @@ namespace xsimd
             template <class T, class A>
             XSIMD_INLINE void reassociation_barrier(batch<T, A>& b, const char* reason) noexcept
             {
-#if XSIMD_WITH_RVV && XSIMD_WITH_INLINE_ASM && ((__GNUC__ >= 15) || (__clang_major__ >= 20))
+#if XSIMD_REASSOCIATIVE_MATH && XSIMD_WITH_RVV && XSIMD_WITH_INLINE_ASM && ((__GNUC__ >= 15) || (__clang_major__ >= 20))
                 __asm__ volatile("" : "+vr"(b.data.value.value));
                 (void)reason;
 #else

test/test_xsimd_api.cpp

@@ -591,7 +591,7 @@ struct xsimd_api_float_types_functions
     void test_exp()
     {
         value_type val(2);
-#ifdef __FAST_MATH__
+#if defined(__FAST_MATH__) || XSIMD_REASSOCIATIVE_MATH
         CHECK_EQ(extract(xsimd::exp(T(val))), doctest::Approx(std::exp(val)));
 #else
         CHECK_EQ(extract(xsimd::exp(T(val))), std::exp(val));