From 6065b3b4240d60fdb377185702ec7be41633d9bf Mon Sep 17 00:00:00 2001
From: Daniel Lemire <daniel@lemire.me>
Date: Thu, 12 Mar 2026 20:40:43 -0400
Subject: [PATCH] disabling loongson LASX runtime dispatching everywhere but
 GCC15+ (#2626)

---
 doc/implementation-selection.md   |  3 +++
 include/simdjson/compiler_check.h |  1 -
 include/simdjson/portability.h    | 19 +++++++++++++++++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/doc/implementation-selection.md b/doc/implementation-selection.md
index 26b38b978..05e4dc109 100644
--- a/doc/implementation-selection.md
+++ b/doc/implementation-selection.md
@@ -33,6 +33,9 @@ compiles *all* the implementations into the executable. On Intel, it will includ
 (icelake, haswell, westmere and fallback), on 64-bit ARM it will include just one since running dispatching is  unnecessary, and on PPC
 it will include 2 (ppc64 and fallback).
 
+On Loongson processors, LASX runtime dispatching is only enabled on GCC 15+, not on LLVM or older versions of GCC.
+Thus unless you compile specifically for LASX or use GCC 15+, you will not benefit from LASX support.
+
 If you know more about where you're going to run and want to save the space, you can disable any of
 these implementations at compile time with `-DSIMDJSON_IMPLEMENTATION_X=0` (where X is ICELAKE, HASWELL,
 WESTMERE, ARM64, PPC64, LSX, LASX and FALLBACK).
diff --git a/include/simdjson/compiler_check.h b/include/simdjson/compiler_check.h
index 72400a30a..655f66318 100644
--- a/include/simdjson/compiler_check.h
+++ b/include/simdjson/compiler_check.h
@@ -119,5 +119,4 @@
 #define SIMDJSON_CONSTEVAL 0
 #endif // defined(__cpp_consteval) && __cpp_consteval >= 201811L && defined(__cpp_lib_constexpr_string) && __cpp_lib_constexpr_string >= 201907L
 #endif // !defined(SIMDJSON_CONSTEVAL)
-
 #endif // SIMDJSON_COMPILER_CHECK_H
diff --git a/include/simdjson/portability.h b/include/simdjson/portability.h
index e6850d0f2..bff2163ba 100644
--- a/include/simdjson/portability.h
+++ b/include/simdjson/portability.h
@@ -66,6 +66,25 @@ using std::size_t;
   #define SIMDJSON_IS_LASX 1 // We can always run both
 #elif defined(__loongarch_sx)
   #define SIMDJSON_IS_LSX 1
+
+// Adjust for runtime dispatching support.
+#if defined(__GNUC__) && !defined(__clang__) && !defined(__INTEL_COMPILER) && !defined(__NVCOMPILER)
+#if __GNUC__ > 15 || (__GNUC__ == 15 && __GNUC_MINOR__ >= 0)
+  // We are ok, we will support runtime dispatch for LASX.
+#else
+  // We disable runtime dispatch for LASX, which means that we will not be able to use LASX
+  // even if it is supported by the hardware.
+  // Loongson users should update to GCC 15 or better.
+  #define SIMDJSON_IMPLEMENTATION_LASX 0
+#endif
+#else
+  // We are not using GCC, so we assume that we can support runtime dispatch for LASX.
+  // https://godbolt.org/z/jcMnrjYhs
+  #define SIMDJSON_IMPLEMENTATION_LASX 0
+#endif
+
+
+
 #endif
 #elif defined(__PPC64__) || defined(_M_PPC64)
 #define SIMDJSON_IS_PPC64 1