Fix GCC module build and MSVC C4127 in the json_view SIMD code

GCC ignores the target attribute in modules, so the SSSE3 dispatch is
disabled for the module interface (the check stays portable, SSE2 is kept).
Make the 8-vs-16 byte unrolling condition in scan_string_run a
preprocessor/template split to avoid a constant condition (C4127).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/scan.hpp

@@ -85,7 +85,9 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned
 #define NLOHMANN_VIEW_STEP(i) if (NLOHMANN_VIEW_LIKELY(plain[p[i]] != 0)) {} else { p += (i); goto stop; }
             NLOHMANN_VIEW_STEP(0) NLOHMANN_VIEW_STEP(1) NLOHMANN_VIEW_STEP(2) NLOHMANN_VIEW_STEP(3)
             NLOHMANN_VIEW_STEP(4) NLOHMANN_VIEW_STEP(5) NLOHMANN_VIEW_STEP(6) NLOHMANN_VIEW_STEP(7)
-            if (!Value || !NLOHMANN_VIEW_VECTOR)
+#if NLOHMANN_VIEW_VECTOR
+            if (!Value) // (a plain `!Value || !NLOHMANN_VIEW_VECTOR` is a constant condition for MSVC, C4127)
+#endif
             {
                 NLOHMANN_VIEW_STEP(8) NLOHMANN_VIEW_STEP(9) NLOHMANN_VIEW_STEP(10) NLOHMANN_VIEW_STEP(11)
                 NLOHMANN_VIEW_STEP(12) NLOHMANN_VIEW_STEP(13) NLOHMANN_VIEW_STEP(14) NLOHMANN_VIEW_STEP(15)

include/nlohmann/detail/view/simd.hpp

@@ -26,6 +26,9 @@
 // used where the CPU has SSSE3 (all x86-64 CPUs since about 2011), else the
 // portable check. JSON_VIEW_USE_SSSE3 skips the CPU check (for code compiled
 // for SSSE3 anyway); JSON_VIEW_NO_SIMD selects the portable code.
+// NLOHMANN_VIEW_NO_TARGET_ATTRIBUTE (set by the GCC module interface, where GCC
+// ignores the target attribute and then rejects the SSSE3 intrinsics) keeps the
+// check portable without disabling the SSE2 code.
 #if !defined(JSON_VIEW_NO_SIMD) && defined(__aarch64__) && (defined(__GNUC__) || defined(__clang__)) && NLOHMANN_VIEW_LITTLE_ENDIAN
     #include <arm_neon.h>
     #define NLOHMANN_VIEW_NEON 1
@@ -44,7 +47,7 @@
 #else
     #define NLOHMANN_VIEW_SSSE3 0 // NOLINT(cppcoreguidelines-macro-to-enum,modernize-macro-to-enum)
 #endif
-#if NLOHMANN_VIEW_SSE2 && !NLOHMANN_VIEW_SSSE3 && ((defined(__clang__) && __clang_major__ >= 4) || (defined(__GNUC__) && !defined(__clang__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))))
+#if NLOHMANN_VIEW_SSE2 && !NLOHMANN_VIEW_SSSE3 && !defined(NLOHMANN_VIEW_NO_TARGET_ATTRIBUTE) && ((defined(__clang__) && __clang_major__ >= 4) || (defined(__GNUC__) && !defined(__clang__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))))
     // (GCC before 4.9 has no SSSE3 intrinsics without -mssse3)
     #include <cpuid.h>
     #include <tmmintrin.h>

single_include/nlohmann/json_view.hpp

@@ -439,6 +439,9 @@ NLOHMANN_JSON_NAMESPACE_END
 // used where the CPU has SSSE3 (all x86-64 CPUs since about 2011), else the
 // portable check. JSON_VIEW_USE_SSSE3 skips the CPU check (for code compiled
 // for SSSE3 anyway); JSON_VIEW_NO_SIMD selects the portable code.
+// NLOHMANN_VIEW_NO_TARGET_ATTRIBUTE (set by the GCC module interface, where GCC
+// ignores the target attribute and then rejects the SSSE3 intrinsics) keeps the
+// check portable without disabling the SSE2 code.
 #if !defined(JSON_VIEW_NO_SIMD) && defined(__aarch64__) && (defined(__GNUC__) || defined(__clang__)) && NLOHMANN_VIEW_LITTLE_ENDIAN
     #include <arm_neon.h>
     #define NLOHMANN_VIEW_NEON 1
@@ -457,7 +460,7 @@ NLOHMANN_JSON_NAMESPACE_END
 #else
     #define NLOHMANN_VIEW_SSSE3 0 // NOLINT(cppcoreguidelines-macro-to-enum,modernize-macro-to-enum)
 #endif
-#if NLOHMANN_VIEW_SSE2 && !NLOHMANN_VIEW_SSSE3 && ((defined(__clang__) && __clang_major__ >= 4) || (defined(__GNUC__) && !defined(__clang__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))))
+#if NLOHMANN_VIEW_SSE2 && !NLOHMANN_VIEW_SSSE3 && !defined(NLOHMANN_VIEW_NO_TARGET_ATTRIBUTE) && ((defined(__clang__) && __clang_major__ >= 4) || (defined(__GNUC__) && !defined(__clang__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))))
     // (GCC before 4.9 has no SSSE3 intrinsics without -mssse3)
     #include <cpuid.h>
     #include <tmmintrin.h>
@@ -815,7 +818,9 @@ NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned
 #define NLOHMANN_VIEW_STEP(i) if (NLOHMANN_VIEW_LIKELY(plain[p[i]] != 0)) {} else { p += (i); goto stop; }
             NLOHMANN_VIEW_STEP(0) NLOHMANN_VIEW_STEP(1) NLOHMANN_VIEW_STEP(2) NLOHMANN_VIEW_STEP(3)
             NLOHMANN_VIEW_STEP(4) NLOHMANN_VIEW_STEP(5) NLOHMANN_VIEW_STEP(6) NLOHMANN_VIEW_STEP(7)
-            if (!Value || !NLOHMANN_VIEW_VECTOR)
+#if NLOHMANN_VIEW_VECTOR
+            if (!Value) // (a plain `!Value || !NLOHMANN_VIEW_VECTOR` is a constant condition for MSVC, C4127)
+#endif
             {
                 NLOHMANN_VIEW_STEP(8) NLOHMANN_VIEW_STEP(9) NLOHMANN_VIEW_STEP(10) NLOHMANN_VIEW_STEP(11)
                 NLOHMANN_VIEW_STEP(12) NLOHMANN_VIEW_STEP(13) NLOHMANN_VIEW_STEP(14) NLOHMANN_VIEW_STEP(15)

src/modules/json.cppm

@@ -17,6 +17,12 @@ module;
 // 3. Or upgrade to a newer GCC version with better modules support.
 // See: https://github.com/nlohmann/json/issues/5103
 
+// GCC ignores the target attribute in modules ("'target' attribute currently
+// unsupported in modules") and then rejects the SSSE3 intrinsics of json_view.
+#if defined(__GNUC__) && !defined(__clang__)
+    #define NLOHMANN_VIEW_NO_TARGET_ATTRIBUTE
+#endif
+
 #include <nlohmann/json.hpp>
 #include <nlohmann/json_literals.hpp>
 #include <nlohmann/json_view.hpp>