Switch to not compiling with a non SIMD capable cpu settings

Cargo.toml

@@ -63,6 +63,8 @@ swar-number-parsing = []
 serde_impl = [ "serde", "serde_json", "halfbrown/serde" ]
 # Support for ARM NEON SIMD
 neon = ["simd-lite"]
+# Allow fallback to non simd CPUs
+allow-non-simd = []
 # for testing allocations
 alloc = ["alloc_counter"]
 # don't inline code - used for debugging

README.md

@@ -26,6 +26,8 @@ To be able to take advantage of simdjson your system needs to be SIMD compatible
 
 simd-json.rs supports AVX2, SSE4.2 and NEON.
 
+Unless the `allow-non-simd` feature is passed simd-json will fail to compile, this is to prevent unexpected slowness in fallback mode that can be hard to understand.
+
 ### allocator
 
 For best performance we highly suggest using [mimalloc](https://crates.io/crates/mimalloc) or [jemalloc](https://crates.io/crates/jemalloc) instead of the system allocator used by default.

src/lib.rs

@@ -160,20 +160,11 @@ pub use crate::avx2::deser::*;
 #[cfg(target_feature = "avx2")]
 use crate::avx2::stage1::{SimdInput, SIMDINPUT_LENGTH, SIMDJSON_PADDING};
 
-#[cfg(all(
-    any(target_arch = "x86", target_arch = "x86_64"),
-    not(target_feature = "avx2")
-))]
+#[cfg(all(target_feature = "sse42", not(target_feature = "avx2")))]
 mod sse42;
-#[cfg(all(
-    any(target_arch = "x86", target_arch = "x86_64"),
-    not(target_feature = "avx2")
-))]
+#[cfg(all(target_feature = "sse42", not(target_feature = "avx2")))]
 pub use crate::sse42::deser::*;
-#[cfg(all(
-    any(target_arch = "x86", target_arch = "x86_64"),
-    not(target_feature = "avx2")
-))]
+#[cfg(all(target_feature = "sse42", not(target_feature = "avx2")))]
 use crate::sse42::stage1::{SimdInput, SIMDINPUT_LENGTH, SIMDJSON_PADDING};
 
 #[cfg(all(target_feature = "neon", feature = "neon"))]
@@ -183,6 +174,41 @@ pub use crate::neon::deser::*;
 #[cfg(all(target_feature = "neon", feature = "neon"))]
 use crate::neon::stage1::{SimdInput, SIMDINPUT_LENGTH, SIMDJSON_PADDING};
 
+// We import this as generics
+#[cfg(all(not(any(
+    target_feature = "sse42",
+    target_feature = "avx2",
+    target_feature = "neon"
+))))]
+mod sse42;
+#[cfg(all(not(any(
+    target_feature = "sse42",
+    target_feature = "avx2",
+    target_feature = "neon"
+))))]
+#[cfg(all(not(any(
+    target_feature = "sse42",
+    target_feature = "avx2",
+    target_feature = "neon"
+))))]
+pub use crate::sse42::deser::*;
+#[cfg(all(not(any(
+    target_feature = "sse42",
+    target_feature = "avx2",
+    target_feature = "neon"
+))))]
+use crate::sse42::stage1::{SimdInput, SIMDINPUT_LENGTH, SIMDJSON_PADDING};
+
+#[cfg(all(
+    not(feature = "allow-non-simd"),
+    not(any(
+        target_feature = "sse42",
+        target_feature = "avx2",
+        target_feature = "neon"
+    ))
+))]
+fn please_compile_with_a_simd_compatible_cpu_setting_read_the_simdjonsrs_readme() -> ! {}
+
 use crate::utf8check::ProcessedUtfBytes;
 
 mod stage2;