Handle a non-Fallback baseline (#105)

Fixes #74 

A couple of notes:
1) For ease, I removed `simd_dispatch` in this PR. As such, I don't want
to land this before [#simd >
v0.3.0](https://xi.zulipchat.com/#narrow/channel/514230-simd/topic/v0.2E3.2E0/with/543274721)
2) The cfg variants in here are gnarly; I believe that's unavoidable to
express what we want. We do need to be clear that we are doing runtime
dispatch properly
3) The automated testing story of this PR is an open question. I have a
manual test, in the new `sh ./check_targets.sh` at the root. I want to
defer running this in CI; I suspect it would be very expensive.

This does not address #92. This does however add a
`force_support_fallback` feature, so it doesn't completely remove the
fallback functionality.

Author: DJMcNab

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ env:
   # If the compilation fails, then the version specified here needs to be bumped up to reality.
   # Be sure to also update the rust-version property in the workspace Cargo.toml file,
   # plus all the README.md files of the affected packages.
-  RUST_MIN_VER: "1.86"
+  RUST_MIN_VER: "1.88"
   # List of packages that will be checked with the minimum supported Rust version.
   # This should be limited to packages that are intended for publishing.
   RUST_MIN_VER_PKGS: "-p fearless_simd"

CHANGELOG.md

@@ -13,7 +13,20 @@ You can find its changes [documented below](#030-2025-10-14).
 
 ## [Unreleased]
 
-This release has an [MSRV][] of 1.86.
+This release has an [MSRV][] of 1.88.
+
+### Changed
+
+- Breaking change: `Level::fallback` has been removed, replaced with `Level::baseline`. ([#105][] by [@DJMcNab][])
+  This corresponds with a change to avoid compiling in support for the fallback level on compilation targets which don't
+  require it; this is most impactful for binary size on WASM, Apple Silicon Macs or Android.
+  A consequence of this is that the available variants on `Level` are now dependent on the target features you are compiling with.
+  The fallback level can be restored with the `force_support_fallback` cargo feature. We don't expect this to be necessary outside
+  of tests.
+
+### Removed
+
+- Breaking change: The (deprecated) `simd_dispatch!` macro. ([#105][] by [@DJMcNab][])
 
 ## [0.3.0][] (2025-10-14)
 
@@ -85,6 +98,7 @@ No changelog was kept for this release.
 [#93]: https://github.com/linebender/fearless_simd/pull/93
 [#96]: https://github.com/linebender/fearless_simd/pull/96
 [#99]: https://github.com/linebender/fearless_simd/pull/99
+[#105]: https://github.com/linebender/fearless_simd/pull/105
 
 [Unreleased]: https://github.com/linebender/fearless_simd/compare/v0.3.0...HEAD
 [0.3.0]: https://github.com/linebender/fearless_simd/compare/v0.3.0...v0.2.0

Cargo.toml

@@ -13,7 +13,7 @@ license = "Apache-2.0 OR MIT"
 repository = "https://github.com/linebender/fearless_simd"
 # Keep in sync with RUST_MIN_VER in .github/workflows/ci.yml, with the relevant README.md files
 # and with the MSRV in the `Unreleased` section of CHANGELOG.md.
-rust-version = "1.86"
+rust-version = "1.88"
 
 [workspace.lints]
 

README.md

@@ -59,7 +59,7 @@ It benefited from conversations with Luca Versari, though he is not responsible
 
 ## Minimum supported Rust Version (MSRV)
 
-This version of Fearless SIMD has been verified to compile with **Rust 1.86** and later.
+This version of Fearless SIMD has been verified to compile with **Rust 1.88** and later.
 
 Future versions of Fearless SIMD might increase the Rust version requirement.
 It will not be treated as a breaking change and as such can even happen with small patch releases.

check_targets.sh

@@ -0,0 +1,41 @@
+# A script to run cargo check for fearless_simd on each supported platform.
+# We currently don't run this in CI, as we expect it would take too long.
+# Before using, you must run:
+# rustup target add aarch64-linux-android x86_64-unknown-linux-gnu i686-pc-windows-msvc wasm32-unknown-unknown riscv64gc-unknown-linux-gnu
+
+# Run using `sh ./check_targets.sh`
+# TODO: Make into an xtask like thing so that windows users can use easily.
+
+# aarch64, both with and without neon support.
+# Note that doing `-neon` is not sound due to the standard library's ABI, but the
+# binary is never executed (nor indeed is it even created), and it's still a useful sanity check.
+RUSTFLAGS=-Ctarget-feature=-neon cargo check -p fearless_simd --target aarch64-linux-android
+RUSTFLAGS=-Ctarget-feature=-neon cargo check -p fearless_simd --target aarch64-linux-android --features force_support_fallback
+cargo check -p fearless_simd --target aarch64-linux-android  --features force_support_fallback
+cargo check -p fearless_simd --target aarch64-linux-android
+
+# x86_64, at all supported static SIMD levels.
+RUSTFLAGS=-Ctarget-feature=+avx2,+fma cargo check -p fearless_simd --target x86_64-unknown-linux-gnu
+RUSTFLAGS=-Ctarget-feature=+avx2,+fma cargo check -p fearless_simd --target x86_64-unknown-linux-gnu --features force_support_fallback
+RUSTFLAGS=-Ctarget-feature=+sse4.2 cargo check -p fearless_simd --target x86_64-unknown-linux-gnu
+RUSTFLAGS=-Ctarget-feature=+sse4.2 cargo check -p fearless_simd --target x86_64-unknown-linux-gnu  --features force_support_fallback
+cargo check -p fearless_simd --target x86_64-unknown-linux-gnu
+cargo check -p fearless_simd --target x86_64-unknown-linux-gnu  --features force_support_fallback
+
+# x86 (i.e. 32 bit) at all supported static SIMD levels.
+RUSTFLAGS=-Ctarget-feature=+avx2,+fma cargo check -p fearless_simd --target i686-pc-windows-msvc
+RUSTFLAGS=-Ctarget-feature=+avx2,+fma cargo check -p fearless_simd --target i686-pc-windows-msvc --features force_support_fallback
+RUSTFLAGS=-Ctarget-feature=+sse4.2 cargo check -p fearless_simd --target i686-pc-windows-msvc
+RUSTFLAGS=-Ctarget-feature=+sse4.2 cargo check -p fearless_simd --target i686-pc-windows-msvc  --features force_support_fallback
+cargo check -p fearless_simd --target i686-pc-windows-msvc
+cargo check -p fearless_simd --target i686-pc-windows-msvc  --features force_support_fallback
+
+# Wasm, both with and without SIMD.
+cargo check -p fearless_simd --target wasm32-unknown-unknown
+cargo check -p fearless_simd --target wasm32-unknown-unknown --features force_support_fallback
+RUSTFLAGS=-Ctarget-feature=+simd128 cargo check -p fearless_simd --target wasm32-unknown-unknown
+RUSTFLAGS=-Ctarget-feature=+simd128 cargo check -p fearless_simd --target wasm32-unknown-unknown --features force_support_fallback
+
+# riscv64, which is importantly a target we don't support any SIMD levels for.
+cargo check -p fearless_simd --target riscv64gc-unknown-linux-gnu
+cargo check -p fearless_simd --target riscv64gc-unknown-linux-gnu --features force_support_fallback

fearless_simd/Cargo.toml

@@ -30,6 +30,10 @@ libm = ["dep:libm"]
 # beyond the basic SIMD operations abstracted on all platforms
 safe_wrappers = []
 
+# Force the "fallback" SIMD level to be supported
+# This is primarily used for tests
+force_support_fallback = []
+
 [lints]
 workspace = true
 

fearless_simd/README.md

@@ -115,6 +115,7 @@ The following crate [feature flags](https://doc.rust-lang.org/cargo/reference/fe
 - `libm`: Use floating point implementations from [libm].
 - `safe_wrappers`: Include safe wrappers for (some) target feature specific intrinsics,
   beyond the basic SIMD operations abstracted on all platforms.
+- `force_support_fallback`: Force scalar fallback, to be supported, even if your compilation target has a better baseline.
 
 At least one of `std` and `libm` is required; `std` overrides `libm`.
 
@@ -124,7 +125,7 @@ At least one of `std` and `libm` is required; `std` overrides `libm`.
 
 ## Minimum supported Rust Version (MSRV)
 
-This version of Fearless SIMD has been verified to compile with **Rust 1.86** and later.
+This version of Fearless SIMD has been verified to compile with **Rust 1.88** and later.
 
 Future versions of Fearless SIMD might increase the Rust version requirement.
 It will not be treated as a breaking change and as such can even happen with small patch releases.

fearless_simd/src/generated/fallback.rs

@@ -83,7 +83,10 @@ impl Simd for Fallback {
     type mask32s = mask32x4<Self>;
     #[inline(always)]
     fn level(self) -> Level {
-        Level::Fallback(self)
+        #[cfg(feature = "force_support_fallback")]
+        return Level::Fallback(self);
+        #[cfg(not(feature = "force_support_fallback"))]
+        Level::baseline()
     }
     #[inline]
     fn vectorize<F: FnOnce() -> R, R>(self, f: F) -> R {

fearless_simd/src/generated/sse4_2.rs

@@ -52,7 +52,12 @@ impl Simd for Sse4_2 {
     type mask32s = mask32x4<Self>;
     #[inline(always)]
     fn level(self) -> Level {
-        Level::Sse4_2(self)
+        #[cfg(not(all(target_feature = "avx2", target_feature = "fma")))]
+        return Level::Sse4_2(self);
+        #[cfg(all(target_feature = "avx2", target_feature = "fma"))]
+        {
+            Level::baseline()
+        }
     }
     #[inline]
     fn vectorize<F: FnOnce() -> R, R>(self, f: F) -> R {