Add a way to avoid f32 implementations that do math as an f64

rust-lang · tgross35 · Feb 7, 2025 · Feb 7, 2025 · Feb 7, 2025 · Feb 7, 2025
commit 2c7495c41bfd5fd2245823f870a2cd97fcfd7f0e
diff --git a/ci/run.sh b/ci/run.sh
@@ -14,6 +14,8 @@ if [ -z "$target" ]; then
     target="$host_target"
 fi
 
+nowiden_tests=$($cmd "$profile" release-checked --features force-soft-floats)
+
 # We enumerate features manually.
 flags="$flags --no-default-features"
 
@@ -112,6 +114,7 @@ $cmd --features unstable-intrinsics --benches
 # Test the same in release mode, which also increases coverage. Also ensure
 # the soft float routines are checked.
 $cmd "$profile" release-checked
+LIBM_F32_NO_WIDEN=1 $cmd "$profile" release-checked -- $nowiden_tests
 $cmd "$profile" release-checked --features force-soft-floats
 $cmd "$profile" release-checked --features unstable-intrinsics
 $cmd "$profile" release-checked --features unstable-intrinsics --benches

diff --git a/configure.rs b/configure.rs
@@ -50,6 +50,7 @@ pub fn emit_libm_config(cfg: &Config) {
     emit_intrinsics_cfg();
     emit_arch_cfg();
     emit_optimization_cfg(cfg);
+    emit_widen_cfg(cfg);
     emit_cfg_shorthands(cfg);
     emit_cfg_env(cfg);
     emit_f16_f128_cfg(cfg);
@@ -96,6 +97,15 @@ fn emit_optimization_cfg(cfg: &Config) {
     }
 }
 
+fn emit_widen_cfg(_cfg: &Config) {
+    println!("cargo:rustc-check-cfg=cfg(f32_no_widen)");
+    println!("cargo:rerun-if-env-changed=LIBM_F32_NO_WIDEN");
+
+    if env::var_os("LIBM_F32_NO_WIDEN").is_some() {
+        println!("cargo:rustc-cfg=f32_no_widen");
+    }
+}
+
 /// Provide an alias for common longer config combinations.
 fn emit_cfg_shorthands(cfg: &Config) {
     println!("cargo:rustc-check-cfg=cfg(x86_no_sse)");

diff --git a/etc/has-nowiden-impl.txt b/etc/has-nowiden-impl.txt
@@ -0,0 +1,2 @@
+# Functions that have a different version based on f32_no_widen
+fmaf
diff --git a/src/math/fmaf.rs b/src/math/fmaf.rs
@@ -5,7 +5,11 @@
 /// according to the rounding mode characterized by the value of FLT_ROUNDS.
 #[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
 pub fn fmaf(x: f32, y: f32, z: f32) -> f32 {
-    super::generic::fma_wide(x, y, z)
+    if cfg!(f32_no_widen) {
+        super::generic::fma(x, y, z)
+    } else {
+        super::generic::fma_wide(x, y, z)
+    }
 }
 
 #[cfg(test)]

diff --git a/src/math/generic/fma.rs b/src/math/generic/fma.rs
@@ -345,6 +345,16 @@ pub trait FmaHelper {
     fn raise_underflow_ret_zero(self) -> Self;
 }
 
+impl FmaHelper for f32 {
+    fn raise_underflow_as_min_positive(self) -> Self {
+        f32::MIN_POSITIVE.copysign(self)
+    }
+
+    fn raise_underflow_ret_zero(self) -> Self {
+        f32::ZERO
+    }
+}
+
 impl FmaHelper for f64 {
     fn raise_underflow_as_min_positive(self) -> Self {
         /* min normal after rounding, underflow depends
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		# Functions that have a different version based on f32_no_widen
		fmaf