TEST ONLY: remove some x87 work-arounds and make some functions extern-C to try to reproduce an issue

Author: RalfJung

library/core/src/num/f128.rs

@@ -439,10 +439,6 @@ impl f128 {
     #[unstable(feature = "f128", issue = "116909")]
     #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
     pub const fn classify(self) -> FpCategory {
-        // Other float types suffer from various platform bugs that violate the usual IEEE semantics
-        // and also make bitwise classification not always work reliably. However, `f128` cannot fit
-        // into any other float types so this is not a concern, and we can rely on bit patterns.
-
         let bits = self.to_bits();
         match (bits & Self::MAN_MASK, bits & Self::EXP_MASK) {
             (0, Self::EXP_MASK) => FpCategory::Infinite,

library/core/src/num/f16.rs

@@ -424,43 +424,13 @@ impl f16 {
     #[unstable(feature = "f16", issue = "116909")]
     #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
     pub const fn classify(self) -> FpCategory {
-        // A previous implementation for f32/f64 tried to only use bitmask-based checks,
-        // using `to_bits` to transmute the float to its bit repr and match on that.
-        // If we only cared about being "technically" correct, that's an entirely legit
-        // implementation.
-        //
-        // Unfortunately, there are platforms out there that do not correctly implement the IEEE
-        // float semantics Rust relies on: some hardware flushes denormals to zero, and some
-        // platforms convert to `f32` to perform operations without properly rounding back (e.g.
-        // WASM, see llvm/llvm-project#96437). These are platforms bugs, and Rust will misbehave on
-        // such platforms, but we can at least try to make things seem as sane as possible by being
-        // careful here.
-        // Cc https://github.com/rust-lang/rust/issues/114479
-        if self.is_infinite() {
-            // Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
-            FpCategory::Infinite
-        } else if self.is_nan() {
-            // And it may not be NaN, as it can simply be an "overextended" finite value.
-            FpCategory::Nan
-        } else {
-            // However, std can't simply compare to zero to check for zero, either,
-            // as correctness requires avoiding equality tests that may be Subnormal == -0.0
-            // because it may be wrong under "denormals are zero" and "flush to zero" modes.
-            // Most of std's targets don't use those, but they are used for thumbv7neon.
-            // So, this does use bitpattern matching for the rest. On x87, due to the incorrect
-            // float codegen on this hardware, this doesn't actually return a right answer for NaN
-            // because it cannot correctly discern between a floating point NaN, and some normal
-            // floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
-            // we are fine.
-            // FIXME(jubilee): This probably could at least answer things correctly for Infinity,
-            // like the f64 version does, but I need to run more checks on how things go on x86.
-            // I fear losing mantissa data that would have answered that differently.
-            let b = self.to_bits();
-            match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
-                (0, 0) => FpCategory::Zero,
-                (_, 0) => FpCategory::Subnormal,
-                _ => FpCategory::Normal,
-            }
+        let b = self.to_bits();
+        match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
+            (0, Self::EXP_MASK) => FpCategory::Infinite,
+            (_, Self::EXP_MASK) => FpCategory::Nan,
+            (0, 0) => FpCategory::Zero,
+            (_, 0) => FpCategory::Subnormal,
+            _ => FpCategory::Normal,
         }
     }
 

library/core/src/num/f32.rs

@@ -652,42 +652,13 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
     pub const fn classify(self) -> FpCategory {
-        // A previous implementation tried to only use bitmask-based checks,
-        // using f32::to_bits to transmute the float to its bit repr and match on that.
-        // If we only cared about being "technically" correct, that's an entirely legit
-        // implementation.
-        //
-        // Unfortunately, there is hardware out there that does not correctly implement the IEEE
-        // float semantics Rust relies on: x87 uses a too-large mantissa and exponent, and some
-        // hardware flushes subnormals to zero. These are platforms bugs, and Rust will misbehave on
-        // such hardware, but we can at least try to make things seem as sane as possible by being
-        // careful here.
-        // Cc https://github.com/rust-lang/rust/issues/114479
-        if self.is_infinite() {
-            // A value may compare unequal to infinity, despite having a "full" exponent mask.
-            FpCategory::Infinite
-        } else if self.is_nan() {
-            // And it may not be NaN, as it can simply be an "overextended" finite value.
-            FpCategory::Nan
-        } else {
-            // However, std can't simply compare to zero to check for zero, either,
-            // as correctness requires avoiding equality tests that may be Subnormal == -0.0
-            // because it may be wrong under "denormals are zero" and "flush to zero" modes.
-            // Most of std's targets don't use those, but they are used for thumbv7neon.
-            // So, this does use bitpattern matching for the rest. On x87, due to the incorrect
-            // float codegen on this hardware, this doesn't actually return a right answer for NaN
-            // because it cannot correctly discern between a floating point NaN, and some normal
-            // floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
-            // we are fine.
-            // FIXME(jubilee): This probably could at least answer things correctly for Infinity,
-            // like the f64 version does, but I need to run more checks on how things go on x86.
-            // I fear losing mantissa data that would have answered that differently.
-            let b = self.to_bits();
-            match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
-                (0, 0) => FpCategory::Zero,
-                (_, 0) => FpCategory::Subnormal,
-                _ => FpCategory::Normal,
-            }
+        let b = self.to_bits();
+        match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
+            (0, Self::EXP_MASK) => FpCategory::Infinite,
+            (_, Self::EXP_MASK) => FpCategory::Nan,
+            (0, 0) => FpCategory::Zero,
+            (_, 0) => FpCategory::Subnormal,
+            _ => FpCategory::Normal,
         }
     }
 
@@ -773,7 +744,7 @@ impl f32 {
     #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
-    pub const fn next_up(self) -> Self {
+    pub const extern "C" fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
         // we do our best to still produce the correct result on such targets.
@@ -822,7 +793,7 @@ impl f32 {
     #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
-    pub const fn next_down(self) -> Self {
+    pub const extern "C" fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
         // we do our best to still produce the correct result on such targets.
@@ -1114,7 +1085,7 @@ impl f32 {
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
-    pub const fn to_bits(self) -> u32 {
+    pub const extern "C" fn to_bits(self) -> u32 {
         // SAFETY: `u32` is a plain old datatype so we can always transmute to it.
         unsafe { mem::transmute(self) }
     }

library/core/src/num/f64.rs

@@ -651,38 +651,13 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_float_classify", issue = "72505")]
     pub const fn classify(self) -> FpCategory {
-        // A previous implementation tried to only use bitmask-based checks,
-        // using f64::to_bits to transmute the float to its bit repr and match on that.
-        // If we only cared about being "technically" correct, that's an entirely legit
-        // implementation.
-        //
-        // Unfortunately, there is hardware out there that does not correctly implement the IEEE
-        // float semantics Rust relies on: x87 uses a too-large exponent, and some hardware flushes
-        // subnormals to zero. These are platforms bugs, and Rust will misbehave on such hardware,
-        // but we can at least try to make things seem as sane as possible by being careful here.
-        // Cc https://github.com/rust-lang/rust/issues/114479
-        //
-        // Thus, a value may compare unequal to infinity, despite having a "full" exponent mask.
-        // And it may not be NaN, as it can simply be an "overextended" finite value.
-        if self.is_nan() {
-            FpCategory::Nan
-        } else {
-            // However, std can't simply compare to zero to check for zero, either,
-            // as correctness requires avoiding equality tests that may be Subnormal == -0.0
-            // because it may be wrong under "denormals are zero" and "flush to zero" modes.
-            // Most of std's targets don't use those, but they are used for thumbv7neon.
-            // So, this does use bitpattern matching for the rest. On x87, due to the incorrect
-            // float codegen on this hardware, this doesn't actually return a right answer for NaN
-            // because it cannot correctly discern between a floating point NaN, and some normal
-            // floating point numbers truncated from an x87 FPU -- but we took care of NaN above, so
-            // we are fine.
-            let b = self.to_bits();
-            match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
-                (0, Self::EXP_MASK) => FpCategory::Infinite,
-                (0, 0) => FpCategory::Zero,
-                (_, 0) => FpCategory::Subnormal,
-                _ => FpCategory::Normal,
-            }
+        let b = self.to_bits();
+        match (b & Self::MAN_MASK, b & Self::EXP_MASK) {
+            (0, Self::EXP_MASK) => FpCategory::Infinite,
+            (_, Self::EXP_MASK) => FpCategory::Nan,
+            (0, 0) => FpCategory::Zero,
+            (_, 0) => FpCategory::Subnormal,
+            _ => FpCategory::Normal,
         }
     }
 
@@ -786,7 +761,7 @@ impl f64 {
     #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
-    pub const fn next_up(self) -> Self {
+    pub const extern "C" fn next_up(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
         // we do our best to still produce the correct result on such targets.
@@ -835,7 +810,7 @@ impl f64 {
     #[inline]
     #[unstable(feature = "float_next_up_down", issue = "91399")]
     #[rustc_const_unstable(feature = "float_next_up_down", issue = "91399")]
-    pub const fn next_down(self) -> Self {
+    pub const extern "C" fn next_down(self) -> Self {
         // Some targets violate Rust's assumption of IEEE semantics, e.g. by flushing
         // denormals to zero. This is in general unsound and unsupported, but here
         // we do our best to still produce the correct result on such targets.
@@ -1155,7 +1130,7 @@ impl f64 {
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use]
     #[inline]
-    pub const fn from_bits(v: u64) -> Self {
+    pub const extern "C" fn from_bits(v: u64) -> Self {
         // It turns out the safety issues with sNaN were overblown! Hooray!
         // SAFETY: `u64` is a plain old datatype so we can always transmute from it.
         unsafe { mem::transmute(v) }

library/std/src/f32/tests.rs

@@ -348,11 +348,11 @@ fn test_is_sign_negative() {
 
 #[test]
 fn test_next_up() {
-    let tiny = f32::from_bits(TINY_BITS);
-    let tiny_up = f32::from_bits(TINY_UP_BITS);
-    let max_down = f32::from_bits(MAX_DOWN_BITS);
-    let largest_subnormal = f32::from_bits(LARGEST_SUBNORMAL_BITS);
-    let smallest_normal = f32::from_bits(SMALLEST_NORMAL_BITS);
+    let tiny = crate::hint::black_box(f32::from_bits(TINY_BITS));
+    let tiny_up = crate::hint::black_box(f32::from_bits(TINY_UP_BITS));
+    let max_down = crate::hint::black_box(f32::from_bits(MAX_DOWN_BITS));
+    let largest_subnormal = crate::hint::black_box(f32::from_bits(LARGEST_SUBNORMAL_BITS));
+    let smallest_normal = crate::hint::black_box(f32::from_bits(SMALLEST_NORMAL_BITS));
     assert_f32_biteq!(f32::NEG_INFINITY.next_up(), f32::MIN);
     assert_f32_biteq!(f32::MIN.next_up(), -max_down);
     assert_f32_biteq!((-1.0 - f32::EPSILON).next_up(), -1.0);