Mark some f16 and f128 functions unstably const

These constifications were blocked on classification functions being
added. Now that those methods are available, constify them.

This brings things more in line with `f32` and `f64`.

Author: tgross35

Diff for: library/core/src/num/f128.rs

@@ -12,6 +12,8 @@
 #![unstable(feature = "f128", issue = "116909")]
 
 use crate::convert::FloatToInt;
+#[cfg(not(test))]
+use crate::intrinsics;
 use crate::mem;
 use crate::num::FpCategory;
 
@@ -780,12 +782,54 @@ impl f128 {
     /// ```
     #[inline]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_bits(self) -> u128 {
-        // SAFETY: `u128` is a plain old datatype so we can always... uh...
-        // ...look, just pretend you forgot what you just read.
-        // Stability concerns.
-        unsafe { mem::transmute(self) }
+    pub const fn to_bits(self) -> u128 {
+        // SAFETY: `u128` is a plain old datatype so we can always transmute to it.
+        // ...sorta.
+        //
+        // It turns out that at runtime, it is possible for a floating point number
+        // to be subject to a floating point mode that alters nonzero subnormal numbers
+        // to zero on reads and writes, aka "denormals are zero" and "flush to zero".
+        // This is not a problem per se, but at least one tier2 platform for Rust
+        // actually exhibits this behavior by default.
+        //
+        // And, of course evaluating to a NaN value is fairly nondeterministic.
+        // More precisely: when NaN should be returned is knowable, but which NaN?
+        // So far that's defined by a combination of LLVM and the CPU, not Rust.
+        // This function, however, allows observing the bitstring of a NaN,
+        // thus introspection on CTFE.
+        //
+        // In order to preserve, at least for the moment, const-to-runtime equivalence,
+        // we reject any of these possible situations from happening.
+        #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+        const fn ct_f128_to_u128(ct: f128) -> u128 {
+            // FIXME(f16_f128): we should use `.classify()` like `f32` and `f64`, but that
+            // is not avaialble on all platforms (needs `netf2` and `unordtf2`). So classify
+            // the bits instead.
+
+            // SAFETY: this direction is a POD transmutation. We just can't return it unless
+            // it is normal, infinite, or zero.
+            let bits = unsafe { mem::transmute::<f128, u128>(ct) };
+            match f128::classify_bits(bits) {
+                FpCategory::Nan => {
+                    panic!("const-eval error: cannot use f128::to_bits on a NaN")
+                }
+                FpCategory::Subnormal => {
+                    panic!("const-eval error: cannot use f128::to_bits on a subnormal number")
+                }
+                FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => bits,
+            }
+        }
+
+        #[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
+        fn rt_f128_to_u128(x: f128) -> u128 {
+            // SAFETY: `u128` is a plain old datatype so we can always... uh...
+            // ...look, just pretend you forgot what you just read.
+            // Stability concerns.
+            unsafe { mem::transmute(x) }
+        }
+        intrinsics::const_eval_select((self,), ct_f128_to_u128, rt_f128_to_u128)
     }
 
     /// Raw transmutation from `u128`.
@@ -830,11 +874,56 @@ impl f128 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f128", issue = "116909")]
-    pub fn from_bits(v: u128) -> Self {
-        // SAFETY: `u128 is a plain old datatype so we can always... uh...
-        // ...look, just pretend you forgot what you just read.
-        // Stability concerns.
-        unsafe { mem::transmute(v) }
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_bits(v: u128) -> Self {
+        // It turns out the safety issues with sNaN were overblown! Hooray!
+        // SAFETY: `u128` is a plain old datatype so we can always transmute from it
+        // ...sorta.
+        //
+        // It turns out that at runtime, it is possible for a floating point number
+        // to be subject to floating point modes that alter nonzero subnormal numbers
+        // to zero on reads and writes, aka "denormals are zero" and "flush to zero".
+        // This is not a problem usually, but at least one tier2 platform for Rust
+        // actually exhibits this behavior by default: thumbv7neon
+        // aka "the Neon FPU in AArch32 state"
+        //
+        // In addition, on x86 targets with SSE or SSE2 disabled and the x87 FPU enabled,
+        // i.e. not soft-float, the way Rust does parameter passing can actually alter
+        // a number that is "not infinity" to have the same exponent as infinity,
+        // in a slightly unpredictable manner.
+        //
+        // And, of course evaluating to a NaN value is fairly nondeterministic.
+        // More precisely: when NaN should be returned is knowable, but which NaN?
+        // So far that's defined by a combination of LLVM and the CPU, not Rust.
+        // This function, however, allows observing the bitstring of a NaN,
+        // thus introspection on CTFE.
+        //
+        // In order to preserve, at least for the moment, const-to-runtime equivalence,
+        // reject any of these possible situations from happening.
+        #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+        const fn ct_u128_to_f128(ct: u128) -> f128 {
+            match f128::classify_bits(ct) {
+                FpCategory::Subnormal => {
+                    panic!("const-eval error: cannot use f128::from_bits on a subnormal number")
+                }
+                FpCategory::Nan => {
+                    panic!("const-eval error: cannot use f128::from_bits on NaN")
+                }
+                FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
+                    // SAFETY: It's not a frumious number
+                    unsafe { mem::transmute::<u128, f128>(ct) }
+                }
+            }
+        }
+
+        #[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
+        fn rt_u128_to_f128(x: u128) -> f128 {
+            // SAFETY: `u128` is a plain old datatype so we can always... uh...
+            // ...look, just pretend you forgot what you just read.
+            // Stability concerns.
+            unsafe { mem::transmute(x) }
+        }
+        intrinsics::const_eval_select((v,), ct_u128_to_f128, rt_u128_to_f128)
     }
 
     /// Return the memory representation of this floating point number as a byte array in
@@ -857,8 +946,9 @@ impl f128 {
     /// ```
     #[inline]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_be_bytes(self) -> [u8; 16] {
+    pub const fn to_be_bytes(self) -> [u8; 16] {
         self.to_bits().to_be_bytes()
     }
 
@@ -882,8 +972,9 @@ impl f128 {
     /// ```
     #[inline]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_le_bytes(self) -> [u8; 16] {
+    pub const fn to_le_bytes(self) -> [u8; 16] {
         self.to_bits().to_le_bytes()
     }
 
@@ -918,8 +1009,9 @@ impl f128 {
     /// ```
     #[inline]
     #[unstable(feature = "f128", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_ne_bytes(self) -> [u8; 16] {
+    pub const fn to_ne_bytes(self) -> [u8; 16] {
         self.to_bits().to_ne_bytes()
     }
 
@@ -945,7 +1037,8 @@ impl f128 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f128", issue = "116909")]
-    pub fn from_be_bytes(bytes: [u8; 16]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_be_bytes(bytes: [u8; 16]) -> Self {
         Self::from_bits(u128::from_be_bytes(bytes))
     }
 
@@ -971,7 +1064,8 @@ impl f128 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f128", issue = "116909")]
-    pub fn from_le_bytes(bytes: [u8; 16]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_le_bytes(bytes: [u8; 16]) -> Self {
         Self::from_bits(u128::from_le_bytes(bytes))
     }
 
@@ -1007,7 +1101,8 @@ impl f128 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f128", issue = "116909")]
-    pub fn from_ne_bytes(bytes: [u8; 16]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_ne_bytes(bytes: [u8; 16]) -> Self {
         Self::from_bits(u128::from_ne_bytes(bytes))
     }
 

Diff for: library/core/src/num/f16.rs

@@ -12,6 +12,8 @@
 #![unstable(feature = "f16", issue = "116909")]
 
 use crate::convert::FloatToInt;
+#[cfg(not(test))]
+use crate::intrinsics;
 use crate::mem;
 use crate::num::FpCategory;
 
@@ -786,12 +788,54 @@ impl f16 {
     /// ```
     #[inline]
     #[unstable(feature = "f16", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_bits(self) -> u16 {
-        // SAFETY: `u16` is a plain old datatype so we can always... uh...
-        // ...look, just pretend you forgot what you just read.
-        // Stability concerns.
-        unsafe { mem::transmute(self) }
+    pub const fn to_bits(self) -> u16 {
+        // SAFETY: `u16` is a plain old datatype so we can always transmute to it.
+        // ...sorta.
+        //
+        // It turns out that at runtime, it is possible for a floating point number
+        // to be subject to a floating point mode that alters nonzero subnormal numbers
+        // to zero on reads and writes, aka "denormals are zero" and "flush to zero".
+        // This is not a problem per se, but at least one tier2 platform for Rust
+        // actually exhibits this behavior by default.
+        //
+        // And, of course evaluating to a NaN value is fairly nondeterministic.
+        // More precisely: when NaN should be returned is knowable, but which NaN?
+        // So far that's defined by a combination of LLVM and the CPU, not Rust.
+        // This function, however, allows observing the bitstring of a NaN,
+        // thus introspection on CTFE.
+        //
+        // In order to preserve, at least for the moment, const-to-runtime equivalence,
+        // we reject any of these possible situations from happening.
+        #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+        const fn ct_f16_to_u16(ct: f16) -> u16 {
+            // FIXME(f16_f128): we should use `.classify()` like `f32` and `f64`, but we don't
+            // yet want to rely on that on all platforms (ABI issues). So just classify the
+            // bits instead.
+
+            // SAFETY: this direction is a POD transmutation. We just can't return it unless
+            // it is normal, infinite, or zero.
+            let bits = unsafe { mem::transmute::<f16, u16>(ct) };
+            match f16::classify_bits(bits) {
+                FpCategory::Nan => {
+                    panic!("const-eval error: cannot use f16::to_bits on a NaN")
+                }
+                FpCategory::Subnormal => {
+                    panic!("const-eval error: cannot use f16::to_bits on a subnormal number")
+                }
+                FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => bits,
+            }
+        }
+
+        #[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
+        fn rt_f16_to_u16(x: f16) -> u16 {
+            // SAFETY: `u16` is a plain old datatype so we can always... uh...
+            // ...look, just pretend you forgot what you just read.
+            // Stability concerns.
+            unsafe { mem::transmute(x) }
+        }
+        intrinsics::const_eval_select((self,), ct_f16_to_u16, rt_f16_to_u16)
     }
 
     /// Raw transmutation from `u16`.
@@ -835,11 +879,56 @@ impl f16 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f16", issue = "116909")]
-    pub fn from_bits(v: u16) -> Self {
-        // SAFETY: `u16` is a plain old datatype so we can always... uh...
-        // ...look, just pretend you forgot what you just read.
-        // Stability concerns.
-        unsafe { mem::transmute(v) }
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_bits(v: u16) -> Self {
+        // It turns out the safety issues with sNaN were overblown! Hooray!
+        // SAFETY: `u16` is a plain old datatype so we can always transmute from it
+        // ...sorta.
+        //
+        // It turns out that at runtime, it is possible for a floating point number
+        // to be subject to floating point modes that alter nonzero subnormal numbers
+        // to zero on reads and writes, aka "denormals are zero" and "flush to zero".
+        // This is not a problem usually, but at least one tier2 platform for Rust
+        // actually exhibits this behavior by default: thumbv7neon
+        // aka "the Neon FPU in AArch32 state"
+        //
+        // In addition, on x86 targets with SSE or SSE2 disabled and the x87 FPU enabled,
+        // i.e. not soft-float, the way Rust does parameter passing can actually alter
+        // a number that is "not infinity" to have the same exponent as infinity,
+        // in a slightly unpredictable manner.
+        //
+        // And, of course evaluating to a NaN value is fairly nondeterministic.
+        // More precisely: when NaN should be returned is knowable, but which NaN?
+        // So far that's defined by a combination of LLVM and the CPU, not Rust.
+        // This function, however, allows observing the bitstring of a NaN,
+        // thus introspection on CTFE.
+        //
+        // In order to preserve, at least for the moment, const-to-runtime equivalence,
+        // reject any of these possible situations from happening.
+        #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+        const fn ct_u16_to_f16(ct: u16) -> f16 {
+            match f16::classify_bits(ct) {
+                FpCategory::Subnormal => {
+                    panic!("const-eval error: cannot use f16::from_bits on a subnormal number")
+                }
+                FpCategory::Nan => {
+                    panic!("const-eval error: cannot use f16::from_bits on NaN")
+                }
+                FpCategory::Infinite | FpCategory::Normal | FpCategory::Zero => {
+                    // SAFETY: It's not a frumious number
+                    unsafe { mem::transmute::<u16, f16>(ct) }
+                }
+            }
+        }
+
+        #[inline(always)] // See https://github.com/rust-lang/compiler-builtins/issues/491
+        fn rt_u16_to_f16(x: u16) -> f16 {
+            // SAFETY: `u16` is a plain old datatype so we can always... uh...
+            // ...look, just pretend you forgot what you just read.
+            // Stability concerns.
+            unsafe { mem::transmute(x) }
+        }
+        intrinsics::const_eval_select((v,), ct_u16_to_f16, rt_u16_to_f16)
     }
 
     /// Return the memory representation of this floating point number as a byte array in
@@ -858,8 +947,9 @@ impl f16 {
     /// ```
     #[inline]
     #[unstable(feature = "f16", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_be_bytes(self) -> [u8; 2] {
+    pub const fn to_be_bytes(self) -> [u8; 2] {
         self.to_bits().to_be_bytes()
     }
 
@@ -879,8 +969,9 @@ impl f16 {
     /// ```
     #[inline]
     #[unstable(feature = "f16", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_le_bytes(self) -> [u8; 2] {
+    pub const fn to_le_bytes(self) -> [u8; 2] {
         self.to_bits().to_le_bytes()
     }
 
@@ -913,8 +1004,9 @@ impl f16 {
     /// ```
     #[inline]
     #[unstable(feature = "f16", issue = "116909")]
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
-    pub fn to_ne_bytes(self) -> [u8; 2] {
+    pub const fn to_ne_bytes(self) -> [u8; 2] {
         self.to_bits().to_ne_bytes()
     }
 
@@ -936,7 +1028,8 @@ impl f16 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f16", issue = "116909")]
-    pub fn from_be_bytes(bytes: [u8; 2]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_be_bytes(bytes: [u8; 2]) -> Self {
         Self::from_bits(u16::from_be_bytes(bytes))
     }
 
@@ -958,7 +1051,8 @@ impl f16 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f16", issue = "116909")]
-    pub fn from_le_bytes(bytes: [u8; 2]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_le_bytes(bytes: [u8; 2]) -> Self {
         Self::from_bits(u16::from_le_bytes(bytes))
     }
 
@@ -991,7 +1085,8 @@ impl f16 {
     #[inline]
     #[must_use]
     #[unstable(feature = "f16", issue = "116909")]
-    pub fn from_ne_bytes(bytes: [u8; 2]) -> Self {
+    #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
+    pub const fn from_ne_bytes(bytes: [u8; 2]) -> Self {
         Self::from_bits(u16::from_ne_bytes(bytes))
     }