From a1939c58ff8b85333ac80af87a58a9ea075db0b1 Mon Sep 17 00:00:00 2001 From: Trevor Gross Date: Thu, 27 Jun 2024 03:21:54 -0400 Subject: [PATCH] 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`. --- library/core/src/num/f128.rs | 127 ++++++++++++++++++++++++++++++----- library/core/src/num/f16.rs | 127 ++++++++++++++++++++++++++++++----- 2 files changed, 222 insertions(+), 32 deletions(-) diff --git a/library/core/src/num/f128.rs b/library/core/src/num/f128.rs index ed8fd4d992209..26627250dbaba 100644 --- a/library/core/src/num/f128.rs +++ b/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::(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::(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 --git a/library/core/src/num/f16.rs b/library/core/src/num/f16.rs index b5cc2c4aa6b38..a6be722b57306 100644 --- a/library/core/src/num/f16.rs +++ b/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::(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::(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)) }