Auto merge of #1325 - RalfJung:float_to_int_unchecked, r=RalfJung

implement float_to_int_unchecked

@hanna-kruppe would be great if you could have a look at this.

`float.rs` tests legal casts. `test_cast` checks that both `as` casts and unchecked casts work (i.e., these are not saturating). The `compile-fail` tests should ensure that illegal casts via the intrinsic are detected as such.

Fixes #1264

Author: bors

src/shims/intrinsics.rs

@@ -1,9 +1,10 @@
 use std::iter;
 use std::convert::TryFrom;
 
+use rustc_ast::ast::FloatTy;
 use rustc_middle::{mir, ty};
-use rustc_apfloat::Float;
-use rustc_target::abi::{Align, LayoutOf};
+use rustc_apfloat::{Float, Round};
+use rustc_target::abi::{Align, LayoutOf, Size};
 
 use crate::*;
 
@@ -279,6 +280,22 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(Scalar::from_u64(f.powi(i).to_bits()), dest)?;
             }
 
+            "float_to_int_unchecked" => {
+                let val = this.read_immediate(args[0])?;
+
+                let res = match val.layout.ty.kind {
+                    ty::Float(FloatTy::F32) => {
+                        this.float_to_int_unchecked(val.to_scalar()?.to_f32()?, dest.layout.ty)?
+                    }
+                    ty::Float(FloatTy::F64) => {
+                        this.float_to_int_unchecked(val.to_scalar()?.to_f64()?, dest.layout.ty)?
+                    }
+                    _ => bug!("`float_to_int_unchecked` called with non-float input type {:?}", val.layout.ty),
+                };
+
+                this.write_scalar(res, dest)?;
+            }
+
             // Atomic operations
             #[rustfmt::skip]
             | "atomic_load"
@@ -493,4 +510,55 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         this.go_to_block(ret);
         Ok(())
     }
+
+    fn float_to_int_unchecked<F>(
+        &self,
+        f: F,
+        dest_ty: ty::Ty<'tcx>,
+    ) -> InterpResult<'tcx, Scalar<Tag>>
+    where
+        F: Float + Into<Scalar<Tag>>
+    {
+        let this = self.eval_context_ref();
+
+        // Step 1: cut off the fractional part of `f`. The result of this is
+        // guaranteed to be precisely representable in IEEE floats.
+        let f = f.round_to_integral(Round::TowardZero).value;
+
+        // Step 2: Cast the truncated float to the target integer type and see if we lose any information in this step.
+        Ok(match dest_ty.kind {
+            // Unsigned
+            ty::Uint(t) => {
+                let width = t.bit_width().unwrap_or_else(|| this.pointer_size().bits());
+                let res = f.to_u128(usize::try_from(width).unwrap());
+                if res.status.is_empty() {
+                    // No status flags means there was no further rounding or other loss of precision.
+                    Scalar::from_uint(res.value, Size::from_bits(width))
+                } else {
+                    // `f` was not representable in this integer type.
+                    throw_ub_format!(
+                        "`float_to_int_unchecked` intrinsic called on {} which cannot be represented in target type `{:?}`",
+                        f, dest_ty,
+                    );
+                }
+            }
+            // Signed
+            ty::Int(t) => {
+                let width = t.bit_width().unwrap_or_else(|| this.pointer_size().bits());
+                let res = f.to_i128(usize::try_from(width).unwrap());
+                if res.status.is_empty() {
+                    // No status flags means there was no further rounding or other loss of precision.
+                    Scalar::from_int(res.value, Size::from_bits(width))
+                } else {
+                    // `f` was not representable in this integer type.
+                    throw_ub_format!(
+                        "`float_to_int_unchecked` intrinsic called on {} which cannot be represented in target type `{:?}`",
+                        f, dest_ty,
+                    );
+                }
+            }
+            // Nothing else
+            _ => bug!("`float_to_int_unchecked` called with non-int output type {:?}", dest_ty),
+        })
+    }
 }

tests/compile-fail/intrinsics/copy_overlapping.rs

@@ -1,4 +1,3 @@
-//error-pattern: copy_nonoverlapping called on overlapping ranges
 #![feature(intrinsics)]
 
 // Directly call intrinsic to avoid debug assertions in libstd
@@ -11,6 +10,6 @@ fn main() {
     unsafe {
         let a = data.as_mut_ptr();
         let b = a.wrapping_offset(1) as *mut _;
-        copy_nonoverlapping(a, b, 2);
+        copy_nonoverlapping(a, b, 2); //~ ERROR copy_nonoverlapping called on overlapping ranges
     }
 }

tests/compile-fail/intrinsics/copy_unaligned.rs

@@ -1,4 +1,3 @@
-//error-pattern: accessing memory with alignment 1, but alignment 2 is required
 #![feature(intrinsics)]
 
 // Directly call intrinsic to avoid debug assertions in libstd
@@ -10,5 +9,5 @@ fn main() {
     let mut data = [0u16; 8];
     let ptr = (&mut data[0] as *mut u16 as *mut u8).wrapping_add(1) as *mut u16;
     // Even copying 0 elements to something unaligned should error
-    unsafe { copy_nonoverlapping(&data[5], ptr, 0); }
+    unsafe { copy_nonoverlapping(&data[5], ptr, 0); } //~ ERROR accessing memory with alignment 1, but alignment 2 is required
 }

tests/compile-fail/intrinsics/float_to_int_32_inf1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(f32::INFINITY); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_32_infneg1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(f32::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_32_nan.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(f32::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}

tests/compile-fail/intrinsics/float_to_int_32_nanneg.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(-f32::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}

tests/compile-fail/intrinsics/float_to_int_32_neg.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>(-1.000000001f32); } //~ ERROR: cannot be represented in target type `u32`
+}

tests/compile-fail/intrinsics/float_to_int_32_too_big1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(2147483648.0f32); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_32_too_big2.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, u32>((u32::MAX-127) as f32); } //~ ERROR: cannot be represented in target type `u32`
+}

tests/compile-fail/intrinsics/float_to_int_32_too_small1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f32, i32>(-2147483904.0f32); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_64_inf1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(f64::INFINITY); } //~ ERROR: cannot be represented in target type `u128`
+}

tests/compile-fail/intrinsics/float_to_int_64_infneg1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(f64::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `u128`
+}

tests/compile-fail/intrinsics/float_to_int_64_infneg2.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(f64::NEG_INFINITY); } //~ ERROR: cannot be represented in target type `i128`
+}

tests/compile-fail/intrinsics/float_to_int_64_nan.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u32>(f64::NAN); } //~ ERROR: cannot be represented in target type `u32`
+}

tests/compile-fail/intrinsics/float_to_int_64_neg.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(-1.0000000000001f64); } //~ ERROR: cannot be represented in target type `u128`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_big1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i32>(2147483648.0f64); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_big2.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i64>(9223372036854775808.0f64); } //~ ERROR: cannot be represented in target type `i64`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_big3.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u64>(18446744073709551616.0f64); } //~ ERROR: cannot be represented in target type `u64`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_big4.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, u128>(u128::MAX as f64); } //~ ERROR: cannot be represented in target type `u128`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_big5.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(240282366920938463463374607431768211455.0f64); } //~ ERROR: cannot be represented in target type `i128`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_small1.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i32>(-2147483649.0f64); } //~ ERROR: cannot be represented in target type `i32`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_small2.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i64>(-9223372036854777856.0f64); } //~ ERROR: cannot be represented in target type `i64`
+}

tests/compile-fail/intrinsics/float_to_int_64_too_small3.rs

@@ -0,0 +1,10 @@
+#![feature(intrinsics)]
+
+// Directly call intrinsic to avoid debug assertions in libstd
+extern "rust-intrinsic" {
+    fn float_to_int_unchecked<Float: Copy, Int: Copy>(value: Float) -> Int;
+}
+
+fn main() {
+    unsafe { float_to_int_unchecked::<f64, i128>(-240282366920938463463374607431768211455.0f64); } //~ ERROR: cannot be represented in target type `i128`
+}