Auto merge of #45205 - rkruppe:saturating-casts, r=eddyb

Saturating casts between integers and floats

Introduces a new flag, `-Z saturating-float-casts`, which makes code generation for int->float and float->int casts safe (`undef`-free), implementing [the saturating semantics laid out by](https://github.com/rust-lang/rust/issues/10184#issuecomment-299229143) @jorendorff for float->int casts and overflowing to infinity for `u128::MAX` -> `f32`.
Constant evaluation in trans was changed to behave like HIR const eval already did, i.e., saturate for u128->f32 and report an error for problematic float->int casts.

Many thanks to @eddyb, whose APFloat port simplified many parts of this patch, and made HIR constant evaluation recognize dangerous float casts as mentioned above.
Also thanks to @ActuallyaDeviloper whose branchless implementation served as inspiration for this implementation.

cc #10184 #41799
fixes #45134

11 files changed, 535 insertions, 16 deletions

diff --git a/src/Cargo.lock b/src/Cargo.lock
index c4314b41e39..0263c74595f 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1869,6 +1869,7 @@ dependencies = [
  "rustc 0.0.0",
  "rustc-demangle 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_allocator 0.0.0",
+ "rustc_apfloat 0.0.0",
  "rustc_back 0.0.0",
  "rustc_const_math 0.0.0",
  "rustc_data_structures 0.0.0",
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 4ed2bce5ab1..6939f8a5599 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -1135,6 +1135,9 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "control whether #[inline] functions are in all cgus"),
     tls_model: Option<String> = (None, parse_opt_string, [TRACKED],
          "choose the TLS model to use (rustc --print tls-models for details)"),
+    saturating_float_casts: bool = (false, parse_bool, [TRACKED],
+        "make casts between integers and floats safe: clip out-of-range inputs to the min/max \
+         integer or to infinity respectively, and turn `NAN` into 0 when casting to integers"),
 }
 
 pub fn default_lib_output() -> CrateType {
diff --git a/src/librustc_apfloat/lib.rs b/src/librustc_apfloat/lib.rs
index 9e348f62223..09c9cecdcee 100644
--- a/src/librustc_apfloat/lib.rs
+++ b/src/librustc_apfloat/lib.rs
@@ -96,7 +96,7 @@ impl Status {
 }
 
 impl<T> StatusAnd<T> {
-    fn map<F: FnOnce(T) -> U, U>(self, f: F) -> StatusAnd<U> {
+    pub fn map<F: FnOnce(T) -> U, U>(self, f: F) -> StatusAnd<U> {
         StatusAnd {
             status: self.status,
             value: f(self.value),
@@ -378,7 +378,7 @@ pub trait Float
     fn from_bits(input: u128) -> Self;
     fn from_i128_r(input: i128, round: Round) -> StatusAnd<Self> {
         if input < 0 {
-            Self::from_u128_r(-input as u128, -round).map(|r| -r)
+            Self::from_u128_r(input.wrapping_neg() as u128, -round).map(|r| -r)
         } else {
             Self::from_u128_r(input as u128, round)
         }
diff --git a/src/librustc_const_math/float.rs b/src/librustc_const_math/float.rs
index b67048939e4..9d820ea8cbe 100644
--- a/src/librustc_const_math/float.rs
+++ b/src/librustc_const_math/float.rs
@@ -203,3 +203,11 @@ impl ::std::ops::Neg for ConstFloat {
         ConstFloat { bits, ty: self.ty }
     }
 }
+
+/// This is `f32::MAX + (0.5 ULP)` as an integer. Numbers greater or equal to this
+/// are rounded to infinity when converted to `f32`.
+///
+/// NB: Computed as maximum significand with an extra 1 bit added (for the half ULP)
+/// shifted by the maximum exponent (accounting for normalization).
+pub const MAX_F32_PLUS_HALF_ULP: u128 = ((1 << (Single::PRECISION + 1)) - 1)
+                                        << (Single::MAX_EXP - Single::PRECISION as i16);
diff --git a/src/librustc_trans/Cargo.toml b/src/librustc_trans/Cargo.toml
index 5b7879ea58e..f797464c1f8 100644
--- a/src/librustc_trans/Cargo.toml
+++ b/src/librustc_trans/Cargo.toml
@@ -19,6 +19,7 @@ owning_ref = "0.3.3"
 rustc-demangle = "0.1.4"
 rustc = { path = "../librustc" }
 rustc_allocator = { path = "../librustc_allocator" }
+rustc_apfloat = { path = "../librustc_apfloat" }
 rustc_back = { path = "../librustc_back" }
 rustc_const_math = { path = "../librustc_const_math" }
 rustc_data_structures = { path = "../librustc_data_structures" }
diff --git a/src/librustc_trans/lib.rs b/src/librustc_trans/lib.rs
index c0460fb4852..29394af3396 100644
--- a/src/librustc_trans/lib.rs
+++ b/src/librustc_trans/lib.rs
@@ -24,6 +24,7 @@
 #![feature(custom_attribute)]
 #![allow(unused_attributes)]
 #![feature(i128_type)]
+#![feature(i128)]
 #![feature(libc)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
@@ -43,6 +44,7 @@ extern crate libc;
 extern crate owning_ref;
 #[macro_use] extern crate rustc;
 extern crate rustc_allocator;
+extern crate rustc_apfloat;
 extern crate rustc_back;
 extern crate rustc_data_structures;
 extern crate rustc_incremental;
diff --git a/src/librustc_trans/mir/constant.rs b/src/librustc_trans/mir/constant.rs
index cea7b9585d8..6573e507bd3 100644
--- a/src/librustc_trans/mir/constant.rs
+++ b/src/librustc_trans/mir/constant.rs
@@ -11,7 +11,7 @@
 use llvm::{self, ValueRef};
 use rustc::middle::const_val::{ConstEvalErr, ConstVal, ErrKind};
 use rustc_const_math::ConstInt::*;
-use rustc_const_math::{ConstInt, ConstMathErr};
+use rustc_const_math::{ConstInt, ConstMathErr, MAX_F32_PLUS_HALF_ULP};
 use rustc::hir::def_id::DefId;
 use rustc::infer::TransNormalize;
 use rustc::traits;
@@ -21,6 +21,7 @@ use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc::ty::layout::{self, LayoutTyper};
 use rustc::ty::cast::{CastTy, IntTy};
 use rustc::ty::subst::{Kind, Substs, Subst};
+use rustc_apfloat::{ieee, Float, Status};
 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
 use {adt, base, machine};
 use abi::{self, Abi};
@@ -689,20 +690,18 @@ impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
                                     llvm::LLVMConstIntCast(llval, ll_t_out.to_ref(), s)
                                 }
                                 (CastTy::Int(_), CastTy::Float) => {
-                                    if signed {
-                                        llvm::LLVMConstSIToFP(llval, ll_t_out.to_ref())
-                                    } else {
-                                        llvm::LLVMConstUIToFP(llval, ll_t_out.to_ref())
-                                    }
+                                    cast_const_int_to_float(self.ccx, llval, signed, ll_t_out)
                                 }
                                 (CastTy::Float, CastTy::Float) => {
                                     llvm::LLVMConstFPCast(llval, ll_t_out.to_ref())
                                 }
                                 (CastTy::Float, CastTy::Int(IntTy::I)) => {
-                                    llvm::LLVMConstFPToSI(llval, ll_t_out.to_ref())
+                                    cast_const_float_to_int(self.ccx, &operand,
+                                                            true, ll_t_out, span)
                                 }
                                 (CastTy::Float, CastTy::Int(_)) => {
-                                    llvm::LLVMConstFPToUI(llval, ll_t_out.to_ref())
+                                    cast_const_float_to_int(self.ccx, &operand,
+                                                            false, ll_t_out, span)
                                 }
                                 (CastTy::Ptr(_), CastTy::Ptr(_)) |
                                 (CastTy::FnPtr, CastTy::Ptr(_)) |
@@ -955,6 +954,64 @@ pub fn const_scalar_checked_binop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 }
 
+unsafe fn cast_const_float_to_int(ccx: &CrateContext,
+                                  operand: &Const,
+                                  signed: bool,
+                                  int_ty: Type,
+                                  span: Span) -> ValueRef {
+    let llval = operand.llval;
+    let float_bits = match operand.ty.sty {
+        ty::TyFloat(fty) => fty.bit_width(),
+        _ => bug!("cast_const_float_to_int: operand not a float"),
+    };
+    // Note: this breaks if llval is a complex constant expression rather than a simple constant.
+    // One way that might happen would be if addresses could be turned into integers in constant
+    // expressions, but that doesn't appear to be possible?
+    // In any case, an ICE is better than producing undef.
+    let llval_bits = consts::bitcast(llval, Type::ix(ccx, float_bits as u64));
+    let bits = const_to_opt_u128(llval_bits, false).unwrap_or_else(|| {
+        panic!("could not get bits of constant float {:?}",
+               Value(llval));
+    });
+    let int_width = int_ty.int_width() as usize;
+    // Try to convert, but report an error for overflow and NaN. This matches HIR const eval.
+    let cast_result = match float_bits {
+        32 if signed => ieee::Single::from_bits(bits).to_i128(int_width).map(|v| v as u128),
+        64 if signed => ieee::Double::from_bits(bits).to_i128(int_width).map(|v| v as u128),
+        32 => ieee::Single::from_bits(bits).to_u128(int_width),
+        64 => ieee::Double::from_bits(bits).to_u128(int_width),
+        n => bug!("unsupported float width {}", n),
+    };
+    if cast_result.status.contains(Status::INVALID_OP) {
+        let err = ConstEvalErr { span: span, kind: ErrKind::CannotCast };
+        err.report(ccx.tcx(), span, "expression");
+    }
+    C_big_integral(int_ty, cast_result.value)
+}
+
+unsafe fn cast_const_int_to_float(ccx: &CrateContext,
+                                  llval: ValueRef,
+                                  signed: bool,
+                                  float_ty: Type) -> ValueRef {
+    // Note: this breaks if llval is a complex constant expression rather than a simple constant.
+    // One way that might happen would be if addresses could be turned into integers in constant
+    // expressions, but that doesn't appear to be possible?
+    // In any case, an ICE is better than producing undef.
+    let value = const_to_opt_u128(llval, signed).unwrap_or_else(|| {
+        panic!("could not get z128 value of constant integer {:?}",
+               Value(llval));
+    });
+    if signed {
+        llvm::LLVMConstSIToFP(llval, float_ty.to_ref())
+    } else if float_ty.float_width() == 32 && value >= MAX_F32_PLUS_HALF_ULP {
+        // We're casting to f32 and the value is > f32::MAX + 0.5 ULP -> round up to infinity.
+        let infinity_bits = C_u32(ccx, ieee::Single::INFINITY.to_bits() as u32);
+        consts::bitcast(infinity_bits, float_ty)
+    } else {
+        llvm::LLVMConstUIToFP(llval, float_ty.to_ref())
+    }
+}
+
 impl<'a, 'tcx> MirContext<'a, 'tcx> {
     pub fn trans_constant(&mut self,
                           bcx: &Builder<'a, 'tcx>,
diff --git a/src/librustc_trans/mir/rvalue.rs b/src/librustc_trans/mir/rvalue.rs
index 777b86387e8..19131a68d86 100644
--- a/src/librustc_trans/mir/rvalue.rs
+++ b/src/librustc_trans/mir/rvalue.rs
@@ -15,11 +15,15 @@ use rustc::ty::layout::{Layout, LayoutTyper};
 use rustc::mir::tcx::LvalueTy;
 use rustc::mir;
 use rustc::middle::lang_items::ExchangeMallocFnLangItem;
+use rustc_apfloat::{ieee, Float, Status, Round};
+use rustc_const_math::MAX_F32_PLUS_HALF_ULP;
+use std::{u128, i128};
 
 use base;
 use builder::Builder;
 use callee;
-use common::{self, val_ty, C_bool, C_i32, C_null, C_usize, C_uint};
+use common::{self, val_ty, C_bool, C_i32, C_u32, C_u64, C_null, C_usize, C_uint, C_big_integral};
+use consts;
 use adt;
 use machine;
 use monomorphize;
@@ -333,14 +337,12 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                                 bcx.ptrtoint(llval, ll_t_out),
                             (CastTy::Int(_), CastTy::Ptr(_)) =>
                                 bcx.inttoptr(llval, ll_t_out),
-                            (CastTy::Int(_), CastTy::Float) if signed =>
-                                bcx.sitofp(llval, ll_t_out),
                             (CastTy::Int(_), CastTy::Float) =>
-                                bcx.uitofp(llval, ll_t_out),
+                                cast_int_to_float(&bcx, signed, llval, ll_t_in, ll_t_out),
                             (CastTy::Float, CastTy::Int(IntTy::I)) =>
-                                bcx.fptosi(llval, ll_t_out),
+                                cast_float_to_int(&bcx, true, llval, ll_t_in, ll_t_out),
                             (CastTy::Float, CastTy::Int(_)) =>
-                                bcx.fptoui(llval, ll_t_out),
+                                cast_float_to_int(&bcx, false, llval, ll_t_in, ll_t_out),
                             _ => bug!("unsupported cast: {:?} to {:?}", operand.ty, cast_ty)
                         };
                         OperandValue::Immediate(newval)
@@ -815,3 +817,158 @@ fn get_overflow_intrinsic(oop: OverflowOp, bcx: &Builder, ty: Ty) -> ValueRef {
 
     bcx.ccx.get_intrinsic(&name)
 }
+
+fn cast_int_to_float(bcx: &Builder,
+                     signed: bool,
+                     x: ValueRef,
+                     int_ty: Type,
+                     float_ty: Type) -> ValueRef {
+    // Most integer types, even i128, fit into [-f32::MAX, f32::MAX] after rounding.
+    // It's only u128 -> f32 that can cause overflows (i.e., should yield infinity).
+    // LLVM's uitofp produces undef in those cases, so we manually check for that case.
+    let is_u128_to_f32 = !signed && int_ty.int_width() == 128 && float_ty.float_width() == 32;
+    if is_u128_to_f32 && bcx.sess().opts.debugging_opts.saturating_float_casts {
+        // All inputs greater or equal to (f32::MAX + 0.5 ULP) are rounded to infinity,
+        // and for everything else LLVM's uitofp works just fine.
+        let max = C_big_integral(int_ty, MAX_F32_PLUS_HALF_ULP);
+        let overflow = bcx.icmp(llvm::IntUGE, x, max);
+        let infinity_bits = C_u32(bcx.ccx, ieee::Single::INFINITY.to_bits() as u32);
+        let infinity = consts::bitcast(infinity_bits, float_ty);
+        bcx.select(overflow, infinity, bcx.uitofp(x, float_ty))
+    } else {
+        if signed {
+            bcx.sitofp(x, float_ty)
+        } else {
+            bcx.uitofp(x, float_ty)
+        }
+    }
+}
+
+fn cast_float_to_int(bcx: &Builder,
+                     signed: bool,
+                     x: ValueRef,
+                     float_ty: Type,
+                     int_ty: Type) -> ValueRef {
+    let fptosui_result = if signed {
+        bcx.fptosi(x, int_ty)
+    } else {
+        bcx.fptoui(x, int_ty)
+    };
+
+    if !bcx.sess().opts.debugging_opts.saturating_float_casts {
+        return fptosui_result;
+    }
+    // LLVM's fpto[su]i returns undef when the input x is infinite, NaN, or does not fit into the
+    // destination integer type after rounding towards zero. This `undef` value can cause UB in
+    // safe code (see issue #10184), so we implement a saturating conversion on top of it:
+    // Semantically, the mathematical value of the input is rounded towards zero to the next
+    // mathematical integer, and then the result is clamped into the range of the destination
+    // integer type. Positive and negative infinity are mapped to the maximum and minimum value of
+    // the destination integer type. NaN is mapped to 0.
+    //
+    // Define f_min and f_max as the largest and smallest (finite) floats that are exactly equal to
+    // a value representable in int_ty.
+    // They are exactly equal to int_ty::{MIN,MAX} if float_ty has enough significand bits.
+    // Otherwise, int_ty::MAX must be rounded towards zero, as it is one less than a power of two.
+    // int_ty::MIN, however, is either zero or a negative power of two and is thus exactly
+    // representable. Note that this only works if float_ty's exponent range is sufficently large.
+    // f16 or 256 bit integers would break this property. Right now the smallest float type is f32
+    // with exponents ranging up to 127, which is barely enough for i128::MIN = -2^127.
+    // On the other hand, f_max works even if int_ty::MAX is greater than float_ty::MAX. Because
+    // we're rounding towards zero, we just get float_ty::MAX (which is always an integer).
+    // This already happens today with u128::MAX = 2^128 - 1 > f32::MAX.
+    fn compute_clamp_bounds<F: Float>(signed: bool, int_ty: Type) -> (u128, u128) {
+        let rounded_min = F::from_i128_r(int_min(signed, int_ty), Round::TowardZero);
+        assert_eq!(rounded_min.status, Status::OK);
+        let rounded_max = F::from_u128_r(int_max(signed, int_ty), Round::TowardZero);
+        assert!(rounded_max.value.is_finite());
+        (rounded_min.value.to_bits(), rounded_max.value.to_bits())
+    }
+    fn int_max(signed: bool, int_ty: Type) -> u128 {
+        let shift_amount = 128 - int_ty.int_width();
+        if signed {
+            i128::MAX as u128 >> shift_amount
+        } else {
+            u128::MAX >> shift_amount
+        }
+    }
+    fn int_min(signed: bool, int_ty: Type) -> i128 {
+        if signed {
+            i128::MIN >> (128 - int_ty.int_width())
+        } else {
+            0
+        }
+    }
+    let float_bits_to_llval = |bits| {
+        let bits_llval = match float_ty.float_width() {
+            32 => C_u32(bcx.ccx, bits as u32),
+            64 => C_u64(bcx.ccx, bits as u64),
+            n => bug!("unsupported float width {}", n),
+        };
+        consts::bitcast(bits_llval, float_ty)
+    };
+    let (f_min, f_max) = match float_ty.float_width() {
+        32 => compute_clamp_bounds::<ieee::Single>(signed, int_ty),
+        64 => compute_clamp_bounds::<ieee::Double>(signed, int_ty),
+        n => bug!("unsupported float width {}", n),
+    };
+    let f_min = float_bits_to_llval(f_min);
+    let f_max = float_bits_to_llval(f_max);
+    // To implement saturation, we perform the following steps:
+    //
+    // 1. Cast x to an integer with fpto[su]i. This may result in undef.
+    // 2. Compare x to f_min and f_max, and use the comparison results to select:
+    //  a) int_ty::MIN if x < f_min or x is NaN
+    //  b) int_ty::MAX if x > f_max
+    //  c) the result of fpto[su]i otherwise
+    // 3. If x is NaN, return 0.0, otherwise return the result of step 2.
+    //
+    // This avoids resulting undef because values in range [f_min, f_max] by definition fit into the
+    // destination type. It creates an undef temporary, but *producing* undef is not UB. Our use of
+    // undef does not introduce any non-determinism either.
+    // More importantly, the above procedure correctly implements saturating conversion.
+    // Proof (sketch):
+    // If x is NaN, 0 is returned by definition.
+    // Otherwise, x is finite or infinite and thus can be compared with f_min and f_max.
+    // This yields three cases to consider:
+    // (1) if x in [f_min, f_max], the result of fpto[su]i is returned, which agrees with
+    //     saturating conversion for inputs in that range.
+    // (2) if x > f_max, then x is larger than int_ty::MAX. This holds even if f_max is rounded
+    //     (i.e., if f_max < int_ty::MAX) because in those cases, nextUp(f_max) is already larger
+    //     than int_ty::MAX. Because x is larger than int_ty::MAX, the return value of int_ty::MAX
+    //     is correct.
+    // (3) if x < f_min, then x is smaller than int_ty::MIN. As shown earlier, f_min exactly equals
+    //     int_ty::MIN and therefore the return value of int_ty::MIN is correct.
+    // QED.
+
+    // Step 1 was already performed above.
+
+    // Step 2: We use two comparisons and two selects, with %s1 being the result:
+    //     %less_or_nan = fcmp ult %x, %f_min
+    //     %greater = fcmp olt %x, %f_max
+    //     %s0 = select %less_or_nan, int_ty::MIN, %fptosi_result
+    //     %s1 = select %greater, int_ty::MAX, %s0
+    // Note that %less_or_nan uses an *unordered* comparison. This comparison is true if the
+    // operands are not comparable (i.e., if x is NaN). The unordered comparison ensures that s1
+    // becomes int_ty::MIN if x is NaN.
+    // Performance note: Unordered comparison can be lowered to a "flipped" comparison and a
+    // negation, and the negation can be merged into the select. Therefore, it not necessarily any
+    // more expensive than a ordered ("normal") comparison. Whether these optimizations will be
+    // performed is ultimately up to the backend, but at least x86 does perform them.
+    let less_or_nan = bcx.fcmp(llvm::RealULT, x, f_min);
+    let greater = bcx.fcmp(llvm::RealOGT, x, f_max);
+    let int_max = C_big_integral(int_ty, int_max(signed, int_ty));
+    let int_min = C_big_integral(int_ty, int_min(signed, int_ty) as u128);
+    let s0 = bcx.select(less_or_nan, int_min, fptosui_result);
+    let s1 = bcx.select(greater, int_max, s0);
+
+    // Step 3: NaN replacement.
+    // For unsigned types, the above step already yielded int_ty::MIN == 0 if x is NaN.
+    // Therefore we only need to execute this step for signed integer types.
+    if signed {
+        // LLVM has no isNaN predicate, so we use (x == x) instead
+        bcx.select(bcx.fcmp(llvm::RealOEQ, x, x), s1, C_uint(int_ty, 0))
+    } else {
+        s1
+    }
+}
diff --git a/src/test/codegen/unchecked-float-casts.rs b/src/test/codegen/unchecked-float-casts.rs
new file mode 100644
index 00000000000..64ab19cceee
--- /dev/null
+++ b/src/test/codegen/unchecked-float-casts.rs
@@ -0,0 +1,65 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -C no-prepopulate-passes
+
+// This file tests that we don't generate any code for saturation if
+// -Z saturating-float-casts is not enabled.
+
+#![crate_type = "lib"]
+#![feature(i128_type)]
+
+// CHECK-LABEL: @f32_to_u32
+#[no_mangle]
+pub fn f32_to_u32(x: f32) -> u32 {
+    // CHECK: fptoui
+    // CHECK-NOT: fcmp
+    // CHECK-NOT: icmp
+    // CHECK-NOT: select
+    x as u32
+}
+
+// CHECK-LABEL: @f32_to_i32
+#[no_mangle]
+pub fn f32_to_i32(x: f32) -> i32 {
+    // CHECK: fptosi
+    // CHECK-NOT: fcmp
+    // CHECK-NOT: icmp
+    // CHECK-NOT: select
+    x as i32
+}
+
+#[no_mangle]
+pub fn f64_to_u8(x: f32) -> u16 {
+    // CHECK-NOT: fcmp
+    // CHECK-NOT: icmp
+    // CHECK-NOT: select
+    x as u16
+}
+
+// CHECK-LABEL: @i32_to_f64
+#[no_mangle]
+pub fn i32_to_f64(x: i32) -> f64 {
+    // CHECK: sitofp
+    // CHECK-NOT: fcmp
+    // CHECK-NOT: icmp
+    // CHECK-NOT: select
+    x as f64
+}
+
+// CHECK-LABEL: @u128_to_f32
+#[no_mangle]
+pub fn u128_to_f32(x: u128) -> f32 {
+    // CHECK: uitofp
+    // CHECK-NOT: fcmp
+    // CHECK-NOT: icmp
+    // CHECK-NOT: select
+    x as f32
+}
diff --git a/src/test/compile-fail/float-int-invalid-const-cast.rs b/src/test/compile-fail/float-int-invalid-const-cast.rs
new file mode 100644
index 00000000000..2efefd92691
--- /dev/null
+++ b/src/test/compile-fail/float-int-invalid-const-cast.rs
@@ -0,0 +1,61 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(i128_type)]
+#![allow(const_err)] // this test is only about hard errors
+
+use std::{f32, f64};
+
+// Forces evaluation of constants, triggering hard error
+fn force<T>(_: T) {}
+
+fn main() {
+    { const X: u16 = -1. as u16; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = -100. as u128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: i8 = f32::NAN as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: i32 = f32::NAN as i32; force(X); } //~ ERROR constant evaluation error
+    { const X: u64 = f32::NAN as u64; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = f32::NAN as u128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: i8 = f32::INFINITY as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: u32 = f32::INFINITY as u32; force(X); } //~ ERROR constant evaluation error
+    { const X: i128 = f32::INFINITY as i128; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = f32::INFINITY as u128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: u8 = f32::NEG_INFINITY as u8; force(X); } //~ ERROR constant evaluation error
+    { const X: u16 = f32::NEG_INFINITY as u16; force(X); } //~ ERROR constant evaluation error
+    { const X: i64 = f32::NEG_INFINITY as i64; force(X); } //~ ERROR constant evaluation error
+    { const X: i128 = f32::NEG_INFINITY as i128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: i8 = f64::NAN as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: i32 = f64::NAN as i32; force(X); } //~ ERROR constant evaluation error
+    { const X: u64 = f64::NAN as u64; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = f64::NAN as u128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: i8 = f64::INFINITY as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: u32 = f64::INFINITY as u32; force(X); } //~ ERROR constant evaluation error
+    { const X: i128 = f64::INFINITY as i128; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = f64::INFINITY as u128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: u8 = f64::NEG_INFINITY as u8; force(X); } //~ ERROR constant evaluation error
+    { const X: u16 = f64::NEG_INFINITY as u16; force(X); } //~ ERROR constant evaluation error
+    { const X: i64 = f64::NEG_INFINITY as i64; force(X); } //~ ERROR constant evaluation error
+    { const X: i128 = f64::NEG_INFINITY as i128; force(X); } //~ ERROR constant evaluation error
+
+    { const X: u8 = 256. as u8; force(X); } //~ ERROR constant evaluation error
+    { const X: i8 = -129. as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: i8 = 128. as i8; force(X); } //~ ERROR constant evaluation error
+    { const X: i32 = 2147483648. as i32; force(X); } //~ ERROR constant evaluation error
+    { const X: i32 = -2147483904. as i32; force(X); } //~ ERROR constant evaluation error
+    { const X: u32 = 4294967296. as u32; force(X); } //~ ERROR constant evaluation error
+    { const X: u128 = 1e40 as u128; force(X); } //~ ERROR constant evaluation error
+    { const X: i128 = 1e40 as i128; force(X); } //~ ERROR constant evaluation error
+}
\ No newline at end of file
diff --git a/src/test/run-pass/saturating-float-casts.rs b/src/test/run-pass/saturating-float-casts.rs
new file mode 100644
index 00000000000..6db4d7635f0
--- /dev/null
+++ b/src/test/run-pass/saturating-float-casts.rs
@@ -0,0 +1,164 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z saturating-float-casts
+
+#![feature(test, i128, i128_type, stmt_expr_attributes)]
+#![deny(overflowing_literals)]
+extern crate test;
+
+use std::{f32, f64};
+use std::{u8, i8, u16, i16, u32, i32, u64, i64};
+#[cfg(not(target_os="emscripten"))]
+use std::{u128, i128};
+use test::black_box;
+
+macro_rules! test {
+    ($val:expr, $src_ty:ident -> $dest_ty:ident, $expected:expr) => (
+        // black_box disables constant evaluation to test run-time conversions:
+        assert_eq!(black_box::<$src_ty>($val) as $dest_ty, $expected,
+                    "run-time {} -> {}", stringify!($src_ty), stringify!($dest_ty));
+    );
+
+    ($fval:expr, f* -> $ity:ident, $ival:expr) => (
+        test!($fval, f32 -> $ity, $ival);
+        test!($fval, f64 -> $ity, $ival);
+    )
+}
+
+// This macro tests const eval in addition to run-time evaluation.
+// If and when saturating casts are adopted, this macro should be merged with test!() to ensure
+// that run-time and const eval agree on inputs that currently trigger a const eval error.
+macro_rules! test_c {
+    ($val:expr, $src_ty:ident -> $dest_ty:ident, $expected:expr) => ({
+        test!($val, $src_ty -> $dest_ty, $expected);
+        {
+            const X: $src_ty = $val;
+            const Y: $dest_ty = X as $dest_ty;
+            assert_eq!(Y, $expected,
+                        "const eval {} -> {}", stringify!($src_ty), stringify!($dest_ty));
+        }
+    });
+
+    ($fval:expr, f* -> $ity:ident, $ival:expr) => (
+        test!($fval, f32 -> $ity, $ival);
+        test!($fval, f64 -> $ity, $ival);
+    )
+}
+
+macro_rules! common_fptoi_tests {
+    ($fty:ident -> $($ity:ident)+) => ({ $(
+        test!($fty::NAN, $fty -> $ity, 0);
+        test!($fty::INFINITY, $fty -> $ity, $ity::MAX);
+        test!($fty::NEG_INFINITY, $fty -> $ity, $ity::MIN);
+        // These two tests are not solely float->int tests, in particular the latter relies on
+        // `u128::MAX as f32` not being UB. But that's okay, since this file tests int->float
+        // as well, the test is just slightly misplaced.
+        test!($ity::MIN as $fty, $fty -> $ity, $ity::MIN);
+        test!($ity::MAX as $fty, $fty -> $ity, $ity::MAX);
+        test_c!(0., $fty -> $ity, 0);
+        test_c!($fty::MIN_POSITIVE, $fty -> $ity, 0);
+        test!(-0.9, $fty -> $ity, 0);
+        test_c!(1., $fty -> $ity, 1);
+        test_c!(42., $fty -> $ity, 42);
+    )+ });
+
+    (f* -> $($ity:ident)+) => ({
+        common_fptoi_tests!(f32 -> $($ity)+);
+        common_fptoi_tests!(f64 -> $($ity)+);
+    })
+}
+
+macro_rules! fptoui_tests {
+    ($fty: ident -> $($ity: ident)+) => ({ $(
+        test!(-0., $fty -> $ity, 0);
+        test!(-$fty::MIN_POSITIVE, $fty -> $ity, 0);
+        test!(-0.99999994, $fty -> $ity, 0);
+        test!(-1., $fty -> $ity, 0);
+        test!(-100., $fty -> $ity, 0);
+        test!(#[allow(overflowing_literals)] -1e50, $fty -> $ity, 0);
+        test!(#[allow(overflowing_literals)] -1e130, $fty -> $ity, 0);
+    )+ });
+
+    (f* -> $($ity:ident)+) => ({
+        fptoui_tests!(f32 -> $($ity)+);
+        fptoui_tests!(f64 -> $($ity)+);
+    })
+}
+
+pub fn main() {
+    common_fptoi_tests!(f* -> i8 i16 i32 i64 u8 u16 u32 u64);
+    fptoui_tests!(f* -> u8 u16 u32 u64);
+    // FIXME emscripten does not support i128
+    #[cfg(not(target_os="emscripten"))] {
+        common_fptoi_tests!(f* -> i128 u128);
+        fptoui_tests!(f* -> u128);
+    }
+
+    // The following tests cover edge cases for some integer types.
+
+    // # u8
+    test_c!(254., f* -> u8, 254);
+    test!(256., f* -> u8, 255);
+
+    // # i8
+    test_c!(-127., f* -> i8, -127);
+    test!(-129., f* -> i8, -128);
+    test_c!(126., f* -> i8, 126);
+    test!(128., f* -> i8, 127);
+
+    // # i32
+    // -2147483648. is i32::MIN (exactly)
+    test_c!(-2147483648., f* -> i32, i32::MIN);
+    // 2147483648. is i32::MAX rounded up
+    test!(2147483648., f32 -> i32, 2147483647);
+    // With 24 significand bits, floats with magnitude in [2^30 + 1, 2^31] are rounded to
+    // multiples of 2^7. Therefore, nextDown(round(i32::MAX)) is 2^31 - 128:
+    test_c!(2147483520., f32 -> i32, 2147483520);
+    // Similarly, nextUp(i32::MIN) is i32::MIN + 2^8 and nextDown(i32::MIN) is i32::MIN - 2^7
+    test!(-2147483904., f* -> i32, i32::MIN);
+    test_c!(-2147483520., f* -> i32, -2147483520);
+
+    // # u32
+    // round(MAX) and nextUp(round(MAX))
+    test_c!(4294967040., f* -> u32, 4294967040);
+    test!(4294967296., f* -> u32, 4294967295);
+
+    // # u128
+    #[cfg(not(target_os="emscripten"))]
+    {
+        // float->int:
+        test_c!(f32::MAX, f32 -> u128, 0xffffff00000000000000000000000000);
+        // nextDown(f32::MAX) = 2^128 - 2 * 2^104
+        const SECOND_LARGEST_F32: f32 = 340282326356119256160033759537265639424.;
+        test_c!(SECOND_LARGEST_F32, f32 -> u128, 0xfffffe00000000000000000000000000);
+
+        // int->float:
+        // f32::MAX - 0.5 ULP and smaller should be rounded down
+        test_c!(0xfffffe00000000000000000000000000, u128 -> f32, SECOND_LARGEST_F32);
+        test_c!(0xfffffe7fffffffffffffffffffffffff, u128 -> f32, SECOND_LARGEST_F32);
+        test_c!(0xfffffe80000000000000000000000000, u128 -> f32, SECOND_LARGEST_F32);
+        // numbers within < 0.5 ULP of f32::MAX it should be rounded to f32::MAX
+        test_c!(0xfffffe80000000000000000000000001, u128 -> f32, f32::MAX);
+        test_c!(0xfffffeffffffffffffffffffffffffff, u128 -> f32, f32::MAX);
+        test_c!(0xffffff00000000000000000000000000, u128 -> f32, f32::MAX);
+        test_c!(0xffffff00000000000000000000000001, u128 -> f32, f32::MAX);
+        test_c!(0xffffff7fffffffffffffffffffffffff, u128 -> f32, f32::MAX);
+        // f32::MAX + 0.5 ULP and greater should be rounded to infinity
+        test_c!(0xffffff80000000000000000000000000, u128 -> f32, f32::INFINITY);
+        test_c!(0xffffff80000000f00000000000000000, u128 -> f32, f32::INFINITY);
+        test_c!(0xffffff87ffffffffffffffff00000001, u128 -> f32, f32::INFINITY);
+
+        // u128->f64 should not be affected by the u128->f32 checks
+        test_c!(0xffffff80000000000000000000000000, u128 -> f64,
+              340282356779733661637539395458142568448.0);
+        test_c!(u128::MAX, u128 -> f64, 340282366920938463463374607431768211455.0);
+    }
+}