Make saturating u128 -> f32 casts the default behavior

... rather than being gated by -Z saturating-float-casts.
There are several reasons for this:

1. Const eval already implements this behavior.
2. Unlike with float->int casts, this behavior is uncontroversially the
right behavior and it is not as performance critical. Thus there is no
particular need to make the bug fix for u128->f32 casts opt-in.
3. Having two orthogonal features under one flag is silly, and never
should have happened in the first place.
4. Benchmarking float->int casts with the -Z flag should not pick up
performance changes due to the u128->f32 casts (assuming there are any).

Fixes #41799

src/librustc/session/config.rs

@@ -1138,8 +1138,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     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"),
+        "make float->int casts UB-free: numbers outside the integer type's range are clipped to \
+         the max/min integer respectively, and NaN is mapped to 0"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_trans/mir/rvalue.rs

@@ -827,7 +827,7 @@ fn cast_int_to_float(bcx: &Builder,
     // 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 {
+    if is_u128_to_f32 {
         // 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);

src/test/codegen/unchecked-float-casts.rs

@@ -37,29 +37,10 @@ pub fn f32_to_i32(x: f32) -> i32 {
 }
 
 #[no_mangle]
-pub fn f64_to_u8(x: f32) -> u16 {
+pub fn f64_to_u16(x: f64) -> u16 {
+    // CHECK: fptoui
     // 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
-}

src/test/run-pass/saturating-float-casts.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Tests saturating float->int casts. See u128-as-f32.rs for the opposite direction.
 // compile-flags: -Z saturating-float-casts
 
 #![feature(test, i128, i128_type, stmt_expr_attributes)]
@@ -139,26 +140,5 @@ pub fn main() {
         // 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);
     }
 }

src/test/run-pass/u128-as-f32.rs

@@ -0,0 +1,58 @@
+// 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.
+
+// ignore-emscripten u128 not supported
+
+#![feature(test, i128, i128_type)]
+#![deny(overflowing_literals)]
+extern crate test;
+
+use std::f32;
+use std::u128;
+use test::black_box;
+
+macro_rules! test {
+    ($val:expr, $src_ty:ident -> $dest_ty:ident, $expected:expr) => ({
+        {
+            const X: $src_ty = $val;
+            const Y: $dest_ty = X as $dest_ty;
+            assert_eq!(Y, $expected,
+                        "const eval {} -> {}", stringify!($src_ty), stringify!($dest_ty));
+        }
+        // black_box disables constant evaluation to test run-time conversions:
+        assert_eq!(black_box::<$src_ty>($val) as $dest_ty, $expected,
+                    "run-time {} -> {}", stringify!($src_ty), stringify!($dest_ty));
+    });
+}
+
+pub fn main() {
+    // nextDown(f32::MAX) = 2^128 - 2 * 2^104
+    const SECOND_LARGEST_F32: f32 = 340282326356119256160033759537265639424.;
+
+    // f32::MAX - 0.5 ULP and smaller should be rounded down
+    test!(0xfffffe00000000000000000000000000, u128 -> f32, SECOND_LARGEST_F32);
+    test!(0xfffffe7fffffffffffffffffffffffff, u128 -> f32, SECOND_LARGEST_F32);
+    test!(0xfffffe80000000000000000000000000, u128 -> f32, SECOND_LARGEST_F32);
+    // numbers within < 0.5 ULP of f32::MAX it should be rounded to f32::MAX
+    test!(0xfffffe80000000000000000000000001, u128 -> f32, f32::MAX);
+    test!(0xfffffeffffffffffffffffffffffffff, u128 -> f32, f32::MAX);
+    test!(0xffffff00000000000000000000000000, u128 -> f32, f32::MAX);
+    test!(0xffffff00000000000000000000000001, u128 -> f32, f32::MAX);
+    test!(0xffffff7fffffffffffffffffffffffff, u128 -> f32, f32::MAX);
+    // f32::MAX + 0.5 ULP and greater should be rounded to infinity
+    test!(0xffffff80000000000000000000000000, u128 -> f32, f32::INFINITY);
+    test!(0xffffff80000000f00000000000000000, u128 -> f32, f32::INFINITY);
+    test!(0xffffff87ffffffffffffffff00000001, u128 -> f32, f32::INFINITY);
+
+    // u128->f64 should not be affected by the u128->f32 checks
+    test!(0xffffff80000000000000000000000000, u128 -> f64,
+          340282356779733661637539395458142568448.0);
+    test!(u128::MAX, u128 -> f64, 340282366920938463463374607431768211455.0);
+}