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

1 files changed, 65 insertions, 0 deletions

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
+}