Auto merge of #59148 - lcnr:unchecked_maths, r=eddyb

add support for unchecked math

add compiler support for
```rust
/// Returns the result of an unchecked addition, resulting in
/// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
pub fn unchecked_add<T>(x: T, y: T) -> T;

/// Returns the result of an unchecked substraction, resulting in
/// undefined behavior when `x - y > T::max_value()` or `x - y < T::min_value()`.
pub fn unchecked_sub<T>(x: T, y: T) -> T;

/// Returns the result of an unchecked multiplication, resulting in
/// undefined behavior when `x * y > T::max_value()` or `x * y < T::min_value()`.
pub fn unchecked_mul<T>(x: T, y: T) -> T;
```

cc https://github.com/rust-lang/rfcs/issues/2508

1 files changed, 46 insertions, 0 deletions

diff --git a/src/test/codegen/unchecked_math.rs b/src/test/codegen/unchecked_math.rs
new file mode 100644
index 00000000000..419c120ede9
--- /dev/null
+++ b/src/test/codegen/unchecked_math.rs
@@ -0,0 +1,46 @@
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::*;
+
+// CHECK-LABEL: @unchecked_add_signed
+#[no_mangle]
+pub unsafe fn unchecked_add_signed(a: i32, b: i32) -> i32 {
+    // CHECK: add nsw
+    unchecked_add(a, b)
+}
+
+// CHECK-LABEL: @unchecked_add_unsigned
+#[no_mangle]
+pub unsafe fn unchecked_add_unsigned(a: u32, b: u32) -> u32 {
+    // CHECK: add nuw
+    unchecked_add(a, b)
+}
+
+// CHECK-LABEL: @unchecked_sub_signed
+#[no_mangle]
+pub unsafe fn unchecked_sub_signed(a: i32, b: i32) -> i32 {
+    // CHECK: sub nsw
+    unchecked_sub(a, b)
+}
+
+// CHECK-LABEL: @unchecked_sub_unsigned
+#[no_mangle]
+pub unsafe fn unchecked_sub_unsigned(a: u32, b: u32) -> u32 {
+    // CHECK: sub nuw
+    unchecked_sub(a, b)
+}
+
+// CHECK-LABEL: @unchecked_mul_signed
+#[no_mangle]
+pub unsafe fn unchecked_mul_signed(a: i32, b: i32) -> i32 {
+    // CHECK: mul nsw
+    unchecked_mul(a, b)
+}
+
+// CHECK-LABEL: @unchecked_mul_unsigned
+#[no_mangle]
+pub unsafe fn unchecked_mul_unsigned(a: u32, b: u32) -> u32 {
+    // CHECK: mul nuw
+    unchecked_mul(a, b)
+}