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

11 files changed, 198 insertions, 2 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 782a7ba4559..31a4e380a3d 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -1240,6 +1240,21 @@ extern "rust-intrinsic" {
     /// y < 0 or y >= N, where N is the width of T in bits.
     pub fn unchecked_shr<T>(x: T, y: T) -> T;
 
+    /// Returns the result of an unchecked addition, resulting in
+    /// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
+    #[cfg(not(stage0))]
+    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()`.
+    #[cfg(not(stage0))]
+    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()`.
+    #[cfg(not(stage0))]
+    pub fn unchecked_mul<T>(x: T, y: T) -> T;
+
     /// Performs rotate left.
     /// The stabilized versions of this intrinsic are available on the integer
     /// primitives via the `rotate_left` method. For example,
diff --git a/src/librustc_codegen_llvm/builder.rs b/src/librustc_codegen_llvm/builder.rs
index f37fd0cb833..102e9e38612 100644
--- a/src/librustc_codegen_llvm/builder.rs
+++ b/src/librustc_codegen_llvm/builder.rs
@@ -265,6 +265,12 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         neg(x) => LLVMBuildNeg,
         fneg(x) => LLVMBuildFNeg,
         not(x) => LLVMBuildNot,
+        unchecked_sadd(x, y) => LLVMBuildNSWAdd,
+        unchecked_uadd(x, y) => LLVMBuildNUWAdd,
+        unchecked_ssub(x, y) => LLVMBuildNSWSub,
+        unchecked_usub(x, y) => LLVMBuildNUWSub,
+        unchecked_smul(x, y) => LLVMBuildNSWMul,
+        unchecked_umul(x, y) => LLVMBuildNUWMul,
     }
 
     fn fadd_fast(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
diff --git a/src/librustc_codegen_llvm/intrinsic.rs b/src/librustc_codegen_llvm/intrinsic.rs
index 98a829b2aff..42aa9989346 100644
--- a/src/librustc_codegen_llvm/intrinsic.rs
+++ b/src/librustc_codegen_llvm/intrinsic.rs
@@ -334,7 +334,8 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
             "ctlz" | "ctlz_nonzero" | "cttz" | "cttz_nonzero" | "ctpop" | "bswap" |
             "bitreverse" | "add_with_overflow" | "sub_with_overflow" |
             "mul_with_overflow" | "overflowing_add" | "overflowing_sub" | "overflowing_mul" |
-            "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" | "exact_div" |
+            "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" |
+            "unchecked_add" | "unchecked_sub" | "unchecked_mul" | "exact_div" |
             "rotate_left" | "rotate_right" | "saturating_add" | "saturating_sub" => {
                 let ty = arg_tys[0];
                 match int_type_width_signed(ty, self) {
@@ -430,6 +431,27 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
                                 } else {
                                     self.lshr(args[0].immediate(), args[1].immediate())
                                 },
+                            "unchecked_add" => {
+                                if signed {
+                                    self.unchecked_sadd(args[0].immediate(), args[1].immediate())
+                                } else {
+                                    self.unchecked_uadd(args[0].immediate(), args[1].immediate())
+                                }
+                            },
+                            "unchecked_sub" => {
+                                if signed {
+                                    self.unchecked_ssub(args[0].immediate(), args[1].immediate())
+                                } else {
+                                    self.unchecked_usub(args[0].immediate(), args[1].immediate())
+                                }
+                            },
+                            "unchecked_mul" => {
+                                if signed {
+                                    self.unchecked_smul(args[0].immediate(), args[1].immediate())
+                                } else {
+                                    self.unchecked_umul(args[0].immediate(), args[1].immediate())
+                                }
+                            },
                             "rotate_left" | "rotate_right" => {
                                 let is_left = name == "rotate_left";
                                 let val = args[0].immediate();
diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index f88923fc9f1..a71243c7c82 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -1002,6 +1002,36 @@ extern "C" {
                          RHS: &'a Value,
                          Name: *const c_char)
                          -> &'a Value;
+    pub fn LLVMBuildNSWAdd(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
+    pub fn LLVMBuildNUWAdd(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
+    pub fn LLVMBuildNSWSub(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
+    pub fn LLVMBuildNUWSub(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
+    pub fn LLVMBuildNSWMul(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
+    pub fn LLVMBuildNUWMul(B: &Builder<'a>,
+                           LHS: &'a Value,
+                           RHS: &'a Value,
+                           Name: *const c_char)
+                           -> &'a Value;
     pub fn LLVMBuildAnd(B: &Builder<'a>,
                         LHS: &'a Value,
                         RHS: &'a Value,
diff --git a/src/librustc_codegen_ssa/traits/builder.rs b/src/librustc_codegen_ssa/traits/builder.rs
index 0c4c4547a79..a11d1ba9231 100644
--- a/src/librustc_codegen_ssa/traits/builder.rs
+++ b/src/librustc_codegen_ssa/traits/builder.rs
@@ -88,6 +88,12 @@ pub trait BuilderMethods<'a, 'tcx: 'a>:
     fn shl(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn lshr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn ashr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_sadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_uadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_ssub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_usub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_smul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn unchecked_umul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn and(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn or(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn xor(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
diff --git a/src/librustc_typeck/check/intrinsic.rs b/src/librustc_typeck/check/intrinsic.rs
index c6191e6b579..0b14ff1db59 100644
--- a/src/librustc_typeck/check/intrinsic.rs
+++ b/src/librustc_typeck/check/intrinsic.rs
@@ -305,7 +305,8 @@ pub fn check_intrinsic_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             "unchecked_shl" | "unchecked_shr" |
             "rotate_left" | "rotate_right" =>
                 (1, vec![param(0), param(0)], param(0)),
-
+            "unchecked_add" | "unchecked_sub" | "unchecked_mul" =>
+                (1, vec![param(0), param(0)], param(0)),
             "overflowing_add" | "overflowing_sub" | "overflowing_mul" =>
                 (1, vec![param(0), param(0)], param(0)),
             "saturating_add" | "saturating_sub" =>
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)
+}
diff --git a/src/test/ui/intrinsics/unchecked_math_unsafe.rs b/src/test/ui/intrinsics/unchecked_math_unsafe.rs
new file mode 100644
index 00000000000..a034b45f530
--- /dev/null
+++ b/src/test/ui/intrinsics/unchecked_math_unsafe.rs
@@ -0,0 +1,8 @@
+#![feature(core_intrinsics)]
+
+fn main() {
+    let (x, y) = (1u32, 2u32);
+    let add = std::intrinsics::unchecked_add(x, y); //~ ERROR call to unsafe function
+    let sub = std::intrinsics::unchecked_sub(x, y); //~ ERROR call to unsafe function
+    let mul = std::intrinsics::unchecked_mul(x, y); //~ ERROR call to unsafe function
+}
diff --git a/src/test/ui/intrinsics/unchecked_math_unsafe.stderr b/src/test/ui/intrinsics/unchecked_math_unsafe.stderr
new file mode 100644
index 00000000000..4066cf8efb8
--- /dev/null
+++ b/src/test/ui/intrinsics/unchecked_math_unsafe.stderr
@@ -0,0 +1,27 @@
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
+  --> $DIR/unchecked_math_unsafe.rs:5:15
+   |
+LL |     let add = std::intrinsics::unchecked_add(x, y);
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
+  --> $DIR/unchecked_math_unsafe.rs:6:15
+   |
+LL |     let sub = std::intrinsics::unchecked_sub(x, y);
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
+  --> $DIR/unchecked_math_unsafe.rs:7:15
+   |
+LL |     let mul = std::intrinsics::unchecked_mul(x, y);
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0133`.
diff --git a/src/test/ui/intrinsics/unchecked_math_unstable.rs b/src/test/ui/intrinsics/unchecked_math_unstable.rs
new file mode 100644
index 00000000000..8869063d1cc
--- /dev/null
+++ b/src/test/ui/intrinsics/unchecked_math_unstable.rs
@@ -0,0 +1,8 @@
+fn main() {
+    let (x, y) = (1u32, 2u32);
+    unsafe {
+        let add = std::intrinsics::unchecked_add(x, y); //~ ERROR use of unstable library feature
+        let sub = std::intrinsics::unchecked_sub(x, y); //~ ERROR use of unstable library feature
+        let mul = std::intrinsics::unchecked_mul(x, y); //~ ERROR use of unstable library feature
+    }
+}
diff --git a/src/test/ui/intrinsics/unchecked_math_unstable.stderr b/src/test/ui/intrinsics/unchecked_math_unstable.stderr
new file mode 100644
index 00000000000..6f5429127c6
--- /dev/null
+++ b/src/test/ui/intrinsics/unchecked_math_unstable.stderr
@@ -0,0 +1,27 @@
+error[E0658]: use of unstable library feature 'core_intrinsics': intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library
+  --> $DIR/unchecked_math_unstable.rs:4:19
+   |
+LL |         let add = std::intrinsics::unchecked_add(x, y);
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(core_intrinsics)] to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'core_intrinsics': intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library
+  --> $DIR/unchecked_math_unstable.rs:5:19
+   |
+LL |         let sub = std::intrinsics::unchecked_sub(x, y);
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(core_intrinsics)] to the crate attributes to enable
+
+error[E0658]: use of unstable library feature 'core_intrinsics': intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library
+  --> $DIR/unchecked_math_unstable.rs:6:19
+   |
+LL |         let mul = std::intrinsics::unchecked_mul(x, y);
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(core_intrinsics)] to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.