Auto merge of #961 - rust-lang:exact_div_reuse, r=RalfJung

Use the upstream `exact_div` implementation

introduced in rust-lang/rust#63810

Author: bors

rust-version

@@ -1 +1 @@
-aa69777ea2902208b24b3fd77767d577ceaf6386
+6c1b220fd747bf244f04b380e4d4ae005068f706

src/shims/intrinsics.rs

@@ -313,23 +313,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 this.write_scalar(Scalar::from_f64(res), dest)?;
             }
 
-            "exact_div" => {
-                // Performs an exact division, resulting in undefined behavior where
-                // `x % y != 0` or `y == 0` or `x == T::min_value() && y == -1`
-                let a = this.read_immediate(args[0])?;
-                let b = this.read_immediate(args[1])?;
-                // check x % y != 0
-                if this.overflowing_binary_op(mir::BinOp::Rem, a, b)?.0.to_bits(dest.layout.size)? != 0 {
-                    // Check if `b` is -1, which is the "min_value / -1" case.
-                    let minus1 = Scalar::from_int(-1, dest.layout.size);
-                    return Err(if b.to_scalar().unwrap() == minus1 {
-                        err_ub_format!("exact_div: result of dividing MIN by -1 cannot be represented")
-                    } else {
-                        err_ub_format!("exact_div: {:?} cannot be divided by {:?} without remainder", *a, *b)
-                    }.into());
-                }
-                this.binop_ignore_overflow(mir::BinOp::Div, a, b, dest)?;
-            },
+            "exact_div" =>
+                this.exact_div(
+                    this.read_immediate(args[0])?,
+                    this.read_immediate(args[1])?,
+                    dest,
+                )?,
 
             "forget" => {}
 

tests/compile-fail/exact_div2.rs

@@ -1,5 +1,5 @@
 #![feature(core_intrinsics)]
 fn main() {
     // divison with a remainder
-    unsafe { std::intrinsics::exact_div(2u16, 3); } //~ ERROR Scalar(0x0002) cannot be divided by Scalar(0x0003) without remainder
+    unsafe { std::intrinsics::exact_div(2u16, 3); } //~ ERROR 2 cannot be divided by 3 without remainder
 }

tests/compile-fail/exact_div3.rs

@@ -1,5 +1,5 @@
 #![feature(core_intrinsics)]
 fn main() {
     // signed divison with a remainder
-    unsafe { std::intrinsics::exact_div(-19i8, 2); } //~ ERROR Scalar(0xed) cannot be divided by Scalar(0x02) without remainder
+    unsafe { std::intrinsics::exact_div(-19i8, 2); } //~ ERROR 237 cannot be divided by 2 without remainder
 }