Add new intrinsic is_constant and optimize pow

Fix overflow check

Make MIRI choose the path randomly and rename the intrinsic

Add back test

Add miri test and make it operate on `ptr`

Define `llvm.is.constant` for primitives

Update MIRI comment and fix test in stage2

Add const eval test

Clarify that both branches must have the same side effects

guaranteed non guarantee

use immediate type instead

Co-Authored-By: Ralf Jung <post@ralfj.de>

Author: 2 people

src/shims/intrinsics/mod.rs

@@ -5,6 +5,7 @@ use std::iter;
 
 use log::trace;
 
+use rand::Rng;
 use rustc_apfloat::{Float, Round};
 use rustc_middle::ty::layout::LayoutOf;
 use rustc_middle::{
@@ -141,6 +142,17 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 this.write_pointer(Pointer::new(ptr.provenance, masked_addr), dest)?;
             }
 
+            // We want to return either `true` or `false` at random, or else something like
+            // ```
+            // if !is_val_statically_known(0) { unreachable_unchecked(); }
+            // ```
+            // Would not be considered UB, or the other way around (`is_val_statically_known(0)`).
+            "is_val_statically_known" => {
+                let [_] = check_arg_count(args)?;
+                let branch: bool = this.machine.rng.get_mut().gen();
+                this.write_scalar(Scalar::from_bool(branch), dest)?;
+            }
+
             // Floating-point operations
             "fabsf32" => {
                 let [f] = check_arg_count(args)?;

tests/pass/intrinsics.rs

@@ -33,6 +33,21 @@ fn main() {
     assert_eq!(intrinsics::likely(false), false);
     assert_eq!(intrinsics::unlikely(true), true);
 
+    let mut saw_true = false;
+    let mut saw_false = false;
+
+    for _ in 0..50 {
+        if unsafe { intrinsics::is_val_statically_known(0) } {
+            saw_true = true;
+        } else {
+            saw_false = true;
+        }
+    }
+    assert!(
+        saw_true && saw_false,
+        "`is_val_statically_known` failed to return both true and false. Congrats, you won the lottery!"
+    );
+
     intrinsics::forget(Bomb);
 
     let _v = intrinsics::discriminant_value(&Some(()));