Emit assume(false) as store i1 true, ptr poison, align 1

Store to poison is the canonical form for non-terminator unreachable.

Author: dianqk

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -566,7 +566,20 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
     }
 
     fn assume(&mut self, val: Self::Value) {
-        self.call_intrinsic("llvm.assume", &[val]);
+        match self.const_to_opt_uint(val) {
+            Some(0) => {
+                // See https://github.com/llvm/llvm-project/blob/1347b9a3aa671d610e812579ab5e5f05870586cf/llvm/docs/Frontend/PerformanceTips.rst?plain=1#L204-L211.
+                self.store(
+                    self.const_bool(true),
+                    self.const_poison(self.type_ptr()),
+                    self.tcx().data_layout.i1_align.abi,
+                );
+            }
+            Some(_) => {}
+            None => {
+                self.call_intrinsic("llvm.assume", &[val]);
+            }
+        }
     }
 
     fn expect(&mut self, cond: Self::Value, expected: bool) -> Self::Value {

tests/codegen/assume-false.rs

@@ -0,0 +1,14 @@
+//@ compile-flags: -Cno-prepopulate-passes -Zmir-opt-level=0 -O -Zinline-mir
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn unwrap_unchecked(x: Option<i32>) -> i32 {
+    // CHECK-LABEL: define{{.*}} i32 @unwrap_unchecked
+    // CHECK-NOT: call void @llvm.assume(i1 false)
+    // CHECK: store i1 true, ptr poison, align 1
+    // CHECK-NOT: call void @llvm.assume(i1 false)
+    // CHECK: ret
+    // CHECK }
+    unsafe { x.unwrap_unchecked() }
+}