From ee041f8e9c0a9c04d413ea6171481724eb77aac2 Mon Sep 17 00:00:00 2001
From: DianQK <dianqk@dianqk.net>
Date: Fri, 5 Jul 2024 22:04:22 +0800
Subject: [PATCH] Emit `assume(false)` as `store i1 true, ptr poison, align 1`

Store to poison is the canonical form for non-terminator unreachable.
---
 compiler/rustc_codegen_llvm/src/intrinsic.rs | 15 ++++++++++++++-
 tests/codegen/assume-false.rs                | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/codegen/assume-false.rs

diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 68c3d47e826bf..10498b996fa35 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/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 {
diff --git a/tests/codegen/assume-false.rs b/tests/codegen/assume-false.rs
new file mode 100644
index 0000000000000..cc82ca5bdbbe3
--- /dev/null
+++ b/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() }
+}