From ab91a63d403b0105cacd72809cd292a72984ed99 Mon Sep 17 00:00:00 2001
From: Ben Kimock <kimockb@gmail.com>
Date: Tue, 26 Aug 2025 21:08:03 -0400
Subject: [PATCH] Ignore intrinsic calls in cross-crate-inlining cost model

---
 .../rustc_mir_transform/src/cross_crate_inline.rs     | 11 ++++++++++-
 tests/assembly-llvm/breakpoint.rs                     |  1 +
 tests/assembly-llvm/simd/reduce-fadd-unordered.rs     |  1 +
 .../cross-crate-inlining/auxiliary/leaf.rs            |  5 +++++
 .../cross-crate-inlining/leaf-inlining.rs             |  7 +++++++
 tests/codegen-llvm/default-visibility.rs              |  1 +
 6 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_mir_transform/src/cross_crate_inline.rs b/compiler/rustc_mir_transform/src/cross_crate_inline.rs
index b186c2bd775..df98c07f549 100644
--- a/compiler/rustc_mir_transform/src/cross_crate_inline.rs
+++ b/compiler/rustc_mir_transform/src/cross_crate_inline.rs
@@ -135,7 +135,16 @@ impl<'tcx> Visitor<'tcx> for CostChecker<'_, 'tcx> {
                     }
                 }
             }
-            TerminatorKind::Call { unwind, .. } => {
+            TerminatorKind::Call { ref func, unwind, .. } => {
+                // We track calls because they make our function not a leaf (and in theory, the
+                // number of calls indicates how likely this function is to perturb other CGUs).
+                // But intrinsics don't have a body that gets assigned to a CGU, so they are
+                // ignored.
+                if let Some((fn_def_id, _)) = func.const_fn_def()
+                    && self.tcx.has_attr(fn_def_id, sym::rustc_intrinsic)
+                {
+                    return;
+                }
                 self.calls += 1;
                 if let UnwindAction::Cleanup(_) = unwind {
                     self.landing_pads += 1;
diff --git a/tests/assembly-llvm/breakpoint.rs b/tests/assembly-llvm/breakpoint.rs
index e0cc2d1eebb..d119b68e899 100644
--- a/tests/assembly-llvm/breakpoint.rs
+++ b/tests/assembly-llvm/breakpoint.rs
@@ -9,6 +9,7 @@
 // CHECK-LABEL: use_bp
 // aarch64: brk #0xf000
 // x86_64: int3
+#[inline(never)]
 pub fn use_bp() {
     core::arch::breakpoint();
 }
diff --git a/tests/assembly-llvm/simd/reduce-fadd-unordered.rs b/tests/assembly-llvm/simd/reduce-fadd-unordered.rs
index e872826f6ef..fdd03639da0 100644
--- a/tests/assembly-llvm/simd/reduce-fadd-unordered.rs
+++ b/tests/assembly-llvm/simd/reduce-fadd-unordered.rs
@@ -16,6 +16,7 @@ use std::simd::*;
 // It would emit about an extra fadd, depending on the architecture.
 
 // CHECK-LABEL: reduce_fadd_negative_zero
+#[inline(never)]
 pub unsafe fn reduce_fadd_negative_zero(v: f32x4) -> f32 {
     // x86_64: addps
     // x86_64-NEXT: movshdup
diff --git a/tests/codegen-llvm/cross-crate-inlining/auxiliary/leaf.rs b/tests/codegen-llvm/cross-crate-inlining/auxiliary/leaf.rs
index d059a3d0a73..7b5679c3f4d 100644
--- a/tests/codegen-llvm/cross-crate-inlining/auxiliary/leaf.rs
+++ b/tests/codegen-llvm/cross-crate-inlining/auxiliary/leaf.rs
@@ -18,3 +18,8 @@ pub fn stem_fn() -> String {
 fn inner() -> String {
     String::from("test")
 }
+
+// This function's optimized MIR contains a call, but it is to an intrinsic.
+pub fn leaf_with_intrinsic(a: &[u64; 2], b: &[u64; 2]) -> bool {
+    a == b
+}
diff --git a/tests/codegen-llvm/cross-crate-inlining/leaf-inlining.rs b/tests/codegen-llvm/cross-crate-inlining/leaf-inlining.rs
index 37132312ca9..5e7912791ad 100644
--- a/tests/codegen-llvm/cross-crate-inlining/leaf-inlining.rs
+++ b/tests/codegen-llvm/cross-crate-inlining/leaf-inlining.rs
@@ -18,3 +18,10 @@ pub fn stem_outer() -> String {
     // CHECK: call {{.*}}stem_fn
     leaf::stem_fn()
 }
+
+// Check that we inline functions that call intrinsics
+#[no_mangle]
+pub fn leaf_with_intrinsic_outer(a: &[u64; 2], b: &[u64; 2]) -> bool {
+    // CHECK-NOT: call {{.*}}leaf_with_intrinsic
+    leaf::leaf_with_intrinsic(a, b)
+}
diff --git a/tests/codegen-llvm/default-visibility.rs b/tests/codegen-llvm/default-visibility.rs
index 88ff9fee254..28238e5ef12 100644
--- a/tests/codegen-llvm/default-visibility.rs
+++ b/tests/codegen-llvm/default-visibility.rs
@@ -32,6 +32,7 @@ pub static tested_symbol: [u8; 6] = *b"foobar";
 // INTERPOSABLE: @{{.*}}default_visibility{{.*}}tested_symbol{{.*}} = constant
 // DEFAULT:      @{{.*}}default_visibility{{.*}}tested_symbol{{.*}} = constant
 
+#[inline(never)]
 pub fn do_memcmp(left: &[u8], right: &[u8]) -> i32 {
     left.cmp(right) as i32
 }