Auto merge of #77972 - Mark-Simulacrum:side-effect-loop, r=nagisa

Prevent miscompilation in trivial loop {}

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.

Performance results indicate this is not a major regression, if at all, so it should be safe to land.

cc #28728

1 files changed, 5 insertions, 5 deletions

diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 8379fe47225..e78a4ea2e3c 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -334,8 +334,8 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
         self.call(expect, &[cond, self.const_bool(expected)], None)
     }
 
-    fn sideeffect(&mut self) {
-        if self.tcx.sess.opts.debugging_opts.insert_sideeffect {
+    fn sideeffect(&mut self, unconditional: bool) {
+        if unconditional || self.tcx.sess.opts.debugging_opts.insert_sideeffect {
             let fnname = self.get_intrinsic(&("llvm.sideeffect"));
             self.call(fnname, &[], None);
         }
@@ -390,7 +390,7 @@ fn codegen_msvc_try(
 ) {
     let llfn = get_rust_try_fn(bx, &mut |mut bx| {
         bx.set_personality_fn(bx.eh_personality());
-        bx.sideeffect();
+        bx.sideeffect(false);
 
         let mut normal = bx.build_sibling_block("normal");
         let mut catchswitch = bx.build_sibling_block("catchswitch");
@@ -553,7 +553,7 @@ fn codegen_gnu_try(
         //      call %catch_func(%data, %ptr)
         //      ret 1
 
-        bx.sideeffect();
+        bx.sideeffect(false);
 
         let mut then = bx.build_sibling_block("then");
         let mut catch = bx.build_sibling_block("catch");
@@ -615,7 +615,7 @@ fn codegen_emcc_try(
         //      call %catch_func(%data, %catch_data)
         //      ret 1
 
-        bx.sideeffect();
+        bx.sideeffect(false);
 
         let mut then = bx.build_sibling_block("then");
         let mut catch = bx.build_sibling_block("catch");