Auto merge of #82884 - nagisa:nagisa/remove-most-of-sideeffect-inserts, r=nikic

Remove the -Zinsert-sideeffect

This removes all of the code we had in place to work-around LLVM's
handling of forward progress. From this removal excluded is a workaround
where we'd insert a `sideeffect` into clearly infinite loops such as
`loop {}`. This code remains conditionally effective when the LLVM
version is earlier than 12.0, which fixed the forward progress related
miscompilations at their root.

1 files changed, 5 insertions, 9 deletions

diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 668daa52ed2..f445d708c94 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -334,8 +334,11 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
         self.call(expect, &[cond, self.const_bool(expected)], None)
     }
 
-    fn sideeffect(&mut self, unconditional: bool) {
-        if unconditional || self.tcx.sess.opts.debugging_opts.insert_sideeffect {
+    fn sideeffect(&mut self) {
+        // This kind of check would make a ton of sense in the caller, but currently the only
+        // caller of this function is in `rustc_codegen_ssa`, which is agnostic to whether LLVM
+        // codegen backend being used, and so is unable to check the LLVM version.
+        if unsafe { llvm::LLVMRustVersionMajor() } < 12 {
             let fnname = self.get_intrinsic(&("llvm.sideeffect"));
             self.call(fnname, &[], None);
         }
@@ -390,7 +393,6 @@ fn codegen_msvc_try(
 ) {
     let llfn = get_rust_try_fn(bx, &mut |mut bx| {
         bx.set_personality_fn(bx.eh_personality());
-        bx.sideeffect(false);
 
         let mut normal = bx.build_sibling_block("normal");
         let mut catchswitch = bx.build_sibling_block("catchswitch");
@@ -552,9 +554,6 @@ fn codegen_gnu_try(
         //      (%ptr, _) = landingpad
         //      call %catch_func(%data, %ptr)
         //      ret 1
-
-        bx.sideeffect(false);
-
         let mut then = bx.build_sibling_block("then");
         let mut catch = bx.build_sibling_block("catch");
 
@@ -614,9 +613,6 @@ fn codegen_emcc_try(
         //      %catch_data[1] = %is_rust_panic
         //      call %catch_func(%data, %catch_data)
         //      ret 1
-
-        bx.sideeffect(false);
-
         let mut then = bx.build_sibling_block("then");
         let mut catch = bx.build_sibling_block("catch");