Support use of asm goto with outputs and options(noreturn)

When labels are present, the `noreturn` option really means that asm block won't fallthrough -- if labels are present, then outputs can still be meaningfully used.
rust-lang · Oct 11, 2024 · 394acf3 · 394acf3
1 parent 4bd723f
commit 394acf3
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Showing 5 changed files with 45 additions and 5 deletions.
diff --git a/compiler/rustc_builtin_macros/src/asm.rs b/compiler/rustc_builtin_macros/src/asm.rs
@@ -300,7 +300,10 @@ pub fn parse_asm_args<'a>(
     if args.options.contains(ast::InlineAsmOptions::PURE) && !have_real_output {
         dcx.emit_err(errors::AsmPureNoOutput { spans: args.options_spans.clone() });
     }
-    if args.options.contains(ast::InlineAsmOptions::NORETURN) && !outputs_sp.is_empty() {
+    if args.options.contains(ast::InlineAsmOptions::NORETURN)
+        && !outputs_sp.is_empty()
+        && labels_sp.is_empty()
+    {
         let err = dcx.create_err(errors::AsmNoReturn { outputs_sp });
         // Bail out now since this is likely to confuse MIR
         return Err(err);

diff --git a/compiler/rustc_codegen_llvm/src/asm.rs b/compiler/rustc_codegen_llvm/src/asm.rs
@@ -334,7 +334,14 @@ impl<'ll, 'tcx> AsmBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
         attributes::apply_to_callsite(result, llvm::AttributePlace::Function, &{ attrs });
 
         // Write results to outputs. We need to do this for all possible control flow.
-        for block in Some(dest).into_iter().chain(labels.iter().copied().map(Some)) {
+        //
+        // Note that `dest` maybe populated with unreachable_block when asm goto with outputs
+        // is used (because we need to codegen callbr which always needs a destination), so
+        // here we use the NORETURN option to determine if `dest` should be used.
+        for block in (if options.contains(InlineAsmOptions::NORETURN) { None } else { Some(dest) })
+            .into_iter()
+            .chain(labels.iter().copied().map(Some))
+        {
             if let Some(block) = block {
                 self.switch_to_block(block);
             }

diff --git a/tests/codegen/asm-goto.rs b/tests/codegen/asm-goto.rs
@@ -43,11 +43,21 @@ pub unsafe fn asm_goto_with_outputs_use_in_label() -> u64 {
 // CHECK-LABEL: @asm_goto_noreturn
 #[no_mangle]
 pub unsafe fn asm_goto_noreturn() -> u64 {
-    let out: u64;
     // CHECK: callbr void asm sideeffect alignstack inteldialect "
     // CHECK-NEXT: to label %unreachable [label %[[JUMPBB:[a-b0-9]+]]]
     asm!("jmp {}", label { return 1; }, options(noreturn));
     // CHECK: [[JUMPBB]]:
     // CHECK-NEXT: ret i64 1
+}
+
+// CHECK-LABEL: @asm_goto_noreturn_with_outputs
+#[no_mangle]
+pub unsafe fn asm_goto_noreturn_with_outputs() -> u64 {
+    let out: u64;
+    // CHECK: [[RES:%[0-9]+]] = callbr i64 asm sideeffect alignstack inteldialect "
+    // CHECK-NEXT: to label %[[FALLTHROUGHBB:[a-b0-9]+]] [label %[[JUMPBB:[a-b0-9]+]]]
+    asm!("mov {}, 1", "jmp {}", out(reg) out, label { return out; });
+    // CHECK: [[JUMPBB]]:
+    // CHECK-NEXT: ret i64 [[RES]]
     out
 }
diff --git a/tests/ui/asm/x86_64/goto.rs b/tests/ui/asm/x86_64/goto.rs
@@ -65,6 +65,25 @@ fn goto_out_jump() {
     }
 }
 
+fn goto_out_jump_noreturn() {
+    unsafe {
+        let mut value = false;
+        let mut out: usize;
+        asm!(
+            "lea {}, [{} + 1]",
+            "jmp {}",
+            out(reg) out,
+            in(reg) 0x12345678usize,
+            label {
+                value = true;
+                assert_eq!(out, 0x12345679);
+            },
+            options(noreturn)
+        );
+        assert!(value);
+    }
+}
+
 // asm goto with outputs cause miscompilation in LLVM when multiple outputs are present.
 // The code sample below is adapted from https://github.com/llvm/llvm-project/issues/74483
 // and does not work with `-C opt-level=0`
@@ -131,6 +150,7 @@ fn main() {
     goto_jump();
     goto_out_fallthrough();
     goto_out_jump();
+    goto_out_jump_noreturn();
     // goto_multi_out();
     goto_noreturn();
     goto_noreturn_diverge();

diff --git a/tests/ui/asm/x86_64/goto.stderr b/tests/ui/asm/x86_64/goto.stderr
@@ -1,5 +1,5 @@
 warning: unreachable statement
-  --> $DIR/goto.rs:124:9
+  --> $DIR/goto.rs:143:9
    |
 LL | /         asm!(
 LL | |             "jmp {}",
@@ -13,7 +13,7 @@ LL |           unreachable!();
    |           ^^^^^^^^^^^^^^ unreachable statement
    |
 note: the lint level is defined here
-  --> $DIR/goto.rs:114:8
+  --> $DIR/goto.rs:133:8
    |
 LL | #[warn(unreachable_code)]
    |        ^^^^^^^^^^^^^^^^