[clang] [CIR] Classify empty records as Ignore in x86_64 callconv (PR #211078)

[via cfe-commits]

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@@ -0,0 +1,50 @@
+// RUN: cir-opt %s -cir-call-conv-lowering=target=x86_64 | FileCheck %s
+
+!s32i = !cir.int<s, 32>
+!rec_E0 = !cir.struct<"E0" {}>
+
+module attributes {
+  dlti.dl_spec = #dlti.dl_spec<
+    #dlti.dl_entry<i32, dense<32>: vector<2xi64>>,
+    #dlti.dl_entry<i64, dense<64>: vector<2xi64>>>
+} {
+
+  // A zero-field record (a C empty struct) classifies as Ignore: the argument
+  // is dropped from the lowered signature.
+  cir.func @take_empty(%arg0: !rec_E0) {
+    cir.return
+  }
+
+  // CHECK: cir.func{{.*}} @take_empty()
+  // CHECK-NEXT: cir.return
+
+  // The empty argument is dropped even when a real argument remains; the real
+  // argument shifts down to the first slot.
+  cir.func @take_mixed(%arg0: !rec_E0, %arg1: !s32i) -> !s32i {
+    cir.return %arg1 : !s32i
+  }
+
+  // CHECK: cir.func{{.*}} @take_mixed(%arg0: !s32i) -> !s32i
+  // CHECK-NEXT: cir.return %arg0 : !s32i
+
+  // An empty-record return is dropped: the function returns void.
+  cir.func @ret_empty() -> !rec_E0 {
+    %0 = cir.alloca "r" align(1) : !cir.ptr<!rec_E0>
+    %1 = cir.load %0 : !cir.ptr<!rec_E0>, !rec_E0
+    cir.return %1 : !rec_E0
+  }
+
+  // CHECK: cir.func{{.*}} @ret_empty()
+  // CHECK-NOT: -> !rec_E0
----------------
adams381 wrote:

The slot survives this pass, so `CHECK-NOT: !rec_E0` fails on the `cir.alloca`/`cir.load`. Switched to positively checking that the return is dropped and the slot survives.

https://github.com/llvm/llvm-project/pull/211078