Add assembly tests verifying the functionality of -Zreg-struct-return for structs of different sizes.

This test covers:
* The callee side, making sure that the structs are correctly loaded into registers when `-Zreg-struct-return` is enabled
* The caller side, making sure that callers do receive returned structs in registers when `-Zreg-struct-return` is enabled

Structs of the size of up to 2 registers (8 bytes) can be returned in registers in x86_32.
Therefore, the tests are done with 3 different struct sizes:
* 2 bytes (register returns should happen)
* 8 bytes (last value where register returns should happen)
* 12 bytes (register returns should not happen even when `-Zreg-struct-return` is enabled)

Author: winstonallo

tests/assembly-llvm/reg-struct-return.rs

@@ -0,0 +1,143 @@
+//! Tests that -Zreg-struct-return changes ABI for small struct returns
+//! from hidden-pointer convention to register-return on x86_32.
+//! This test covers:
+//! * Callee side, verifying that the structs are correctly loaded into registers when
+//!   `-Zreg-struct-return` is activated
+//! * Caller side, verifying callers do receive returned structs in registers when
+//!   `-Zreg-struct-return` is activated
+//@ add-core-stubs
+//@ assembly-output: emit-asm
+//@ compile-flags: -O --target=i686-unknown-linux-gnu -Crelocation-model=static
+//@ revisions: WITH WITHOUT
+//@[WITH] compile-flags: -Zreg-struct-return
+//@ needs-llvm-components: x86
+
+#![feature(no_core)]
+#![no_std]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;
+
+// Verifies ABI changes for small structs, where both fields fit into one register.
+// WITH is expected to use register return, WITHOUT should use hidden pointer.
+mod Small {
+    struct SmallStruct {
+        a: i8,
+        b: i8,
+    }
+
+    unsafe extern "C" {
+        fn small() -> SmallStruct;
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn small_callee() -> SmallStruct {
+        // (42 << 8) | 42 = 10794
+
+        // WITH-LABEL: small_callee
+        // WITH: movw $10794, %ax
+        // WITH: retl
+
+        // WITHOUT-LABEL: small_callee
+        // WITHOUT: movl 4(%esp), %e{{.*}}
+        // WITHOUT: movw $10794, (%e{{.*}})
+        // WITHOUT: retl $4
+        SmallStruct { a: 42, b: 42 }
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn small_caller(dst: &mut SmallStruct) {
+        // WITH-LABEL: small_caller
+        // WITH: calll small
+        // WITH: movw %ax, (%e{{.*}})
+
+        // WITHOUT-LABEL: small_caller
+        // WITHOUT: calll small
+        // WITHOUT: movzwl {{.*}}(%esp), %e[[TMP:..]]
+        // WITHOUT: movw %[[TMP]], (%e{{..}})
+        *dst = small();
+    }
+}
+
+// Verifies ABI changes for a struct of size 8, which is the maximum size
+// for reg-struct-return.
+// WITH is expected to still use register return, WITHOUT should use hidden
+// pointer.
+mod Pivot {
+    struct PivotStruct {
+        a: i32,
+        b: i32,
+    }
+
+    unsafe extern "C" {
+        fn pivot() -> PivotStruct;
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn pivot_callee() -> PivotStruct {
+        // WITH-LABEL: pivot_callee
+        // WITH: movl $42, %e{{.*}}
+        // WITH: movl $42, %e{{.*}}
+        // WITH: retl
+
+        // WITHOUT-LABEL: pivot_callee
+        // WITHOUT: movl 4(%esp), %e{{.*}}
+        // WITHOUT-DAG: movl $42, (%e{{.*}})
+        // WITHOUT-DAG: movl $42, 4(%e{{.*}})
+        // WITHOUT: retl $4
+        PivotStruct { a: 42, b: 42 }
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn pivot_caller(dst: &mut PivotStruct) {
+        // WITH-LABEL: pivot_caller
+        // WITH: calll pivot
+        // WITH-DAG: movl %e{{.*}}, 4(%e{{.*}})
+        // WITH-DAG: movl %e{{.*}}, (%e{{.*}})
+
+        // WITHOUT-LABEL: pivot_caller
+        // WITHOUT: calll pivot
+        // WITHOUT: movsd {{.*}}(%esp), %[[TMP:xmm.]]
+        // WITHOUT: movsd %[[TMP]], (%e{{..}})
+        *dst = pivot();
+    }
+}
+
+// Verifies ABI changes for a struct of size 12, which is larger than the
+// maximum size for reg-struct-return (8 bytes).
+// Here, the hidden pointer convention should be used even when `-Zreg-struct-return` is set.
+mod Large {
+    struct LargeStruct {
+        a: i32,
+        b: i32,
+        c: i32,
+    }
+
+    unsafe extern "C" {
+        fn large() -> LargeStruct;
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn large_callee() -> LargeStruct {
+        // CHECK-LABEL: large_callee
+        // CHECK: movl 4(%esp), %e{{.*}}
+        // CHECK-DAG: movl $42, (%e{{.*}})
+        // CHECK-DAG: movl $42, 4(%e{{.*}})
+        // CHECK-DAG: movl $42, 8(%e{{.*}})
+        // CHECK: retl $4
+        LargeStruct { a: 42, b: 42, c: 42 }
+    }
+
+    #[unsafe(no_mangle)]
+    pub unsafe extern "C" fn large_caller(dst: &mut LargeStruct) {
+        // CHECK-LABEL: large_caller
+        // CHECK: calll large
+        // CHECK-DAG: movl   {{.*}}(%esp), %[[TMP1:e..]]
+        // CHECK-DAG: movl  %[[TMP1]], {{.*}}(%e{{..}})
+        // CHECK-DAG: movsd  {{.*}}(%esp), %[[TMP2:xmm.]]
+        // CHECK-DAG: movsd  %[[TMP2]], {{.*}}(%e{{..}})
+        *dst = large();
+    }
+}