| Issue |
89615
|
| Summary |
Invalid calling convention used for passing structures to varargs functions on ARM64EC
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
julliard
|
On ARM64EC, passing structures to varargs functions uses the arm64 calling convention, but it should use the x64 convention, as documented at https://learn.microsoft.com/en-us/windows/arm/arm64ec-abi#variadic-calling-convention
Below is a short test case showing that a 5-byte structure is passed by value, but according to the ARM64EC ABI it should be passed by reference:
```
struct s { char a, b, c, d, e; };
extern void foo( int a, ... );
void bar(void)
{
struct s s = { 1, 2, 3, 4, 5 };
foo( 0, s );
}
```
LLVM generates this (structure contents in x1):
```
0: d2804021 mov x1, #0x201
4: 910003e4 mov x4, sp
8: 2a1f03e0 mov w0, wzr
c: f2a08061 movk x1, #0x403, lsl #16
10: aa1f03e5 mov x5, xzr
14: f2c000a1 movk x1, #0x5, lsl #32
18: 14000000 b 0x18 <.text+0x18>
```
while MSVC does this (structure contents on stack, pointer in x1):
```
0: f81f0ffe str x30, [sp, #-0x10]!
4: d10043ff sub sp, sp, #0x10
8: 910003e1 mov x1, sp
c: d2800000 mov x0, #0x0
10: 910003e4 mov x4, sp
14: d2800005 mov x5, #0x0
18: 52804028 mov w8, #0x201
1c: 72a08068 movk w8, #0x403, lsl #16
20: b90003e8 str w8, [sp]
24: 528000a8 mov w8, #0x5
28: 390013e8 strb w8, [sp, #0x4]
2c: 94000000 bl 0x2c <.text$mn+0x2c>
30: 910043ff add sp, sp, #0x10
34: f84107fe ldr x30, [sp], #0x10
38: d65f03c0 ret
```
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