Invalid calling convention used for passing structures to varargs functions on ARM64EC

[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

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Alexandre Julliard (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 &lt;.text+0x18&gt;

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 &lt;.text$mn+0x2c&gt;
      30: 910043ff     	add	sp, sp, #<!-- -->0x10
      34: f84107fe     	ldr	x30, [sp], #<!-- -->0x10
      38: d65f03c0     	ret

[efriedma-quic]

I think https://reviews.llvm.org/D125419 addresses this; I guess I missed this one when I was porting the commits forward at some point.