1// SPDX-License-Identifier: Apache-2.0
2// SPDX-FileCopyrightText: 2026 The Ebitengine Authors
3
4//go:build linux
5
6#include "textflag.h"
7#include "go_asm.h"
8#include "funcdata.h"
9
10// S390X ELF ABI callbackasm1 implementation
11// On entry, R0 contains the callback index (set by callbackasm)
12// NOTE: We use R0 instead of R11 because R11 is callee-saved on S390X.
13//
14// S390X stack frame layout:
15// 0(R15) - back chain
16// 48(R15) - register save area (R6-R15)
17// 160(R15) - parameter area
18//
19// S390X uses R2-R6 for integer arguments (5 registers) and F0,F2,F4,F6 for floats (4 registers).
20//
21// Our frame layout (total 264 bytes, 8-byte aligned):
22// 0(R15) - back chain
23// 48(R15) - saved R6-R15 (done by STMG)
24// 160(R15) - callbackArgs struct (32 bytes: index, args, result, stackArgs)
25// 192(R15) - args array start
26//
27// Args array layout:
28// - floats F0,F2,F4,F6 (32 bytes)
29// - ints R2-R6 (40 bytes)
30// Total args array: 72 bytes, ends at 264
31//
32// Stack args in caller's frame start at old_R15+160
33
34#define FRAME_SIZE 264
35#define CB_ARGS 160
36#define ARGS_ARRAY 192
37#define FLOAT_OFF 0
38#define INT_OFF 32
39
40TEXT callbackasm1(SB), NOSPLIT|NOFRAME, $0
41 NO_LOCAL_POINTERS
42
43 // On entry, the trampoline in zcallback_s390x.s left
44 // the callback index in R0 (NOT R11, since R11 is callee-saved).
45 // R6 contains the 5th integer argument.
46
47 // Save R6-R15 in caller's frame (per S390X ABI) BEFORE allocating our frame
48 // STMG stores R6's current value (the 5th arg) at 48(R15)
49 STMG R6, R15, 48(R15)
50
51 // Save current stack pointer (will be back chain)
52 MOVD R15, R1
53
54 // Allocate our stack frame
55 SUB $FRAME_SIZE, R15
56 MOVD R1, 0(R15) // back chain
57
58 // Save R0 (callback index) immediately - it's volatile
59 MOVD R0, (CB_ARGS+0)(R15)
60
61 // Save callback arguments to args array.
62 // Layout: floats first (F0,F2,F4,F6), then ints (R2-R6)
63 FMOVD F0, (ARGS_ARRAY+FLOAT_OFF+0*8)(R15)
64 FMOVD F2, (ARGS_ARRAY+FLOAT_OFF+1*8)(R15)
65 FMOVD F4, (ARGS_ARRAY+FLOAT_OFF+2*8)(R15)
66 FMOVD F6, (ARGS_ARRAY+FLOAT_OFF+3*8)(R15)
67
68 MOVD R2, (ARGS_ARRAY+INT_OFF+0*8)(R15)
69 MOVD R3, (ARGS_ARRAY+INT_OFF+1*8)(R15)
70 MOVD R4, (ARGS_ARRAY+INT_OFF+2*8)(R15)
71 MOVD R5, (ARGS_ARRAY+INT_OFF+3*8)(R15)
72
73 // R6 (5th int arg) was saved at 48(old_R15) by STMG
74 // old_R15 = current R15 + FRAME_SIZE, so R6 is at 48+FRAME_SIZE(R15) = 312(R15)
75 MOVD (48+FRAME_SIZE)(R15), R1
76 MOVD R1, (ARGS_ARRAY+INT_OFF+4*8)(R15)
77
78 // Finish setting up callbackArgs struct at CB_ARGS(R15)
79 // struct { index uintptr; args unsafe.Pointer; result uintptr; stackArgs unsafe.Pointer }
80 // Note: index was already saved earlier
81 ADD $ARGS_ARRAY, R15, R1
82 MOVD R1, (CB_ARGS+8)(R15) // args = address of register args
83 MOVD $0, (CB_ARGS+16)(R15) // result = 0
84
85 // stackArgs points to caller's stack arguments at old_R15+160 = R15+FRAME_SIZE+160
86 ADD $(FRAME_SIZE+160), R15, R1
87 MOVD R1, (CB_ARGS+24)(R15) // stackArgs = &caller_stack_args
88
89 // Call crosscall2 with arguments in registers:
90 // R2 = fn (from callbackWrap_call closure)
91 // R3 = frame (address of callbackArgs)
92 // R5 = ctxt (0)
93 MOVD ·callbackWrap_call(SB), R2
94 MOVD (R2), R2 // dereference closure to get fn
95 ADD $CB_ARGS, R15, R3 // frame = &callbackArgs
96 MOVD $0, R5 // ctxt = 0
97
98 BL crosscall2(SB)
99
100 // Get callback result into R2
101 MOVD (CB_ARGS+16)(R15), R2
102
103 // Deallocate frame
104 ADD $FRAME_SIZE, R15
105
106 // Restore R6-R15 from caller's frame
107 LMG 48(R15), R6, R15
108
109 RET