;
; jchuff-sse2.asm - Huffman entropy encoding (SSE2)
;
; Copyright 2009-2011, 2014-2016 D. R. Commander.
; Copyright 2015 Matthieu Darbois
;
; Based on
; x86 SIMD extension for IJG JPEG library
; Copyright (C) 1999-2006, MIYASAKA Masaru.
; For conditions of distribution and use, see copyright notice in jsimdext.inc
;
; This file should be assembled with NASM (Netwide Assembler),
; can *not* be assembled with Microsoft's MASM or any compatible
; assembler (including Borland's Turbo Assembler).
; NASM is available from http://nasm.sourceforge.net/ or
; http://sourceforge.net/project/showfiles.php?group_id=6208
;
; This file contains an SSE2 implementation for Huffman coding of one block.
; The following code is based directly on jchuff.c; see jchuff.c for more
; details.
;
; [TAB8]

%include "jsimdext.inc"

; --------------------------------------------------------------------------
        SECTION SEG_CONST

        alignz  16
        global  EXTN(jconst_huff_encode_one_block)

EXTN(jconst_huff_encode_one_block):

%include "jpeg_nbits_table.inc"

        alignz  16

; --------------------------------------------------------------------------
        SECTION SEG_TEXT
        BITS    32

; These macros perform the same task as the emit_bits() function in the
; original libjpeg code.  In addition to reducing overhead by explicitly
; inlining the code, additional performance is achieved by taking into
; account the size of the bit buffer and waiting until it is almost full
; before emptying it.  This mostly benefits 64-bit platforms, since 6
; bytes can be stored in a 64-bit bit buffer before it has to be emptied.

%macro EMIT_BYTE 0
        sub put_bits, 8  ; put_bits -= 8;
        mov edx, put_buffer
        mov ecx, put_bits
        shr edx, cl  ; c = (JOCTET)GETJOCTET(put_buffer >> put_bits);
        mov byte [eax], dl  ; *buffer++ = c;
        add eax, 1
        cmp dl, 0xFF  ; need to stuff a zero byte?
        jne %%.EMIT_BYTE_END
        mov byte [eax], 0  ; *buffer++ = 0;
        add eax, 1
%%.EMIT_BYTE_END:
%endmacro

%macro PUT_BITS 1
        add put_bits, ecx  ; put_bits += size;
        shl put_buffer, cl  ; put_buffer = (put_buffer << size);
        or  put_buffer, %1
%endmacro

%macro CHECKBUF15 0
        cmp put_bits, 16  ; if (put_bits > 31) {
        jl %%.CHECKBUF15_END
        mov eax, POINTER [esp+buffer]
        EMIT_BYTE
        EMIT_BYTE
        mov POINTER [esp+buffer], eax
%%.CHECKBUF15_END:
%endmacro

%macro EMIT_BITS 1
        PUT_BITS %1
        CHECKBUF15
%endmacro

%macro kloop_prepare 37  ;(ko, jno0, ..., jno31, xmm0, xmm1, xmm2, xmm3)
    pxor xmm4, xmm4  ; __m128i neg = _mm_setzero_si128();
    pxor xmm5, xmm5  ; __m128i neg = _mm_setzero_si128();
    pxor xmm6, xmm6  ; __m128i neg = _mm_setzero_si128();
    pxor xmm7, xmm7  ; __m128i neg = _mm_setzero_si128();
    pinsrw %34, word [esi + %2  * SIZEOF_WORD], 0  ; xmm_shadow[0] = block[jno0];
    pinsrw %35, word [esi + %10 * SIZEOF_WORD], 0  ; xmm_shadow[8] = block[jno8];
    pinsrw %36, word [esi + %18 * SIZEOF_WORD], 0  ; xmm_shadow[16] = block[jno16];
    pinsrw %37, word [esi + %26 * SIZEOF_WORD], 0  ; xmm_shadow[24] = block[jno24];
    pinsrw %34, word [esi + %3  * SIZEOF_WORD], 1  ; xmm_shadow[1] = block[jno1];
    pinsrw %35, word [esi + %11 * SIZEOF_WORD], 1  ; xmm_shadow[9] = block[jno9];
    pinsrw %36, word [esi + %19 * SIZEOF_WORD], 1  ; xmm_shadow[17] = block[jno17];
    pinsrw %37, word [esi + %27 * SIZEOF_WORD], 1  ; xmm_shadow[25] = block[jno25];
    pinsrw %34, word [esi + %4  * SIZEOF_WORD], 2  ; xmm_shadow[2] = block[jno2];
    pinsrw %35, word [esi + %12 * SIZEOF_WORD], 2  ; xmm_shadow[10] = block[jno10];
    pinsrw %36, word [esi + %20 * SIZEOF_WORD], 2  ; xmm_shadow[18] = block[jno18];
    pinsrw %37, word [esi + %28 * SIZEOF_WORD], 2  ; xmm_shadow[26] = block[jno26];
    pinsrw %34, word [esi + %5  * SIZEOF_WORD], 3  ; xmm_shadow[3] = block[jno3];
    pinsrw %35, word [esi + %13 * SIZEOF_WORD], 3  ; xmm_shadow[11] = block[jno11];
    pinsrw %36, word [esi + %21 * SIZEOF_WORD], 3  ; xmm_shadow[19] = block[jno19];
    pinsrw %37, word [esi + %29 * SIZEOF_WORD], 3  ; xmm_shadow[27] = block[jno27];
    pinsrw %34, word [esi + %6  * SIZEOF_WORD], 4  ; xmm_shadow[4] = block[jno4];
    pinsrw %35, word [esi + %14 * SIZEOF_WORD], 4  ; xmm_shadow[12] = block[jno12];
    pinsrw %36, word [esi + %22 * SIZEOF_WORD], 4  ; xmm_shadow[20] = block[jno20];
    pinsrw %37, word [esi + %30 * SIZEOF_WORD], 4  ; xmm_shadow[28] = block[jno28];
    pinsrw %34, word [esi + %7  * SIZEOF_WORD], 5  ; xmm_shadow[5] = block[jno5];
    pinsrw %35, word [esi + %15 * SIZEOF_WORD], 5  ; xmm_shadow[13] = block[jno13];
    pinsrw %36, word [esi + %23 * SIZEOF_WORD], 5  ; xmm_shadow[21] = block[jno21];
    pinsrw %37, word [esi + %31 * SIZEOF_WORD], 5  ; xmm_shadow[29] = block[jno29];
    pinsrw %34, word [esi + %8  * SIZEOF_WORD], 6  ; xmm_shadow[6] = block[jno6];
    pinsrw %35, word [esi + %16 * SIZEOF_WORD], 6  ; xmm_shadow[14] = block[jno14];
    pinsrw %36, word [esi + %24 * SIZEOF_WORD], 6  ; xmm_shadow[22] = block[jno22];
    pinsrw %37, word [esi + %32 * SIZEOF_WORD], 6  ; xmm_shadow[30] = block[jno30];
    pinsrw %34, word [esi + %9  * SIZEOF_WORD], 7  ; xmm_shadow[7] = block[jno7];
    pinsrw %35, word [esi + %17 * SIZEOF_WORD], 7  ; xmm_shadow[15] = block[jno15];
    pinsrw %36, word [esi + %25 * SIZEOF_WORD], 7  ; xmm_shadow[23] = block[jno23];
%if %1 != 32
    pinsrw %37, word [esi + %33 * SIZEOF_WORD], 7  ; xmm_shadow[31] = block[jno31];
%else
    pinsrw %37, ecx, 7  ; xmm_shadow[31] = block[jno31];
%endif
    pcmpgtw xmm4, %34  ; neg = _mm_cmpgt_epi16(neg, x1);
    pcmpgtw xmm5, %35  ; neg = _mm_cmpgt_epi16(neg, x1);
    pcmpgtw xmm6, %36  ; neg = _mm_cmpgt_epi16(neg, x1);
    pcmpgtw xmm7, %37  ; neg = _mm_cmpgt_epi16(neg, x1);
    paddw %34, xmm4   ; x1 = _mm_add_epi16(x1, neg);
    paddw %35, xmm5   ; x1 = _mm_add_epi16(x1, neg);
    paddw %36, xmm6  ; x1 = _mm_add_epi16(x1, neg);
    paddw %37, xmm7  ; x1 = _mm_add_epi16(x1, neg);
    pxor %34, xmm4    ; x1 = _mm_xor_si128(x1, neg);
    pxor %35, xmm5    ; x1 = _mm_xor_si128(x1, neg);
    pxor %36, xmm6   ; x1 = _mm_xor_si128(x1, neg);
    pxor %37, xmm7   ; x1 = _mm_xor_si128(x1, neg);
    pxor xmm4, %34    ; neg = _mm_xor_si128(neg, x1);
    pxor xmm5, %35    ; neg = _mm_xor_si128(neg, x1);
    pxor xmm6, %36   ; neg = _mm_xor_si128(neg, x1);
    pxor xmm7, %37   ; neg = _mm_xor_si128(neg, x1);
    movdqa XMMWORD [esp + t1 + %1 * SIZEOF_WORD], %34  ; _mm_storeu_si128((__m128i *)(t1 + ko), x1);
    movdqa XMMWORD [esp + t1 + (%1 + 8) * SIZEOF_WORD], %35  ; _mm_storeu_si128((__m128i *)(t1 + ko + 8), x1);
    movdqa XMMWORD [esp + t1 + (%1 + 16) * SIZEOF_WORD], %36  ; _mm_storeu_si128((__m128i *)(t1 + ko + 16), x1);
    movdqa XMMWORD [esp + t1 + (%1 + 24) * SIZEOF_WORD], %37  ; _mm_storeu_si128((__m128i *)(t1 + ko + 24), x1);
    movdqa XMMWORD [esp + t2 + %1 * SIZEOF_WORD], xmm4  ; _mm_storeu_si128((__m128i *)(t2 + ko), neg);
    movdqa XMMWORD [esp + t2 + (%1 + 8) * SIZEOF_WORD], xmm5  ; _mm_storeu_si128((__m128i *)(t2 + ko + 8), neg);
    movdqa XMMWORD [esp + t2 + (%1 + 16) * SIZEOF_WORD], xmm6  ; _mm_storeu_si128((__m128i *)(t2 + ko + 16), neg);
    movdqa XMMWORD [esp + t2 + (%1 + 24) * SIZEOF_WORD], xmm7  ; _mm_storeu_si128((__m128i *)(t2 + ko + 24), neg);
%endmacro

;
; Encode a single block's worth of coefficients.
;
; GLOBAL(JOCTET*)
; jsimd_huff_encode_one_block_sse2 (working_state *state, JOCTET *buffer,
;                                   JCOEFPTR block, int last_dc_val,
;                                   c_derived_tbl *dctbl, c_derived_tbl *actbl)
;

; eax + 8 = working_state *state
; eax + 12 = JOCTET *buffer
; eax + 16 = JCOEFPTR block
; eax + 20 = int last_dc_val
; eax + 24 = c_derived_tbl *dctbl
; eax + 28 = c_derived_tbl *actbl

%define pad             6*SIZEOF_DWORD  ; Align to 16 bytes
%define t1              pad
%define t2              t1+(DCTSIZE2*SIZEOF_WORD)
%define block           t2+(DCTSIZE2*SIZEOF_WORD)
%define actbl           block+SIZEOF_DWORD
%define buffer          actbl+SIZEOF_DWORD
%define temp            buffer+SIZEOF_DWORD
%define temp2           temp+SIZEOF_DWORD
%define temp3           temp2+SIZEOF_DWORD
%define temp4           temp3+SIZEOF_DWORD
%define temp5           temp4+SIZEOF_DWORD
%define gotptr          temp5+SIZEOF_DWORD  ; void *gotptr
%define put_buffer      ebx
%define put_bits        edi

        align   16
        global  EXTN(jsimd_huff_encode_one_block_sse2)

EXTN(jsimd_huff_encode_one_block_sse2):
        push    ebp
        mov     eax,esp                         ; eax = original ebp
        sub     esp, byte 4
        and     esp, byte (-SIZEOF_XMMWORD)     ; align to 128 bits
        mov     [esp],eax
        mov     ebp,esp                         ; ebp = aligned ebp
        sub     esp, temp5+9*SIZEOF_DWORD-pad
        push    ebx
        push    ecx
;       push    edx             ; need not be preserved
        push    esi
        push    edi
        push    ebp

        mov esi, POINTER [eax+8]        ; (working_state *state)
        mov put_buffer,  DWORD [esi+8]  ; put_buffer = state->cur.put_buffer;
        mov put_bits,    DWORD [esi+12]  ; put_bits = state->cur.put_bits;
        push esi  ; esi is now scratch

        get_GOT edx                       ; get GOT address
        movpic POINTER [esp+gotptr], edx  ; save GOT address

        mov ecx, POINTER [eax+28]
        mov edx, POINTER [eax+16]
        mov esi, POINTER [eax+12]
        mov POINTER [esp+actbl],  ecx
        mov POINTER [esp+block],  edx
        mov POINTER [esp+buffer], esi

        ; Encode the DC coefficient difference per section F.1.2.1
        mov esi, POINTER [esp+block]        ; block
        movsx ecx, word [esi]  ; temp = temp2 = block[0] - last_dc_val;
        sub   ecx, DWORD [eax+20]
        mov   esi, ecx

        ; This is a well-known technique for obtaining the absolute value
        ; without a branch.  It is derived from an assembly language technique
        ; presented in "How to Optimize for the Pentium Processors",
        ; Copyright (c) 1996, 1997 by Agner Fog.
        mov edx, ecx
        sar edx, 31   ; temp3 = temp >> (CHAR_BIT * sizeof(int) - 1);
        xor ecx, edx ; temp ^= temp3;
        sub ecx, edx ; temp -= temp3;

        ; For a negative input, want temp2 = bitwise complement of abs(input)
        ; This code assumes we are on a two's complement machine
        add esi, edx  ; temp2 += temp3;
        mov DWORD [esp+temp], esi  ; backup temp2 in temp

        ; Find the number of bits needed for the magnitude of the coefficient
        movpic ebp, POINTER [esp+gotptr]   ; load GOT address (ebp)
        movzx edx, byte [GOTOFF(ebp, jpeg_nbits_table + ecx)]  ; nbits = JPEG_NBITS(temp);
        mov DWORD [esp+temp2], edx  ; backup nbits in temp2

        ; Emit the Huffman-coded symbol for the number of bits
        mov    ebp, POINTER [eax+24]  ; After this point, arguments are not accessible anymore
        mov    eax,  INT [ebp + edx * 4]  ; code = dctbl->ehufco[nbits];
        movzx  ecx, byte [ebp + edx + 1024]  ; size = dctbl->ehufsi[nbits];
        EMIT_BITS eax  ; EMIT_BITS(code, size)

        mov ecx, DWORD [esp+temp2]  ; restore nbits

        ; Mask off any extra bits in code
        mov eax, 1
        shl eax, cl
        dec eax
        and eax, DWORD [esp+temp]  ; temp2 &= (((JLONG) 1)<<nbits) - 1;

        ; Emit that number of bits of the value, if positive,
        ; or the complement of its magnitude, if negative.
        EMIT_BITS eax  ; EMIT_BITS(temp2, nbits)

        ; Prepare data
        xor ecx, ecx
        mov esi, POINTER [esp+block]
        kloop_prepare  0,  1,  8,  16, 9,  2,  3,  10, 17, 24, 32, 25, \
                       18, 11, 4,  5,  12, 19, 26, 33, 40, 48, 41, 34, \
                       27, 20, 13, 6,  7,  14, 21, 28, 35, \
                       xmm0, xmm1, xmm2, xmm3
        kloop_prepare  32, 42, 49, 56, 57, 50, 43, 36, 29, 22, 15, 23, \
                       30, 37, 44, 51, 58, 59, 52, 45, 38, 31, 39, 46, \
                       53, 60, 61, 54, 47, 55, 62, 63, 63, \
                       xmm0, xmm1, xmm2, xmm3

        pxor xmm7, xmm7
        movdqa xmm0, XMMWORD [esp + t1 + 0 * SIZEOF_WORD]   ; __m128i tmp0 = _mm_loadu_si128((__m128i *)(t1 + 0));
        movdqa xmm1, XMMWORD [esp + t1 + 8 * SIZEOF_WORD]   ; __m128i tmp1 = _mm_loadu_si128((__m128i *)(t1 + 8));
        movdqa xmm2, XMMWORD [esp + t1 + 16 * SIZEOF_WORD]  ; __m128i tmp2 = _mm_loadu_si128((__m128i *)(t1 + 16));
        movdqa xmm3, XMMWORD [esp + t1 + 24 * SIZEOF_WORD]  ; __m128i tmp3 = _mm_loadu_si128((__m128i *)(t1 + 24));
        pcmpeqw xmm0, xmm7  ; tmp0 = _mm_cmpeq_epi16(tmp0, zero);
        pcmpeqw xmm1, xmm7  ; tmp1 = _mm_cmpeq_epi16(tmp1, zero);
        pcmpeqw xmm2, xmm7  ; tmp2 = _mm_cmpeq_epi16(tmp2, zero);
        pcmpeqw xmm3, xmm7  ; tmp3 = _mm_cmpeq_epi16(tmp3, zero);
        packsswb xmm0, xmm1  ; tmp0 = _mm_packs_epi16(tmp0, tmp1);
        packsswb xmm2, xmm3  ; tmp2 = _mm_packs_epi16(tmp2, tmp3);
        pmovmskb edx, xmm0  ; index  = ((uint64_t)_mm_movemask_epi8(tmp0)) << 0;
        pmovmskb ecx, xmm2  ; index  = ((uint64_t)_mm_movemask_epi8(tmp2)) << 16;
        shl ecx, 16
        or  edx, ecx
        not edx  ; index = ~index;

        lea esi, [esp+t1]
        mov ebp, POINTER [esp+actbl]  ; ebp = actbl

.BLOOP:
        bsf ecx, edx  ; r = __builtin_ctzl(index);
        jz .ELOOP
        lea esi, [esi+ecx*2]  ; k += r;
        shr edx, cl  ; index >>= r;
        mov DWORD [esp+temp3], edx
.BRLOOP:
        cmp ecx, 16  ; while (r > 15) {
        jl .ERLOOP
        sub ecx, 16 ; r -= 16;
        mov DWORD [esp+temp], ecx
        mov   eax, INT [ebp + 240 * 4]  ; code_0xf0 = actbl->ehufco[0xf0];
        movzx ecx, byte [ebp + 1024 + 240]  ; size_0xf0 = actbl->ehufsi[0xf0];
        EMIT_BITS eax  ; EMIT_BITS(code_0xf0, size_0xf0)
        mov ecx, DWORD [esp+temp]
        jmp .BRLOOP
.ERLOOP:
        movsx eax, word [esi]  ; temp = t1[k];
        movpic edx, POINTER [esp+gotptr]   ; load GOT address (edx)
        movzx eax, byte [GOTOFF(edx, jpeg_nbits_table + eax)]  ; nbits = JPEG_NBITS(temp);
        mov DWORD [esp+temp2], eax
        ; Emit Huffman symbol for run length / number of bits
        shl ecx, 4  ; temp3 = (r << 4) + nbits;
        add ecx, eax
        mov   eax,  INT [ebp + ecx * 4]  ; code = actbl->ehufco[temp3];
        movzx ecx, byte [ebp + ecx + 1024]  ; size = actbl->ehufsi[temp3];
        EMIT_BITS eax

        movsx edx, word [esi+DCTSIZE2*2]  ; temp2 = t2[k];
        ; Mask off any extra bits in code
        mov ecx, DWORD [esp+temp2]
        mov eax, 1
        shl eax, cl
        dec eax
        and eax, edx  ; temp2 &= (((JLONG) 1)<<nbits) - 1;
        EMIT_BITS eax  ; PUT_BITS(temp2, nbits)
        mov edx, DWORD [esp+temp3]
        add esi, 2  ; ++k;
        shr edx, 1  ; index >>= 1;

        jmp .BLOOP
.ELOOP:
        movdqa xmm0, XMMWORD [esp + t1 + 32 * SIZEOF_WORD]  ; __m128i tmp0 = _mm_loadu_si128((__m128i *)(t1 + 0));
        movdqa xmm1, XMMWORD [esp + t1 + 40 * SIZEOF_WORD]  ; __m128i tmp1 = _mm_loadu_si128((__m128i *)(t1 + 8));
        movdqa xmm2, XMMWORD [esp + t1 + 48 * SIZEOF_WORD]  ; __m128i tmp2 = _mm_loadu_si128((__m128i *)(t1 + 16));
        movdqa xmm3, XMMWORD [esp + t1 + 56 * SIZEOF_WORD]  ; __m128i tmp3 = _mm_loadu_si128((__m128i *)(t1 + 24));
        pcmpeqw xmm0, xmm7  ; tmp0 = _mm_cmpeq_epi16(tmp0, zero);
        pcmpeqw xmm1, xmm7  ; tmp1 = _mm_cmpeq_epi16(tmp1, zero);
        pcmpeqw xmm2, xmm7  ; tmp2 = _mm_cmpeq_epi16(tmp2, zero);
        pcmpeqw xmm3, xmm7  ; tmp3 = _mm_cmpeq_epi16(tmp3, zero);
        packsswb xmm0, xmm1  ; tmp0 = _mm_packs_epi16(tmp0, tmp1);
        packsswb xmm2, xmm3  ; tmp2 = _mm_packs_epi16(tmp2, tmp3);
        pmovmskb edx, xmm0  ; index  = ((uint64_t)_mm_movemask_epi8(tmp0)) << 0;
        pmovmskb ecx, xmm2  ; index  = ((uint64_t)_mm_movemask_epi8(tmp2)) << 16;
        shl ecx, 16
        or  edx, ecx
        not edx  ; index = ~index;

        lea eax, [esp + t1 + (DCTSIZE2/2) * 2]
        sub eax, esi
        shr eax, 1
        bsf ecx, edx  ; r = __builtin_ctzl(index);
        jz .ELOOP2
        shr edx, cl  ; index >>= r;
        add ecx, eax
        lea esi, [esi+ecx*2]  ; k += r;
        mov DWORD [esp+temp3], edx
        jmp .BRLOOP2
.BLOOP2:
        bsf ecx, edx  ; r = __builtin_ctzl(index);
        jz .ELOOP2
        lea esi, [esi+ecx*2]  ; k += r;
        shr edx, cl  ; index >>= r;
        mov DWORD [esp+temp3], edx
.BRLOOP2:
        cmp ecx, 16  ; while (r > 15) {
        jl .ERLOOP2
        sub ecx, 16  ; r -= 16;
        mov DWORD [esp+temp], ecx
        mov   eax, INT [ebp + 240 * 4]  ; code_0xf0 = actbl->ehufco[0xf0];
        movzx ecx, byte [ebp + 1024 + 240]  ; size_0xf0 = actbl->ehufsi[0xf0];
        EMIT_BITS eax  ; EMIT_BITS(code_0xf0, size_0xf0)
        mov ecx, DWORD [esp+temp]
        jmp .BRLOOP2
.ERLOOP2:
        movsx eax, word [esi]  ; temp = t1[k];
        bsr eax, eax  ; nbits = 32 - __builtin_clz(temp);
        inc eax
        mov DWORD [esp+temp2], eax
        ; Emit Huffman symbol for run length / number of bits
        shl ecx, 4  ; temp3 = (r << 4) + nbits;
        add ecx, eax
        mov   eax,  INT [ebp + ecx * 4]  ; code = actbl->ehufco[temp3];
        movzx ecx, byte [ebp + ecx + 1024]  ; size = actbl->ehufsi[temp3];
        EMIT_BITS eax

        movsx edx, word [esi+DCTSIZE2*2]  ; temp2 = t2[k];
        ; Mask off any extra bits in code
        mov ecx, DWORD [esp+temp2]
        mov eax, 1
        shl eax, cl
        dec eax
        and eax, edx  ; temp2 &= (((JLONG) 1)<<nbits) - 1;
        EMIT_BITS eax  ; PUT_BITS(temp2, nbits)
        mov edx, DWORD [esp+temp3]
        add esi, 2  ; ++k;
        shr edx, 1  ; index >>= 1;

        jmp .BLOOP2
.ELOOP2:
        ; If the last coef(s) were zero, emit an end-of-block code
        lea edx, [esp + t1 + (DCTSIZE2-1) * 2]  ; r = DCTSIZE2-1-k;
        cmp edx, esi  ; if (r > 0) {
        je .EFN
        mov   eax,  INT [ebp]  ; code = actbl->ehufco[0];
        movzx ecx, byte [ebp + 1024]  ; size = actbl->ehufsi[0];
        EMIT_BITS eax
.EFN:
        mov eax, [esp+buffer]
        pop esi
        ; Save put_buffer & put_bits
        mov DWORD [esi+8], put_buffer  ; state->cur.put_buffer = put_buffer;
        mov DWORD [esi+12], put_bits  ; state->cur.put_bits = put_bits;

        pop     ebp
        pop     edi
        pop     esi
;       pop     edx             ; need not be preserved
        pop     ecx
        pop     ebx
        mov     esp,ebp         ; esp <- aligned ebp
        pop     esp             ; esp <- original ebp
        pop     ebp
        ret

; For some reason, the OS X linker does not honor the request to align the
; segment unless we do this.
        align   16