diff --git a/libjpegtwrp/armv6_idct.S b/libjpegtwrp/armv6_idct.S
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+++ b/libjpegtwrp/armv6_idct.S
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+/*
+ * Copyright (C) 2010 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/*
+ * This is a fast-and-accurate implementation of inverse Discrete Cosine
+ * Transform (IDCT) for ARMv6+. It also performs dequantization of the input
+ * coefficients just like other methods.
+ *
+ * This implementation is based on the scaled 1-D DCT algorithm proposed by
+ * Arai, Agui, and Nakajima. The following code is based on the figure 4-8
+ * on page 52 of the JPEG textbook by Pennebaker and Mitchell. Coefficients
+ * are (almost) directly mapped into registers.
+ *
+ * The accuracy is achieved by using SMULWy and SMLAWy instructions. Both
+ * multiply 32 bits by 16 bits and store the top 32 bits of the result. It
+ * makes 32-bit fixed-point arithmetic possible without overflow. That is
+ * why jpeg_idct_ifast(), which is written in C, cannot be improved.
+ *
+ * More tricks are used to gain more speed. First of all, we use as many
+ * registers as possible. ARM processor has 16 registers including sp (r13)
+ * and pc (r15), so only 14 registers can be used without limitations. In
+ * general, we let r0 to r7 hold the coefficients; r10 and r11 hold four
+ * 16-bit constants; r12 and r14 hold two of the four arguments; and r8 hold
+ * intermediate value. In the second pass, r9 is the loop counter. In the
+ * first pass, r8 to r11 are used to hold quantization values, so the loop
+ * counter is held by sp. Yes, the stack pointer. Since it must be aligned
+ * to 4-byte boundary all the time, we align it to 32-byte boundary and use
+ * bit 3 to bit 5. As the result, we actually use 14.1 registers. :-)
+ *
+ * Second, we rearrange quantization values to access them sequentially. The
+ * table is first transposed, and the new columns are placed in the order of
+ * 7, 5, 1, 3, 0, 2, 4, 6. Thus we can use LDMDB to load four values at a
+ * time. Rearranging coefficients also helps, but that requires to change a
+ * dozen of files, which seems not worth it. In addition, we choose to scale
+ * up quantization values by 13 bits, so the coefficients are scaled up by
+ * 16 bits after both passes. Then we can pack and saturate them two at a
+ * time using PKHTB and USAT16 instructions.
+ *
+ * Third, we reorder the instructions to avoid bubbles in the pipeline. This
+ * is done by hand accroding to the cycle timings and the interlock behavior
+ * described in the technical reference manual of ARM1136JF-S. We also take
+ * advantage of dual issue processors by interleaving instructions with
+ * dependencies. It has been benchmarked on four devices and all the results
+ * showed distinguishable improvements. Note that PLD instructions actually
+ * slow things down, so they are removed at the last minute. In the future,
+ * this might be futher improved using a system profiler.
+ */
+
+#ifdef __arm__
+#include <machine/cpu-features.h>
+#endif
+
+#if __ARM_ARCH__ >= 6
+
+// void armv6_idct(short *coefs, int *quans, unsigned char *rows, int col)
+    .arm
+    .text
+    .align
+    .global armv6_idct
+    .func   armv6_idct
+
+armv6_idct:
+    // Push everything except sp (r13) and pc (r15).
+    stmdb   sp!, {r4, r5, r6, r7, r8, r9, r10, r11, r12, r14}
+
+    // r12 = quans, r14 = coefs.
+    sub     r4, sp, #236
+    bic     sp, r4, #31
+    add     r5, sp, #224
+    add     r12, r1, #256
+    stm     r5, {r2, r3, r4}
+    add     r14, r0, #16
+
+pass1_head:
+    // Load quantization values. (q[0, 2, 4, 6])
+    ldmdb   r12!, {r8, r9, r10, r11}
+
+    // Load coefficients. (c[4, 1, 2, 3, 0, 5, 6, 7])
+    ldrsh   r4, [r14, #-2] !
+    ldrsh   r1, [r14, #16]
+    ldrsh   r2, [r14, #32]
+    ldrsh   r3, [r14, #48]
+    ldrsh   r0, [r14, #64]
+    ldrsh   r5, [r14, #80]
+    ldrsh   r6, [r14, #96]
+    ldrsh   r7, [r14, #112]
+
+    // r4 = q[0] * c[0];
+    mul     r4, r8, r4
+
+    // Check if ACs are all zero.
+    cmp     r0, #0
+    orreqs  r8, r1, r2
+    orreqs  r8, r3, r5
+    orreqs  r8, r6, r7
+    beq     pass1_zero
+
+    // Step 1: Dequantizations.
+
+    // r2 = q[2] * c[2];
+    // r0 = q[4] * c[4] + r4;
+    // r6 = q[6] * c[6] + r2;
+    mul     r2, r9, r2
+    mla     r0, r10, r0, r4
+    mla     r6, r11, r6, r2
+
+    // Load quantization values. (q[7, 5, 1, 3])
+    ldmdb   r12!, {r8, r9, r10, r11}
+
+    // r4 = r4 * 2 - r0 = -(r0 - r4 * 2);
+    // r2 = r2 * 2 - r6 = -(r6 - r2 * 2);
+    rsb     r4, r0, r4, lsl #1
+    rsb     r2, r6, r2, lsl #1
+
+    // r7 = q[7] * c[7];
+    // r5 = q[5] * c[5];
+    // r1 = q[1] * c[1] + r7;
+    // r3 = q[3] * c[3] + r5;
+    mul     r7, r8, r7
+    mul     r5, r9, r5
+    mla     r1, r10, r1, r7
+    mla     r3, r11, r3, r5
+
+    // Load constants.
+    ldrd    r10, constants
+
+    // Step 2: Rotations and Butterflies.
+
+    // r7 = r1 - r7 * 2;
+    // r1 = r1 - r3;
+    // r5 = r5 * 2 - r3 = -(r3 - r5 * 2);
+    // r3 = r1 + r3 * 2;
+    // r8 = r5 + r7;
+    sub     r7, r1, r7, lsl #1
+    sub     r1, r1, r3
+    rsb     r5, r3, r5, lsl #1
+    add     r3, r1, r3, lsl #1
+    add     r8, r5, r7
+
+    // r2 = r2 * 1.41421 = r2 * 27146 / 65536 + r2;
+    // r8 = r8 * 1.84776 / 8 = r8 * 15137 / 65536;
+    // r1 = r1 * 1.41421 = r1 * 27146 / 65536 + r1;
+    smlawt  r2, r2, r10, r2
+    smulwb  r8, r8, r10
+    smlawt  r1, r1, r10, r1
+
+    // r0 = r0 + r6;
+    // r2 = r2 - r6;
+    // r6 = r0 - r6 * 2;
+    add     r0, r0, r6
+    sub     r2, r2, r6
+    sub     r6, r0, r6, lsl #1
+
+    // r5 = r5 * -2.61313 / 8 + r8 = r5 * -21407 / 65536 + r8;
+    // r8 = r7 * -1.08239 / 8 + r8 = r7 * -8867 / 65536 + r8;
+    smlawt  r5, r5, r11, r8
+    smlawb  r8, r7, r11, r8
+
+    // r4 = r4 + r2;
+    // r0 = r0 + r3;
+    // r2 = r4 - r2 * 2;
+    add     r4, r4, r2
+    add     r0, r0, r3
+    sub     r2, r4, r2, lsl #1
+
+    // r7 = r5 * 8 - r3 = -(r3 - r5 * 8);
+    // r3 = r0 - r3 * 2;
+    // r1 = r1 - r7;
+    // r4 = r4 + r7;
+    // r5 = r8 * 8 - r1 = -(r1 - r8 * 8);
+    // r7 = r4 - r7 * 2;
+    rsb     r7, r3, r5, lsl #3
+    sub     r3, r0, r3, lsl #1
+    sub     r1, r1, r7
+    add     r4, r4, r7
+    rsb     r5, r1, r8, lsl #3
+    sub     r7, r4, r7, lsl #1
+
+    // r2 = r2 + r1;
+    // r6 = r6 + r5;
+    // r1 = r2 - r1 * 2;
+    // r5 = r6 - r5 * 2;
+    add     r2, r2, r1
+    add     r6, r6, r5
+    sub     r1, r2, r1, lsl #1
+    sub     r5, r6, r5, lsl #1
+
+    // Step 3: Reorder and Save.
+
+    str     r0, [sp, #-4] !
+    str     r4, [sp, #32]
+    str     r2, [sp, #64]
+    str     r6, [sp, #96]
+    str     r5, [sp, #128]
+    str     r1, [sp, #160]
+    str     r7, [sp, #192]
+    str     r3, [sp, #224]
+    b       pass1_tail
+
+    // Precomputed 16-bit constants: 27146, 15137, -21407, -8867.
+    // Put them in the middle since LDRD only accepts offsets from -255 to 255.
+    .align  3
+constants:
+    .word   0x6a0a3b21
+    .word   0xac61dd5d
+
+pass1_zero:
+    str     r4, [sp, #-4] !
+    str     r4, [sp, #32]
+    str     r4, [sp, #64]
+    str     r4, [sp, #96]
+    str     r4, [sp, #128]
+    str     r4, [sp, #160]
+    str     r4, [sp, #192]
+    str     r4, [sp, #224]
+    sub     r12, r12, #16
+
+pass1_tail:
+    ands    r9, sp, #31
+    bne     pass1_head
+
+    // r12 = rows, r14 = col.
+    ldr     r12, [sp, #256]
+    ldr     r14, [sp, #260]
+
+    // Load constants.
+    ldrd    r10, constants
+
+pass2_head:
+    // Load coefficients. (c[0, 1, 2, 3, 4, 5, 6, 7])
+    ldmia   sp!, {r0, r1, r2, r3, r4, r5, r6, r7}
+
+    // r0 = r0 + 0x00808000;
+    add     r0, r0, #0x00800000
+    add     r0, r0, #0x00008000
+
+    // Step 1: Analog to the first pass.
+
+    // r0 = r0 + r4;
+    // r6 = r6 + r2;
+    add     r0, r0, r4
+    add     r6, r6, r2
+
+    // r4 = r0 - r4 * 2;
+    // r2 = r2 * 2 - r6 = -(r6 - r2 * 2);
+    sub     r4, r0, r4, lsl #1
+    rsb     r2, r6, r2, lsl #1
+
+    // r1 = r1 + r7;
+    // r3 = r3 + r5;
+    add     r1, r1, r7
+    add     r3, r3, r5
+
+    // Step 2: Rotations and Butterflies.
+
+    // r7 = r1 - r7 * 2;
+    // r1 = r1 - r3;
+    // r5 = r5 * 2 - r3 = -(r3 - r5 * 2);
+    // r3 = r1 + r3 * 2;
+    // r8 = r5 + r7;
+    sub     r7, r1, r7, lsl #1
+    sub     r1, r1, r3
+    rsb     r5, r3, r5, lsl #1
+    add     r3, r1, r3, lsl #1
+    add     r8, r5, r7
+
+    // r2 = r2 * 1.41421 = r2 * 27146 / 65536 + r2;
+    // r8 = r8 * 1.84776 / 8 = r8 * 15137 / 65536;
+    // r1 = r1 * 1.41421 = r1 * 27146 / 65536 + r1;
+    smlawt  r2, r2, r10, r2
+    smulwb  r8, r8, r10
+    smlawt  r1, r1, r10, r1
+
+    // r0 = r0 + r6;
+    // r2 = r2 - r6;
+    // r6 = r0 - r6 * 2;
+    add     r0, r0, r6
+    sub     r2, r2, r6
+    sub     r6, r0, r6, lsl #1
+
+    // r5 = r5 * -2.61313 / 8 + r8 = r5 * -21407 / 65536 + r8;
+    // r8 = r7 * -1.08239 / 8 + r8 = r7 * -8867 / 65536 + r8;
+    smlawt  r5, r5, r11, r8
+    smlawb  r8, r7, r11, r8
+
+    // r4 = r4 + r2;
+    // r0 = r0 + r3;
+    // r2 = r4 - r2 * 2;
+    add     r4, r4, r2
+    add     r0, r0, r3
+    sub     r2, r4, r2, lsl #1
+
+    // r7 = r5 * 8 - r3 = -(r3 - r5 * 8);
+    // r3 = r0 - r3 * 2;
+    // r1 = r1 - r7;
+    // r4 = r4 + r7;
+    // r5 = r8 * 8 - r1 = -(r1 - r8 * 8);
+    // r7 = r4 - r7 * 2;
+    rsb     r7, r3, r5, lsl #3
+    sub     r3, r0, r3, lsl #1
+    sub     r1, r1, r7
+    add     r4, r4, r7
+    rsb     r5, r1, r8, lsl #3
+    sub     r7, r4, r7, lsl #1
+
+    // r2 = r2 + r1;
+    // r6 = r6 + r5;
+    // r1 = r2 - r1 * 2;
+    // r5 = r6 - r5 * 2;
+    add     r2, r2, r1
+    add     r6, r6, r5
+    sub     r1, r2, r1, lsl #1
+    sub     r5, r6, r5, lsl #1
+
+    // Step 3: Reorder and Save.
+
+    // Load output pointer.
+    ldr     r8, [r12], #4
+
+    // For little endian: r6, r2, r4, r0, r3, r7, r1, r5.
+    pkhtb   r6, r6, r4, asr #16
+    pkhtb   r2, r2, r0, asr #16
+    pkhtb   r3, r3, r1, asr #16
+    pkhtb   r7, r7, r5, asr #16
+    usat16  r6, #8, r6
+    usat16  r2, #8, r2
+    usat16  r3, #8, r3
+    usat16  r7, #8, r7
+    orr     r0, r2, r6, lsl #8
+    orr     r1, r7, r3, lsl #8
+
+#ifdef __ARMEB__
+    // Reverse bytes for big endian.
+    rev     r0, r0
+    rev     r1, r1
+#endif
+
+    // Use STR instead of STRD to support unaligned access.
+    str     r0, [r8, r14] !
+    str     r1, [r8, #4]
+
+pass2_tail:
+    adds    r9, r9, #0x10000000
+    bpl     pass2_head
+
+    ldr     sp, [sp, #8]
+    add     sp, sp, #236
+
+    ldmia   sp!, {r4, r5, r6, r7, r8, r9, r10, r11, r12, r14}
+    bx      lr
+    .endfunc
+
+#endif