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armv6_idct.S
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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.
*/
// void armv6_idct(short *coefs, int *quans, unsigned char *rows, int col)
.arm
.syntax unified
.text
.align
.global 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
orrseq r8, r1, r2
orrseq r8, r3, r5
orrseq 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 to r10, r11.
ldrd r10, r11, 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 to r10, r11.
ldrd r10, r11, 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