On Thu, 3 Nov 2016, Janne Grunau wrote:

On 2016-11-02 14:58:41 +0200, Martin Storsjö wrote:
---
 libavcodec/aarch64/Makefile              |   2 +
 libavcodec/aarch64/vp9dsp_init_aarch64.c | 139 ++++++
 libavcodec/aarch64/vp9mc_neon.S          | 733 +++++++++++++++++++++++++++++++
 libavcodec/vp9.h                         |   1 +
 libavcodec/vp9dsp.c                      |   2 +
 5 files changed, 877 insertions(+)
 create mode 100644 libavcodec/aarch64/vp9dsp_init_aarch64.c
 create mode 100644 libavcodec/aarch64/vp9mc_neon.S

diff --git a/libavcodec/aarch64/Makefile b/libavcodec/aarch64/Makefile
index 764bedc..6f1227a 100644
--- a/libavcodec/aarch64/Makefile
+++ b/libavcodec/aarch64/Makefile
@@ -17,6 +17,7 @@ OBJS-$(CONFIG_DCA_DECODER)              += 
aarch64/dcadsp_init.o
 OBJS-$(CONFIG_RV40_DECODER)             += aarch64/rv40dsp_init_aarch64.o
 OBJS-$(CONFIG_VC1_DECODER)              += aarch64/vc1dsp_init_aarch64.o
 OBJS-$(CONFIG_VORBIS_DECODER)           += aarch64/vorbisdsp_init.o
+OBJS-$(CONFIG_VP9_DECODER)              += aarch64/vp9dsp_init_aarch64.o

 # ARMv8 optimizations

@@ -43,3 +44,4 @@ NEON-OBJS-$(CONFIG_MPEGAUDIODSP)        += 
aarch64/mpegaudiodsp_neon.o
 NEON-OBJS-$(CONFIG_DCA_DECODER)         += aarch64/dcadsp_neon.o               
\
                                            aarch64/synth_filter_neon.o
 NEON-OBJS-$(CONFIG_VORBIS_DECODER)      += aarch64/vorbisdsp_neon.o
+NEON-OBJS-$(CONFIG_VP9_DECODER)         += aarch64/vp9mc_neon.o
diff --git a/libavcodec/aarch64/vp9dsp_init_aarch64.c 
b/libavcodec/aarch64/vp9dsp_init_aarch64.c
new file mode 100644
index 0000000..3d414af
--- /dev/null
+++ b/libavcodec/aarch64/vp9dsp_init_aarch64.c

+    LOCAL_ALIGNED_16(uint8_t, temp, [((sz < 64 ? 2 * sz : 64) + 8) * sz]);     
   \

((1 + (sz < 64)) * sz + 8) * sz

nit, looks nicer imo. no need to change it

Yeah, that looks nicer - changed.

diff --git a/libavcodec/aarch64/vp9mc_neon.S
b/libavcodec/aarch64/vp9mc_neon.S
new file mode 100644
index 0000000..9ca2a32
--- /dev/null
+++ b/libavcodec/aarch64/vp9mc_neon.S
@@ -0,0 +1,733 @@
+/*
+ * Copyright (c) 2016 Google Inc.
+ *
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include "libavutil/aarch64/asm.S"
+
+const regular_filter, align=4

can't we use the constants from C? the single sign extend will be lost
in the noise. same applies to the arm version

+// All public functions in this file have the following signature:
+// typedef void (*vp9_mc_func)(uint8_t *dst, ptrdiff_t dst_stride,
+//                            const uint8_t *ref, ptrdiff_t ref_stride,
+//                            int h, int mx, int my);
+
+function ff_vp9_copy64_neon, export=1
+1:
+        ldp             x5,  x6,  [x2]
+        stp             x5,  x6,  [x0]
+        ldp             x5,  x6,  [x2, #16]

use more registers we have enough, probably not noticable

No measurable change on the A53, but changed to use more registers, and reordered them to allow for a bit more latency in the loads, in case it would help somewhere.

+        stp             x5,  x6,  [x0, #16]
+        subs            w4,  w4,  #1
+        ldp             x5,  x6,  [x2, #32]
+        stp             x5,  x6,  [x0, #32]
+        ldp             x5,  x6,  [x2, #48]
+        stp             x5,  x6,  [x0, #48]
+        add             x2,  x2,  x3
+        add             x0,  x0,  x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_avg64_neon, export=1
+        mov             x5,  x0
+1:
+        ld1             {v4.16b,  v5.16b,  v6.16b,  v7.16b},  [x2], x3
+        ld1             {v0.16b,  v1.16b,  v2.16b,  v3.16b},  [x0], x1
+        ld1             {v20.16b, v21.16b, v22.16b, v23.16b}, [x2], x3
+        urhadd          v0.16b,  v0.16b,  v4.16b
+        urhadd          v1.16b,  v1.16b,  v5.16b
+        ld1             {v16.16b, v17.16b, v18.16b, v19.16b}, [x0], x1
+        urhadd          v2.16b,  v2.16b,  v6.16b
+        urhadd          v3.16b,  v3.16b,  v7.16b
+        subs            w4,  w4,  #2
+        urhadd          v16.16b, v16.16b, v20.16b
+        urhadd          v17.16b, v17.16b, v21.16b
+        st1             {v0.16b,  v1.16b,  v2.16b,  v3.16b},  [x5], x1
+        urhadd          v18.16b, v18.16b, v22.16b
+        urhadd          v19.16b, v19.16b, v23.16b
+        st1             {v16.16b, v17.16b, v18.16b, v19.16b}, [x5], x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_copy32_neon, export=1
+1:
+        ldp             x5,  x6,  [x2]
+        stp             x5,  x6,  [x0]
+        subs            w4,  w4,  #1
+        ldp             x5,  x6,  [x2, #16]

see 64

Updated similarlly as the 64 version

+        stp             x5,  x6,  [x0, #16]
+        add             x2,  x2,  x3
+        add             x0,  x0,  x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_avg32_neon, export=1
+1:
+        ld1             {v2.16b, v3.16b},  [x2], x3
+        ld1             {v0.16b, v1.16b},  [x0]
+        urhadd          v0.16b,  v0.16b,  v2.16b
+        urhadd          v1.16b,  v1.16b,  v3.16b
+        subs            w4,  w4,  #1
+        st1             {v0.16b, v1.16b},  [x0], x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_copy16_neon, export=1
+        add             x5,  x0,  x1
+        add             x1,  x1,  x1

I think lsl 'x1,  x1,  #1' would be a little clearer

Changed

+        add             x6,  x2,  x3
+        add             x3,  x3,  x3
+1:
+        ld1             {v0.16b},  [x2], x3
+        ld1             {v1.16b},  [x6], x3
+        ld1             {v2.16b},  [x2], x3
+        ld1             {v3.16b},  [x6], x3
+        subs            w4,  w4,  #4
+        st1             {v0.16b},  [x0], x1
+        st1             {v1.16b},  [x5], x1
+        st1             {v2.16b},  [x0], x1
+        st1             {v3.16b},  [x5], x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_avg16_neon, export=1
+1:
+        ld1             {v2.16b},  [x2], x3
+        ld1             {v0.16b},  [x0], x1
+        ld1             {v3.16b},  [x2], x3
+        urhadd          v0.16b,  v0.16b,  v2.16b
+        ld1             {v1.16b},  [x0]
+        sub             x0,  x0,  x1

use another register for st1

Done, although it didn't actually improve anything in my measurements

+        urhadd          v1.16b,  v1.16b,  v3.16b
+        subs            w4,  w4,  #2
+        st1             {v0.16b},  [x0], x1
+        st1             {v1.16b},  [x0], x1
+        b.ne            1b
+        ret
+endfunc

...

+function ff_vp9_copy4_neon, export=1
+1:
+        ld1             {v0.s}[0], [x2], x3
+        ld1             {v1.s}[0], [x2], x3
+        st1             {v0.s}[0], [x0], x1
+        ld1             {v2.s}[0], [x2], x3
+        st1             {v1.s}[0], [x0], x1
+        ld1             {v3.s}[0], [x2], x3
+        subs            w4,  w4,  #4
+        st1             {v2.s}[0], [x0], x1
+        st1             {v3.s}[0], [x0], x1
+        b.ne            1b
+        ret
+endfunc
+
+function ff_vp9_avg4_neon, export=1
+1:
+        ld1r            {v4.2s}, [x2], x3

is there a difference between ld1r and ld1 {}[0]? eiether way we should
just use one variant for loading 4 bytes unless we specificially need
one of them. Iirc the cortex-a8 is the only core were a load to all
lanes is faster than a load to all lanes.

Changed to ld1 {}[0]

+        ld1r            {v1.2s}, [x0], x1
+        ld1r            {v6.2s}, [x2], x3
+        ld1r            {v2.2s}, [x0], x1
+        ld1r            {v7.2s}, [x2], x3
+        ld1r            {v3.2s}, [x0], x1
+        sub             x0,  x0,  x1, lsl #2
+        subs            w4,  w4,  #4
+        urhadd          v0.8b,  v0.8b,  v4.8b
+        urhadd          v1.8b,  v1.8b,  v5.8b
+        urhadd          v2.8b,  v2.8b,  v6.8b
+        urhadd          v3.8b,  v3.8b,  v7.8b

in theory a single urhadd instruction is enough but I doubt it's faster,
doing 2 with 8 values each might though.

Changed to 2 urhadd with 8 values each, and using a different register for output; 1 cycle faster

+        st1             {v0.s}[0], [x0], x1
+        st1             {v1.s}[0], [x0], x1
+        st1             {v2.s}[0], [x0], x1
+        st1             {v3.s}[0], [x0], x1
+        b.ne            1b
+        ret
+endfunc
+
+
+// Extract a vector from src1-src2 and src4-src5 (src1-src3 and src4-src6
+// for size >= 16), and multiply-accumulate into dst1 and dst3 (or
+// dst1-dst2 and dst3-dst4 for size >= 16)
+.macro extmla dst1, dst2, dst3, dst4, src1, src2, src3, src4, src5, src6, 
offset, size
+        ext             v20.16b, \src1, \src2, #(2*\offset)
+        ext             v22.16b, \src4, \src5, #(2*\offset)
+.if \size >= 16
+        mla             \dst1, v20.8h, v0.h[\offset]
+        ext             v21.16b, \src2, \src3, #(2*\offset)
+        mla             \dst3, v22.8h, v0.h[\offset]
+        ext             v23.16b, \src5, \src6, #(2*\offset)
+        mla             \dst2, v21.8h, v0.h[\offset]
+        mla             \dst4, v23.8h, v0.h[\offset]
+.else
+        mla             \dst1, v20.8h, v0.h[\offset]
+        mla             \dst3, v22.8h, v0.h[\offset]
+.endif
+.endm
+// The same as above, but don't accumulate straight into the
+// destination, but use a temp register and accumulate with saturation.
+.macro extmulqadd dst1, dst2, dst3, dst4, src1, src2, src3, src4, src5, src6, 
offset, size
+        ext             v20.16b, \src1, \src2, #(2*\offset)
+        ext             v22.16b, \src4, \src5, #(2*\offset)
+.if \size >= 16
+        mul             v20.8h, v20.8h, v0.h[\offset]
+        ext             v21.16b, \src2, \src3, #(2*\offset)
+        mul             v22.8h, v22.8h, v0.h[\offset]
+        ext             v23.16b, \src5, \src6, #(2*\offset)
+        mul             v21.8h, v21.8h, v0.h[\offset]
+        mul             v23.8h, v23.8h, v0.h[\offset]
+.else
+        mul             v20.8h, v20.8h, v0.h[\offset]
+        mul             v22.8h, v22.8h, v0.h[\offset]
+.endif
+        sqadd           \dst1, \dst1, v20.8h
+        sqadd           \dst3, \dst3, v22.8h
+.if \size >= 16
+        sqadd           \dst2, \dst2, v21.8h
+        sqadd           \dst4, \dst4, v23.8h
+.endif
+.endm
+
+
+// Instantiate a horizontal filter function for the given size.
+// This can work on 4, 8 or 16 pixels in parallel; for larger
+// widths it will do 16 pixels at a time and loop horizontally.
+// The actual width is passed in x5, the height in w4 and the
+// filter coefficients in x9. idx2 is the index of the largest
+// filter coefficient (3 or 4) and idx1 is the other one of them.
+.macro do_8tap_h type, size, idx1, idx2
+function \type\()_8tap_\size\()h_\idx1\idx2
+        sub             x2,  x2,  #3
+        add             x6,  x0,  x1
+        add             x7,  x2,  x3
+        add             x1,  x1,  x1
+        add             x3,  x3,  x3
+        // Only size >= 16 loops horizontally and needs
+        // reduced dst stride
+.if \size >= 16
+        sub             x1,  x1,  x5
+.endif
+        // size >= 16 loads two qwords and increments r2,
+        // for size 4/8 it's enough with one qword and no
+        // postincrement
+.if \size >= 16
+        sub             x3,  x3,  x5
+        sub             x3,  x3,  #8
+.endif
+        // Load the filter vector
+        ld1             {v0.8h},  [x9]
+1:
+.if \size >= 16
+        mov             x9,  x5
+.endif
+        // Load src
+.if \size >= 16
+        ld1             {v4.16b},  [x2], #16
+        ld1             {v16.16b}, [x7], #16
+        ld1             {v6.8b},   [x2], #8
+        ld1             {v18.8b},  [x7], #8

ld1 {v4,  v5,  v6}, ...
ld1 {v16, v17, v18}, ...

and adept the rest

Done

+.else
+        ld1             {v4.16b},  [x2]
+        ld1             {v16.16b}, [x7]
+.endif
+        uxtl2           v5.8h,  v4.16b
+        uxtl            v4.8h,  v4.8b
+        uxtl2           v17.8h, v16.16b
+        uxtl            v16.8h, v16.8b
+.if \size >= 16
+        uxtl            v6.8h,  v6.8b
+        uxtl            v18.8h, v18.8b
+.endif
+2:
+
+        // Accumulate, adding idx2 last with a separate
+        // saturating add. The positive filter coefficients
+        // for all indices except idx2 must add up to less
+        // than 127 for this not to overflow.
+        mul             v1.8h,  v4.8h,  v0.h[0]
+        mul             v24.8h, v16.8h, v0.h[0]
+.if \size >= 16
+        mul             v2.8h,  v5.8h,  v0.h[0]
+        mul             v25.8h, v17.8h, v0.h[0]
+.endif
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, 1,     \size
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, 2,     \size
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, \idx1, \size
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, 5,     \size
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, 6,     \size
+        extmla          v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, 7,     \size
+        extmulqadd      v1.8h,  v2.8h,  v24.8h, v25.8h, v4.16b,  v5.16b,  
v6.16b,  v16.16b, v17.16b, v18.16b, \idx2, \size
+
+        // Round, shift and saturate
+        sqrshrun        v1.8b,   v1.8h,  #7
+        sqrshrun        v24.8b,  v24.8h, #7
+.if \size >= 16
+        sqrshrun2       v1.16b,  v2.8h,  #7
+        sqrshrun2       v24.16b, v25.8h, #7
+.endif
+        // Average
+.ifc \type,avg
+.if \size >= 16
+        ld1             {v2.16b}, [x0]
+        ld1             {v3.16b}, [x6]
+        urhadd          v1.16b,  v1.16b,  v2.16b
+        urhadd          v24.16b, v24.16b, v3.16b
+.elseif \size == 8
+        ld1             {v2.8b},  [x0]
+        ld1             {v3.8b},  [x6]
+        urhadd          v1.8b,  v1.8b,  v2.8b
+        urhadd          v24.8b, v24.8b, v3.8b
+.else
+        ld1r            {v2.2s},  [x0]
+        ld1r            {v3.2s},  [x6]
+        urhadd          v1.8b,  v1.8b,  v2.8b
+        urhadd          v24.8b, v24.8b, v3.8b
+.endif
+.endif
+        // Store and loop horizontally (for size >= 16)
+.if \size >= 16
+        st1             {v1.16b},  [x0], #16
+        st1             {v24.16b}, [x6], #16
+        mov             v4.16b,  v6.16b
+        mov             v16.16b, v18.16b
+        subs            x9,  x9,  #16
+        beq             3f

you can branch before the 2 movs

Indeed. And moving the subs to before the st1s also seems to help a little

+        ld1             {v6.16b},  [x2], #16
+        ld1             {v18.16b}, [x7], #16
+        uxtl            v5.8h,  v6.8b
+        uxtl2           v6.8h,  v6.16b
+        uxtl            v17.8h, v18.8b
+        uxtl2           v18.8h, v18.16b
+        b               2b
+.elseif \size == 8
+        st1             {v1.8b},    [x0]
+        st1             {v24.8b},   [x6]
+.else // \size == 4
+        st1             {v1.s}[0],  [x0]
+        st1             {v24.s}[0], [x6]
+.endif
+3:
+        // Loop vertically
+        add             x0,  x0,  x1
+        add             x6,  x6,  x1
+        add             x2,  x2,  x3
+        add             x7,  x7,  x3
+        subs            w4,  w4,  #2
+        b.ne            1b
+        ret
+endfunc
+.endm
+
+.macro do_8tap_h_size size
+do_8tap_h put, \size, 3, 4
+do_8tap_h avg, \size, 3, 4
+do_8tap_h put, \size, 4, 3
+do_8tap_h avg, \size, 4, 3
+.endm
+
+do_8tap_h_size 4
+do_8tap_h_size 8
+do_8tap_h_size 16
+
+.macro do_8tap_h_func type, filter, size
+function ff_vp9_\type\()_\filter\()\size\()_h_neon, export=1
+        movrel          x6,  \filter\()_filter-16
+        cmp             w5,  #8
+        add             x9,  x6,  w5, uxtw #4
+        mov             x5,  #\size
+.if \size >= 16
+        bge             \type\()_8tap_16h_34
+        b               \type\()_8tap_16h_43
+.else
+        bge             \type\()_8tap_\size\()h_34
+        b               \type\()_8tap_\size\()h_43
+.endif
+endfunc
+.endm
+
+.macro do_8tap_h_filters size
+do_8tap_h_func put, regular, \size
+do_8tap_h_func avg, regular, \size
+do_8tap_h_func put, sharp,   \size
+do_8tap_h_func avg, sharp,   \size
+do_8tap_h_func put, smooth,  \size
+do_8tap_h_func avg, smooth,  \size
+.endm
+
+do_8tap_h_filters 64
+do_8tap_h_filters 32
+do_8tap_h_filters 16
+do_8tap_h_filters 8
+do_8tap_h_filters 4
+
+
+// Vertical filters
+
+// Round, shift and saturate and store reg1-reg2 over 4 lines
+.macro do_store4 reg1, reg2, tmp1, tmp2, type
+        sqrshrun        \reg1\().8b,  \reg1\().8h, #7
+        sqrshrun        \reg2\().8b,  \reg2\().8h, #7
+.ifc \type,avg
+        ld1r            {\tmp1\().2s},     [x0], x1
+        ld1r            {\tmp2\().2s},     [x0], x1
+        ld1             {\tmp1\().s}[1],  [x0], x1
+        ld1             {\tmp2\().s}[1],  [x0], x1

use a copy of x0 for loading in avg

This helps surprisingly much, around 4 cycles.

On arm, it is a net gain for the larger filters, but a loss for the size 4 case (since we need to push another register), so I'll skip doing it there.

+        urhadd          \reg1\().8b,  \reg1\().8b,  \tmp1\().8b
+        urhadd          \reg2\().8b,  \reg2\().8b,  \tmp2\().8b
+        sub             x0,  x0,  x1, lsl #2
+.endif
+        st1             {\reg1\().s}[0],  [x0], x1
+        st1             {\reg2\().s}[0],  [x0], x1
+        st1             {\reg1\().s}[1],  [x0], x1
+        st1             {\reg2\().s}[1],  [x0], x1
+.endm
+
+// Round, shift and saturate and store reg1-4
+.macro do_store reg1, reg2, reg3, reg4, tmp1, tmp2, tmp3, tmp4, type
+        sqrshrun        \reg1\().8b,  \reg1\().8h, #7
+        sqrshrun        \reg2\().8b,  \reg2\().8h, #7
+        sqrshrun        \reg3\().8b,  \reg3\().8h, #7
+        sqrshrun        \reg4\().8b,  \reg4\().8h, #7
+.ifc \type,avg
+        ld1             {\tmp1\().8b},  [x0], x1
+        ld1             {\tmp2\().8b},  [x0], x1
+        ld1             {\tmp3\().8b},  [x0], x1
+        ld1             {\tmp4\().8b},  [x0], x1

use a copy of x0

Ditto

+        urhadd          \reg1\().8b,  \reg1\().8b,  \tmp1\().8b
+        urhadd          \reg2\().8b,  \reg2\().8b,  \tmp2\().8b
+        urhadd          \reg3\().8b,  \reg3\().8b,  \tmp3\().8b
+        urhadd          \reg4\().8b,  \reg4\().8b,  \tmp4\().8b
+        sub             x0,  x0,  x1, lsl #2
+.endif
+        st1             {\reg1\().8b},  [x0], x1
+        st1             {\reg2\().8b},  [x0], x1
+        st1             {\reg3\().8b},  [x0], x1
+        st1             {\reg4\().8b},  [x0], x1
+.endm

...

+// Instantiate a vertical filter function for filtering a 4 pixels wide
+// slice. The first half of the registers contain one row, while the second
+// half of a register contains the second-next row (also stored in the first
+// half of the register two steps ahead). The convolution does two outputs
+// at a time; the output of v17-v24 into one, and v18-v25 into another one.
+// The first half of first output is the first output row, the first half
+// of the other output is the second output row. The second halves of the
+// registers are rows 3 and 4.
+// This only is designed to work for 4 or 8 output lines.
+.macro do_8tap_4v type, idx1, idx2
+function \type\()_8tap_4v_\idx1\idx2
+        sub             x2,  x2,  x3, lsl #1
+        sub             x2,  x2,  x3
+        ld1             {v0.8h},  [x6]
+
+        ld1r            {v1.2s},    [x2], x3
+        ld1r            {v2.2s},    [x2], x3
+        ld1r            {v3.2s},    [x2], x3
+        ld1r            {v4.2s},    [x2], x3
+        ld1r            {v5.2s},    [x2], x3
+        ld1r            {v6.2s},    [x2], x3
+        ext             v1.8b,  v1.8b,  v3.8b,  #4

I think '{zip,trn}1 v1.2s, v1.2s, v3.2s' is the clearer pattern on
64-bit. it works with ld1 {v0.s}[0] in the case there is no difference
between single lane ld1 and ld1r

Done

+        ld1r            {v7.2s},    [x2], x3
+        ext             v2.8b,  v2.8b,  v4.8b,  #4
+        ld1r            {v30.2s},   [x2], x3
+        uxtl            v17.8h, v1.8b
+        ext             v3.8b,  v3.8b,  v5.8b,  #4
+        ld1r            {v31.2s},   [x2], x3
+        uxtl            v18.8h, v2.8b
+        ext             v4.8b,  v4.8b,  v6.8b,  #4
+        uxtl            v19.8h, v3.8b
+        ext             v5.8b,  v5.8b,  v7.8b,  #4
+        ld1             {v30.s}[1], [x2], x3

same applies as for the 32-bit version. we should do the load and
combine pattern for all but the last two loads. small cost for height ==
4, larger gain for height == 8

Done

// Martin
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